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1. 143 742 283 INTEGRATED CIRCUIT 7415283 144 770 324 INTEGRATED CIRCUIT LM324 145 770 386 INTEGRATED CIRCUIT LM386 146 770 555 INTEGRATED CIRCUIT LM555 147 738 095 097 1595 97 2 1612 413 148 706 489 INTEGRATED CIRCUIT 76489 149 738 095 INTEGRATED CIRCUIT 811595 150 739 120 INTEGRATED CIRCUIT 8815120 151 706 502 INTEGRATED CIRCUIT 6502A 152 706 522 INTEGRATED CIRCUIT 6522 153 706 845 INTEGRATED CIRCUIT 6845 154 706 850 INTEGRATED CIRCUIT 6850 155 733 691 INTEGRATED CIRCUIT 3691 156 705 050 INTEGRATED CIRCUIT 5 5050 157 201 601 647 VIDEO PROCESSOR 1 158 201 602 648 SERIAL PROCESSOR 1 159 707 002 INTEGRATED CIRCUIT 7002 160 704 816 INTEGRATED CIRCUIT 4816 161 201 629 INTEGRATED CIRCUIT 23128 162 201 628 INTEGRATED CIRCUIT 23128 163 164 800 004 SOCKET D I N 5 WAY 165 825 000 SOCKET UM1233 E36 166 800 002 SOCKET D I N 6 WAY 167 800 001 SOCKET D I N 5 WAY DOMINO 168 800 003 SOCKET D I N 7 WAY 169 800 304 SOCKET D TYPE 15 WAY 170 171 800 059 PLUG 17 WAY 172 800 055 PLUG 10 WAY 173 800 050 PLUG 2 WAY 174 800 051 PLUG 3 WAY 175 800 070 SHUNT 176 800 054 PLUG 8 WAY 177 880 040 SLEEVING BLACK NEOPRENE 178 201 029 HEATSINK 179 870 420 WIRE TCW 180 800 200 FASHION TAB 181 800 006 CONNECTOR IDC 34 WAY
2. lt lt 14 14 lt gt 1 U1 N YY W H UI 15 22 Plate ceramic 11 2 59 ELTCUTE Diagram 8 5 Resistors R Value o o Vo XO NO LO XO LO XO XO LO LO LO LO LO LO LO XO XO e c4 O lt EC IF OT Y Y Y LO ey eno o b EC b EIN S 00 00 ts 4 09 uud 1 1 1 wH de DO 0 oe OY 0 0 VN CN CN TITTEN N N o N 3 333 27 35 277 G 0 a 4 MER MO MX OOOO Z4 qi o MIN gt 2929909939 NINO IN e N GO 002 Z A lt A SP od oc on 14 Hs gt gt gt MM AM eO XA CN ST 1 OY 62 lt 1 00 ON CO 00 SP LO amp O T 00 OY Oc ON PA 02 NO C01 NMM
3. One simple problem that sometimes occurs is that of getting twinkling characters in some of the higher modes of graphics or smeering of the cursor If this was not originally a problem but has developed after some months of use then it may well be that the heat sink on the video processor has become dislodged This can be put right by applying firm pressure to the heat sink and also possibly by applying more heatsink compound between it and the top of the integrated circuit 42 Certain other faults on the VDU display associated with the UHF output can be cured by adding extra decoupling to the supply to the modulator to improve its stability A 10 ohm series resistor with a 4 7 uF capacitor to earth is usually sufficient It has been noticed that problems can occur with some of the 74LS74 ICs especially from certain manufacturers If there is any problem therefore with the cleaness of the clock pulses applied to the 6502A or problems with the PAL encoder circuit or more likely with the RAS signal it would be worth checking the output of these ICs to see if they are driving to the full TTL levels Unfortunately a number of problems also arise when people have tried to do their own upgrades and have made mistakes or used bad soldering techniques In particular a number of people seem to get the printer upgrade on the issue 2 board wrong and therefore this should be checked very c
4. J 313130 NHN13Y 0 51 SNOlLdO dN 1491 25 1 8 5 897 7 WLI 99414 NI 9719 90 54 8 12365 12 34 253 ya 825194 L a uiy 92 0 LMS 031 1033 two 9vd AWO 30vuodn 39014 14V9 5 2 ovd 14 ZHWI wou it diagram 9 6 it diagram ircu 9 7 Power supply c 9 8 Parts list DISC OTY REMARKS EL B ECONET DISC 1 SPKR 55 CASE LOWER ASSEMBLY MODEL ECONET TEM PART No DESCRIPTION 1 A2 210 232 CASE LOWER 2 A2 103 500 MAIN PCB ASSEMBLY MOD 3 A1 103 004 A CASE LOWER ASSY 4 1 103 001 KEYBOARD ASSEMBLY 5 A2 21 111 CUT RIGID PVC LABEL 6 A2 201 098 CUT RIGID PVC LABEL R 7 8 890 000 STICK ON FEET 9 10 800 600 B N C CONNECTOR 75R 11 870 109 VIRE 7 0 2 WHITE 12 13 882 644 No 8x 9
5. CQ QOO Oo 555 timer circuit IC16 provides a reset signal both at power up and also when the BREAK key is pressed There is also a separate reset circuit using a CR combination from the 5 volt power supply 10 and R20 and D1 to provide a signal called Reset A which is fed to IC3 the internal VIA The idea is that although the 555 timer produces a general reset at power up or when the BREAK key is pressed Reset A goes low only on power up By interrogating the interrupt register on IC3 on the occurrence of a general reset the microprocessor can discover Whether it was a cold start ie power up or a warm start ie the BREAK key has been pressed when the system has already been in use for some time 3 2 Memory and address decoding 31 1 4 Kbytes of ROM are catered for in the address map 15 1 4 Kbytes of this ae contained in the operating system IC51 This is in fact a 16K device but 3 4 of it is left unused and it is in this area that the I 0 device memory map is situated Four other ROMs ICs 52 88 100 and 101 are on the main circuit board They may all be 16 Kbyte devices in which case any one of them may be switched into the 16 Kbyte space in the memory map by writing to the ROM select latch IC76 Alternatively four 4 Kbyte ROMs may be in these four sockets in order to fill the 16 Kbyte space assigned In this case a two line to four line decoder half of IC20 is
6. PP 1 3 als 127 259 oca An T TC 20 8 LL xw SV 32 Bo 3E 410 712 1415139 bsy n al n Bor E23 Ac 45 2MHZE d 6 2 45 5221 efst wi Do 2 1 1 5105 s 1501 150R 2s 57 paar FRED HA 4 3 POIRI O ov GER 0586 NOT 3 7 73 FOC 1 x Sofo 74158386 7 338 PROT 1653 3 u 1 1232 ike 74 504 5 4 q 13 wREN 5 51 AD 3 12 via B 23 42 Le i 630 RC DATA a ien 2621 E Via A 2 a WR DAT 24 305 3 A5 ja c 1623 IN sans a E 1083 DA i RA 2 q 2459 8 lt gt LL 38 eex ete P P NT 2 ino n EE a N Mais E aj ce or 12 j RSTAm2DiRsT rael 745 wf ape IMHE OS 3 765 mre ster 10 252 an his 658 n 2 38 0 ean BLP IMMZE vs ds 167 5 EXE 2 jset CONNECTOR 57 ram ME E ATOR eo 6 E eo PLI 17 26 n 1 2 VIDPROC T ipee Fh 2 12 ust 518 n oa E 5 SW EXT ROM CONNECTOR icp p iNTOFF 15 o m amp p SERPROC cr SQ o 11 10 ot 5w5v 0 CRT 19 4 M gt RSTA 5 RAW n vss 0 esp wS DZ o 201 7415
7. S18 North S19 East S20 North 521 2 x East West S22 North S32 West S33 West ix Test using a FIT and if available a PET 4 5 Modification E Add Econet Interface to Model A i The following parts are required 5 off 14 pin DIL IC sockets 1 off 20 pin DIL IC sockets 1 off 28 pin DIL IC sockets 1 off 7415163 IC76 already fitted on model B 1 off 7416123 IC87 Not required if disc already fitted 1 off 68B54 IC89 1 off 7415132 IC91 T wtf 75459 IC93 2 off LM319 ICs 94 95 1 off 74LS244 IC96 off 741574 IC97 off 27 off 10uF Tantalum Capacitor C18 off 10uF Ceramic Capacitor C23 64 EPROM with NFS 2 off Rows of 8 MOLEX pins off 5 pin 180 degree DIN socket SK7 off 8X22K SIL resistor pack 2 not required if DNFS already fitted 20 off 2 tolerance 1 4W resistors as follows R34 10k R40 1 R64 1M5 R44 1M5 R52 1k0 R36 1M5 R45 10k R59 56k R38 100k R46 1k0 R60 56k 00k R48 1k R62 56k R35 10k R41 100k R51 10k R63 56k 29 R39 100k R47 1k5 R61 1k0 ii Solder all of the above passive components onto the main PCB iii Insert all of the above integrated circuits into their sockets iv Cut the wire links at link positions 52 S12 and S13 512 and 513 should already have been cut on Model B s v Set the following link positions using MOLEX jumpers S18 North S19 East S20 North 521 2 x East West S22 North S32 West S33 West er vi On issue 1 2
8. a SP SP SE SP SI SP SP SI LO LO LO LO issue 7 onwards 60 8 8 Not fitted 54 Not used 55 3k3 56 Not used 57 10R 545 58 1508 1 7 59 56k 2 13 7 60 56k 2 13 7 61 1k 13 8 62 56k 2 13 7 63 56k 2 13 7 64 1 5 1347 65 3k3 3 10 66 10k 5 10 67 10k 5 10 68 3k3 2 7 69 3k3 10 8 70 3k3 2 6 71 2 7 14 6 72 3k3 8 9 Not fitted 73 3k3 7 9 Not fitted 74 2k2 1245 75 82k 1 25 35 76 10k 1 39 77 100k 13 5 78 150k 14 4 79 820k 15 5 80 39k 14 5 81 3k3 10 6 82 150k 14 4 83 4 7 54 84 10k 13 9 85 3k3 10 6 86 220 14 5 87 8k2 14 88 812 14 65 89 4 7 T1344 90 4k7 13 5 91 8208 7 6 92 820R 7 6 93 3k3 14 4 94 100R 8 6 95 2 2 14 4 96 3k3 14 4 97 2k2 14 4 98 1k2 45 99 1k2 10 2 100 1 2 11 2 01 1k0 T2 02 100R 6 5 03 1 0 0 2 04 100R 9 8 Not fitted 05 100R 8 6 06 56R 8 4 97 RO 7 5 08 3k3 3 3 09 1k8 2k713 1 SOT 10 68R 14 2 111 68R 14 3 12 68R 14 3 13 68R 14 3 14 18R 1W 11 1 15 4110 442 issue 7 onwards issue 7 onwards issue 4 onwards 61 qq A CO N H Co gt aux 9
9. 181 61 y 1645 cer 245139 1045 745139 Sy Ev MA 39 wv LE 12 5 a T 3 5 E R6 282 18 17 109 gt 8 Ao 47 2 3 2 2 5 39 5 i ipu E 29h 3 ale 5 6 NU 81 595 z 127 7438 81149743 7 568 SER 14 13 5 233 3 n A7 3 ANS L 3 cas 7 4131 us Al vIDFROC DAL 92 14 12 deud CASO 5 B 502 AUDIO 2 ror OUTPUT 18 2 164 756 383 D t GSO 16 1 gt se 4x1 EJ 4816 481543 4816 3 or 14 E ESYNC 12 3 18 T 13 IY ager Dhs ov 12 8 12 13 38145 teste LPSTB 2 ien ale Yr 10 as pru b3 las 292 6 ov 30165 31 01595 11 RAS a 6 T 18 AQ 24 Mage 16 5 1 12 3 cas CAS yr 5 ov 3555 laz Do 2 Saz Y ha or 09 MAE MAS 14 2203 ol Ja Yar 1672 162 8 9 2 ad 3 AD ov 7124 545 2 jac 8 OL D 05 1 29 IMAZ 23 04 3 m 28 MABRY Ys EXTENSION i i OS 27 04 586 a 20 gt 28107 Tif Y SS 1 3 V ANALGIN 5 Ur des Rat 1910 a5 5 fw NRST 2 17 rags jet 95 pg 13 11 RAS a Ras ras 2 A HPGFO WR TTXVOU IMA13 MAS 4 2515 A In 15 1 15 FRED T TUS MAI cs CRTC DTE 5 sp cas Cas gt TRE E IMHZE 3 6 44 i 7 2 00 7 00 b 7 00 B f R108
10. 10 FE08 3 00 Start of user RAM in Start of user RAM in Model A Model B with disc 0E00 1900 Figure 2 Memory map including internal hardware 11 controller video processor Teletext hardware Random Access Memory on the Microcomputer is provided by either 8 or 16 dynamic memory devices ICs 53 68 These devices store 16K bits each and therefore in the Model B the data inputs and outputs of one pair of devices are paralleled for each of the 8 data bits DO to D7 To address 16K bits requires 14 address lines and this is achieved on the 4816 by having 7 inputs and latching in the addresses in two halves by using a row address strobe RAS and a column address strobe CAS Two octal buffers 811595 have to be used to multiplex the appropriate processor address lines onto the RAM address lines ICs 12 and 13 However the 6845 CRT controller IC2 also needs to access the RAM and what is more it accesses it differently depending on whether it is working in the Teletext mode or in one of the other graphics modes Therefore two more pairs of octal buffers are used ICs 10 and 11 for the Teletext mode and ICs 8 and 9 for the other modes the main difference being that in these modes the three least significant address bits are produced by the character row address lines from the CRTC in order to give the bit mapping of the characters in the RAM memory rather than having the ROM character generator as
11. BBC Microcomputer E Service Manual Microcomputer service manual SECTION 1 BBC Microcomputer Models 4 SECTION 2 BBC Microcomputer Model B 51 54 SECTION 3 Additional Upgrades 1770 Disc interface daughter board upgrade 64K Sideways RAM upgrade Part No 0433 001 Issue 1 October 1985 within this publication the term is used abbreviation for British Broadcasting Corporat Copyright ACORN Computers Limited 1985 Neither the whole or any part of the informati ion on contained in or the product described in this manual may be adapted or reproduced in any material form except with the prior written approval of ACORN Computers Limited ACORN Computers The product described in this manual and products for use with 16 are subject to continuous development and information of a technical nature and particula and its use including the information and particulars in this manual are given by ACORN Computers in good faith However 1t 15 acknowledged that there may be errors or manual A list of details of any amendments or manual obtained upon request from improvement All rs of the product missions in this revisions to this ACORN Computers Technical Enquiries ACORN Computers welcome comments and suggestions relating to the product and this ma All correspondence should be addres
12. He ar 01 1 OD 9 IMHZE Ras 34 arty 5 Lao IMHZE c mas Al D m Eursp2 x he 1 36 1176 3 1 n 2 c 87 apa 1036 Sa 1639 PINSK uar 76 25953 I ww 1 Una Ras 45 io a 3213 cas CASH Zas 44 AS B hs 2 DO 00 2 00 23 1240 121 4 A3 12 47 Ha DI 7 42 3 6 ES 5 5 5 n 5303 ME 2 At 24 20 44 4 ACORN 40 24 Bias si Aag 15 Hij RES n TUBE 07 1 As 9112 06 149 05 5 sy 6 04 Rig T NW 10 Las nrl ub 64595 RAS RAS RAS VS 02 al casha cash Di MA A 2 en 72 008 TA 5 1 T o NTUBE d s Raz wj NIRO 2 2 vcc3 32 n RAM ADD W 2 2 21 RA RINW 1 C26 E e TM F 11 5v ov VF YS Tx Tov Tov Tov 741586 12 vou SY 24 586 TANT TANT ov ov ov ov 5v ALL CAPACITORS MARKED ARE DECOUPLING CAPACI cuit diezram 55 CONNECTOR SKS q lt
13. lt 0 2013 pes bos goo so 3l i lt 0 lt 0 Cx ESL VOZI Wall Wall ar 9 4 layout E gt lt 981 su 0 Lei Wall W311 ano 275 lt 0 921 WALI 2 6 WALI WALT 181 WALT lt 291 2071 58311 gt 2 x Pt 191 SWALI Walt TI Riaan a 021 Wall 5 lt lt lt a a lt 7 9311 v giu DT NOTES INSERT WIRE LINKS SHOWN THUS e USING WIRE ITEM 176 1 2 ENSURE POSITION OF LINK SHUNTS ITEM 173 IS CORRECT DRAWING INSERT FASTON TAB ITEM 177 POSITIONS SHOWN THUS A 3 4 6 157 WHEN REQD FIT HEATSINK ITEM 175 9 5 ADDRESS BUS Ad AIS DATA BUS DO 0 5 VSPINT n ael las Rana IVSPROY e sv 2 3 1625 1 12 Ais Al 1622 B 2 7418123 Ala 5 741530 5 Sian 4 sl 1687 ss d sp 1c32 6 i 5 y rero 7 55 14155
14. D Ni Nu NA 3 Ng C NH NB E 15 NA INTOFF STA 2 Rs Nm NA C 35 r an ee Na el ADLC Riw 1 144149 5 NOT FITTED ee 1721 741500 1029 162 3 74L 569 3 4 741532 2 gt Vi b PROC NM O 5 1627 9 1091 s Als 2438 152 745122 1029 N oy lt aise N Al E n 37 24 io UE V lt P po EN ES LEN 16 1 741520 MN sv 4 BE y A 13 2 12 EN Ep NS ELLEN lt lt 2 A RIN 29 a EST 53 Riy NOT REQ LSO 150 a Poe se wan Es N T Rr Rats Riw NT x 630 Al 6 SERIA 07 estie 1 209 QUAL ae ye x N 7 3 Neq E 298 Du LED Kj pea 1 5 8 4x8 10 he 5 T Z r 595 2 2 SERFR gcs zi 5 CTS 297 2 2 28 14 4 5 AE Sy c 39 Lar 22K 2622 12 26 1 5V 1036 GUYS 240510 46 QU 3280238 av iv 23 524 H is Bos ES 33 GREEN 504 BER G IMS Riz 8122 8120 SR ER 828 8135 2 2 5 KS D 31153138 13 ER Bun le 42812 le 59 o
15. lt 2 gt Aa UN OW OO U NB Oo 00 W ww Not u GF YU N N OPA HA Su ns A A 44 S N WWW N NY 8 Hs CO CO CO CO Hs Hs Hs 4 dS 9 8 2 2 2 14 GN 2 8 2 ds ds 0 9 PONWAH AH A A 1 BRO N OY C Not fitted Not fitted Not fitted Not fitted issue 4 onwards issue 4 onwards issue 4 onwards issue 4 onwards 62 8 6 Links Some links have been omitted on later issue boards whilst others have been added LINK PCB Circuit position diagram 1 2 108 2 5 Not used on issues 2 and 3 2 2 161 1244 SM 2 173 14 14 Only fitted on issues 1 2 3 4 12 12 1 9 5 26 195 14 9 Only fitted on issues 1 2 3 6 26 205 15 9 Only fitted on issues 1 2 3 p 30 65 4 9 84 327 015 2 9 Oi 3541287 334 10 10 45 15 1 8 L 7547210 13510 12 97 70 9 8 13 100 67 9 8 14 TOT 53 7 10 15 1
16. 3 boards only the following modifications are requi red Remove the capacitor C17 and replace it with a 2 2nF capacitor Cut the PCB track from IC26 pin 6 to IC96 pins 1 and 19 leaving the track from 26 pin 6 to IC97 2 intact Cut the track from IC89 pin 26 to IC97 pin 4 and link IC26 pin 9 to IC96 pins 1 and 19 and also to 1 97 pin 4 vii Test using a FIT and if available a PET 4 6 Modification F Add 8 inch disc interface to Model B As Modification D Add 5 1 4 inch disc interface but add x Set the following link positions by cutting the indicated PCB track and inserting a wire link LINK CUT TRACK WIRE LINK S4 East Solder side West S10 West Component side East 527 West Solder side East 4 7 Partial upgrading If you want to upgrade a Model A to enable it to run software intended for use with a model B but do not want all the various interfacing facilities then it is only really necessary to add the RAM and the 6522 VIA and change link 525 The VIA is needed as some professional software uses its hardware timers If you want to use sideways ROMs then you will need to add the 7415163 IC76 and be sure that links S12 and S13 are cut 30 5 Selection links and circuit changes 5 1 Selection Link Survey Here is a survey of the options which may be selected on the Microcomputer by selection links 51 to 539 These links may take the form of tracks on the c
17. Note the edge connector 1 is nearest the speaker thus the polarising key is pin 3 and pins 4 and 5 are empty Also check that there are no short circuits between any of the edge connector pins Repeat the tests for the other edge connector v Insert ICs 98 and 99 turn the machine on and type REPEAT SOUND 1 GET 0 0 UNTILO RETURN Now press any alphanumeric key and you should hear the voice synthesis operating If the pitch is wrong follow the instructions in vi below If there is no speech double check the modifications and try again vi The pitch of the speech must be set From Issue 4 PCBs onwards this is a simple matter of adjusting VR2 which is situated just west of IC98 On Issue 1 2 and 3 PCBs the resistor R32 between ICs 98 and 99 may need to be changed to achieve the best result The method for setting the pitch is to connect a frequency meter to pin 3 of IC99 and to adjust VR2 until the meter reads 160 kHz or 100 Hz or as close as is obtainable by changing R32 vii Reassemble the machine Before fitting the ROM socket cover into the the case lid remove the rforated section of the black label above the ROM sockets It may be necessary to trim the label to match the case cutout For early version cases without a rib on the underside behind the keyboard cutout remove the two small lugs on the ROM socket cover before fitting viii
18. 110 52 7 8 19 102 102 7 8 20 123 55 9 8 21 122 9 8 2 22 127 70 9 8 23 114 219 1373 W 0 24 181 195 1353 W 0 29 245 185 42 26 221 68 10 1 21 226 95 1771 TW 28 237 144 12 6 TW 29 237 146 12 716 30 284 20 8 8 3T 2707470 14 3 PW 321 2955 505 9 9 PW 33 295y 67 10 9 PW 34 200 65 9 8 T 35 245 20 9 8 T C 36 260 20 9 8 TC 37 280 20 9 8 TG 38 300 15 9 8 T C 395 2954215 9 8 Not fitted after issue 4 Not fitted after issue 4 when Econet fitted Wire Wire For external issue issue issue issue issue issue 35 ds gt 4 link in Model link in Model A connections onwards onwards onwards onwards onwards onwards 5 3 Circuit Modifications from issue 1 to issue 7 In this next section are listed the mor taken issue momen come Compu 5 3 1 1 Th 9 gt Li s Pi pin 1 Chan circu 3 4 place the circuit design it 7 Since there are so few issue 1 across an issue 1 board and cannot 5 t we will ignore the changes from 1 important changes which have has evolved from issue 1 to boards in circulation at the to 2 and suggest that if you olve any fault which occurs on hat you should consult the Technical ters Ltd Changes from issue 2 to 3 line on PL9
19. A video output is also provided which consists of a summing of the RGB signals in such a way as to give an appropriate grey scale On issue 4 boards onwards the option is given of adding colour to the video signal in order to provide a PAL encoded video output See section 3 4 Moving on round in an anti clockwise direction we come to the two serial interfaces the cassette interface and the RS423 These facilities are both provided by a standard Asynchronous Communications Interface Adaptor the 6850 and a custom designed circuit referred to as the serial processor This processor contains the programmable baud rate generators for transmit and receive which provide the clocks for the ACIA The ACIA itself is responsible for serialising the data providing the control lines for the RSA23 and generating interrupts whilst the serial processor switches these data and control lines between the cassette and RS423 interfaces The serial processor also provides data separator and sinewave synthesis circuits for the cassette recorder as well as a means of detecting the presence of the incoming tone from the recorder See section 3 5 The next section is the analogue input port which is a four channel 12 bit converter which is discussed in more detail in section 3 9 and the interfacing survey see chapter 7 In the NW corner is the Econet section which centres around a 68B54 Advanced Data Link Controll
20. ASSEMBLY DRAWING MODEL B DISC ECONET 1 PER WORKS ORDER RESISTOR 18R 1W 10 CARBON FILM 1 R114 RESISTOR 10R 1 4W 105 CARBON FILM 3 R14 18 57 RESISTOR 56R 10 CARBON FILM 3 8106 124 131 RESISTOR 68R 1 4W 10 CARBON FILM 6 10 113 129 130 RESISTOR 828 1 4 10 CARBON FILM 3 R120 121 122 RESISTOR 100R 1 4W 10 CARBON FILM 5 R94 102 105 119 171 RESISTOR 150R 14W 10 CARBON FILM 4 R22 23 49 RESISTOR 470R 1 4W 10 CARBON FILM 4 R123 128 15518 15 RESISTOR 680R 1 4W 10 CARBON FILM 3 R150 155 157 RESISTOR 820R 1 4W 10 CARBON FILM 3 R91 92 148 RESISTOR 1K 1 4W 10 CARBON FILM 20 R29 33 37 46 52 53 61 101 103 107 115 118 134 135 139 140 145 98 99 100 RESISTOR 1K2 1 4W 10 CARBON FILM 2 R154 127 RESISTOR 1K5 1 4W 10 CARBON FILM 2 R138 146 RESISTOR 2K2 1 4W 10 CARBON FILM 7 R74 95 97 117 136 152 170 RESISTOR 2K7 1 4W 10 CARBON FILM 1 R71 3K3 1 4W 10 CARBON FILM 12 R10 65 68 69 70 81 85 93 96 08 156 160 RESISTOR 1K8 1 4W 10 CARBON FILM 1 R141 RESISTOR 3K9 10 CARBON FILM 4 R116 126 137 147 RESISTOR 4K7 1 4W 10 CARBON FILM 8 R6 28 31 83 89 90 162 173 RESISTOR 5K6 1 4W 10 CARBON FILM 1 R161 RESISTOR 8K2 1 4W 10 CARBON FILM 2 R87 88 RESISTOR 10K 1 4W 10 CARBON FILM 12 R1 3 8 17 27 30 66 67 76 84 172 RESISTOR 12K 1 4W 10 CARBON FILM 1 R143 RESISTOR 15K 1 4W 10 CARBON FILM 1 R144 RESISTOR 22K 1 4W 10 CARBON FILM 3 R4 16 174 RESISTOR 39K 1 4W 10 CARBON FILM 5 R9 15 24 50 80 RESISTOR 82K 1 4W 10 C
21. Test using a FIT and if available a PET 4 4 Modification D Add 5 1 4 inch Disc Interface to Basic Model B i The following parts are required 1 off 8271 IC78 2 off 7438 ICs 79 80 1 off 74LS10 IC82 2 off 7415393 ICs 81 86 2 off CD4013B 83 84 1 4020 10285 7416123 87 Not required if Econet already fitted off 2764 EPROM DFS IC88 or IC100 if Econet fitted but not required if DNFS already fitted 11 Insert the ICs listed above into the sockets provided on the main circuit board 111 On issue 1 or 2 circuit boards only connect the two pads of link position S8 with a wire link 28 If the MOS ROM version 0 1 is fitted in position 1051 then it must be replaced by a 1 2 MOS see modification A v the existing power supply does not incorporate an auxiliary power output socket it must be exchanged for a suitable unit eg ASTEC type vi On issue 1 2 or 3 circuit boards only cut the leg of IC27 pin 9 as close t componen reconnec of insul vii On issue 4 boards onwards ated wire o the PCB as possible and the track connected to it on the t side of the circuit board between IC27 and 1 89 then t the cut IC leg to the East pad of link 59 with a short Length cut the TCW link at position S9 viii Set the following link positions using MOLEX jumpers
22. WEST selects 5 1 4 disc EAST selects 8 disc Changes the pin connection of the index line on the disc interface 11 Selects Econet station ID NORTH is LSB See Econet upgrade instructions section 4 modification E 12 CLOSED ties ROM select line A to OV OPEN ROM select line A driven by IC76 On model A s IC76 is not fitted because sideways ROM S are not used ROM 0 1 52 is permanently selected Do not fit IC76 with this link closed 13 CLOSED ties ROM select line B to OV at IC20 OPEN ROM select line B driven by 76 Do not fit IC76 with this link closed See comments on link 12 14 CLOSED disables ROM output from page FD enables JIM OPEN enables ROM output from page FD disables JIM If link 14 is open then link 15 must be closed and R72 must fitted The purpose of this link was to provide access to an extra page of the OS ROM for development purposes It is unlikely to be used in production machines as it disables the 1MHz bus 15 CLOSED disables fast access to page FD via IC23 OPEN enables fast access to page FD via IC23 Link 15 must be closed if link 14 is open and R72 must be fitted See comments on link 14 16 CLOSED disables fast access to page FC via IC23 OPEN enables fast access to page FC via IC23 Link 16 must be closed if link 17 is open and R73 must be fitted See comments on link 14 17 CLOSED disables ROM o
23. West v Test using a FIT and if available a PET see section 6 2 4 2 Modification B Convert Model A to Model B i The following parts are required 8 off 4816 3 1 61 to 68 1 6522 1 69 2 off 7415244 70 71 1 off 7415245 1 72 off uPD7002 1 73 off 8815120 1 74 053691 1 75 7415163 1 76 741500 1 77 off 6 DIN socket MAB6H SK3 off 5 pin DIN socket MAB5WH 5 4 1 off 15 way D type socket 164801 1 SK6 2 off 34 way header 3431 1302 PL8 PL11 1 off 26 way header 3429 1302 PL9 off 20 way header 3428 1302 PLIO off 40 way header 3432 1302 PL12 25 11 Insert the above ICs into the sockets provided on the main circuit board Solder the connectors on to the printed circuit board 111 Cut the wire links at link positions 512 and 513 Move the MOLEX link at position S25 from South to North iv On issue 1 2 or 3 circuit boards only add a 2k2 ohm resistor between PL9 pin 1 and 5v on the solder side of the circuit board using a resistor with sleeved leads 5v is available at IC85 pin 16 33 mm due North of pin 1 v On issue 1 2 or 3 circuit boards only cut the track connected to PL9 pin 23 this may have previously been cut then link IC69 pin 40 to PL9 pin 19 This modification may have been made and if so a check should be made to ensure that it has been correctly performed CT vi On issue 1 and 2 circuit b
24. ie if the RAM can be written to but not read then the character at location 57001 will not cycle through 0 to 255 since the read instruction will be in error All these routines are working in machine code at high speed and therefore it is easy to use an oscilloscope to probe around the circuit to see what has gone wrong FOR 5 0 TO 2 REM ie 3 tests available PROCtest 5 NEXT SAVE ROMIMAG 3000 2000 END DEFPROCtest N offset amp 400 N 0 amp 3000 offsets PS amp F800 tables P 6200 amp offset 6800 REM RESET vector opt 5 IF 5 0 PROC strobe select lines IF 5 1 PROC DRAM test N 2 PROC RNW exercise WN H 0000 1001 CO N H CO C9 CO 47 170 amp 3200 offset amp 4433283F 180 amp 3204 offset amp 1B19021E 190 amp 3208 offset amp 13721293 200 amp 320C offset amp 002C002C 210 220 230 DEFPROC strobe select lines 240 OPT opt 250 260 loop 270 LDA 6 00 IC2 pin 25 CRTC 280 LDA 6 08 1 4 pin 9 ACIA 290 LDA amp FE10 Y 167 9 SER PROC 300 LDA amp FE18 IC96 pin 1 STATID or IC97 pin 4 INTOFF 310 LDA amp FE20 IC97 pin 2 INTON 320 STA amp 20 106 pin
25. itself are also given The measuring from the SW Comments CRT controller Internal VIA ACIA Teletext ROM Video ULA Serial ULA Sound generator Previously 741500 lection se are appear at more 741574 206 174 49 741500 286 183 50 741500 298 183 51 27128 214 24 52 27128 233 24 53 4816 287 69 54 4816 287 93 55 4816 275 93 56 4816 298 146 57 4816 287 146 58 4816 275 146 59 4816 264 146 60 4816 252 146 61 4816 298 68 62 4816 298 93 63 4816 264 93 64 4816 298 122 65 4816 287 122 66 4816 275 122 67 4816 264 122 68 4816 252 122 69 6522 160 29 70 7415244 137 24 7415244 184 29 7415245 199 29 1 07002100 173 8815120 183 201 v H N Co Co N N oes n 13 1 NNNN 1 O 00 3691 207 199 7415163 70 44 74500 38 14 8271 60 75 7438 42 46 6 6 7438 57 4 7415393 82 4 CO PO PO PO CO AE ES Os amp 4S CO N CO NO CO N PO Hs CONDO Hs 4 0 00 0 005 LM319 20 193 7415244 82 185 741574 4 171 5610015 75 TMS5220 35 75 27128 272 24 27128 290 24 GOIN SIR
26. that have the 220nF output capacitor on the cassette system a smaller value may be necessary The resistor is necessary to adjust the relative phase of the two tones of the cassette signal 41 a number of occasions the tracks on the right hand side of the keyboard PCB have become broken in transit This occurred more frequently on earlier versions and less so since the newly modified case has been used but if any of the keys on the right hand side are inoperative then this is a likely cause Excessively hard use of the keyboard may also cause solder pads to lift A group of non functional keys would indicate that this has happened This could be checked quite simply by the use of a meter to test continuity A number of people complain of interference on the sound signal This is caused by the pick up of digital noise as the track goes from the 1MHz extension bus input to the audio stages The solution is to connect a 10k resistor across from this line to ground This should only be necessary on issues 1 to 3 of the PCB It can be done by connecting the resistor between pin 8 of IC20 which is ground to the plated through hole just to the south of that pin It is necessary to Scratch away the solder resist very carefully from around this hole before you can successfully solder into it U 1020 1086 NN ide Pin 8 Ne dd Plated through hole Figure 12 Reducing noise on 1MHz audio input
27. 182 800 008 CONNECTOR IDC 26 WAY 183 800 009 CONNECTOR IDC 20 WAY 184 800 007 CONNECTOR IDC 40 WAY 185 186 742 123 INTEGRATED CIRCUIT 7415123 187 742 132 INTEGRATED CIRCUIT 7415132 188 742 393 INTEGRATED CIRCUIT 7415393 189 735 159 INTEGRATED CIRCUIT 75159 190 706 854 INTEGRATED CIRCUIT 68B54 191 754 013 INTEGRATED CIRCUIT 4013 192 754 020 INTEGRATED CIRCUIT 4020 193 708 271 INTEGRATED CIRCUIT 8271 194 770 319 INTEGRATED CIRCUIT LM319 195 201 666 INTEGRATED CIRCUIT 27128 84 6 IC7 N JHH PREP BPR pono 2 1639 1017 35 1019 1016 EGRATED CIRCUIT 81 IC18 IC8 11 IC74 IC3 69 IC2 ICA IC75 IC5 EGRATED CIRCUIT EGRATED CIRCUIT IC73 53 68 1 51 OPERATING SYSTEM IC52 BASIC 5 7 5 1 MODULATOR SK3 5 4 SK5 SK6 113 KEYBOARD 14 SERIAL ROM PL15 S21 2 39 520 22 25 26 31 33 520 21 2 22 25 26 31 33 511 7 511 FIT R114 1 18 N 19 E PRENAI 110 FIT 1 6 IF 201 601 8 11 L9 L12 IC87 IC91 IC81 86 1093 1689 1683 84 1685 1678 1694 95 1088 DNFS ROM 9 9 Glossary of abbreviations ACK ACIA ADC ADLC ADSR a sound ASCII code for BASIC BBC BNC used for 1 2 5 5 51 1
28. 3 VID PROC 330 STA amp FE30 IC76 pin 9 ROMSEL 340 LDA amp FE40 pin 23 VIA 350 LDA amp FE60 IC69 pin 23 VIA B 360 LDA amp FE80 IC78 pin 24 FDC 370 LDA amp FEAO IC89 pin 9 ADLC 380 LDA amp FECO IC73 pin 23 390 LDA amp FEEO PL 12 pin 8 TUBE 400 LDA FC00 PL 11 pin 10 FRED 410 LDA FD00 PL 11 pin 12 JIM 420 JMP loop 430 440 ENDPROC 450 460 DEFPROC DRAM test 470 OPTopt 480 LDX amp OF 490 STX amp FE20 Write to Vidproc 500 510 setup 6845 520 LDA table X table of 6845 data 530 STX 6 00 Register number 540 STA amp FEO1 Contents of register 550 DEX 560 BPL setup 6845 570 LDA 3 580 STA amp FEFE send down TUBE 590 600 restart 610 LDY 0 620 630 loop2 640 NOP 650 TYA 660 STA amp 7C00 Y 670 STA amp 7D00 Y 680 STA amp 7E00 Y 690 STA amp 7F00 Y 700 INY 710 BNE loop2 720 730 LDA amp 42 740 STA amp FE20 Write to Vidproc 750 JMP restart 48 770 780 790 800 810 820 830 840 850 860 870 880 890 900 910 220 930 940 950 960 970 980 990 1000 1010 1020 1030 1040 1050 1060 1070 1080 1090 1100 ENDPROC DEFPROC_RNW exercise OPT opts LDX STX 0F FE20 Write to Vidproc setup 6845 LDA STX STA DEX BPL table X table of 6845 data amp FEOO Register number amp FEO1 Contents of register setup 6845 3 amp FEFE send d
29. 320 X 256 2 pixels 4 bits pixel 16 colours 160 X 256 16 X 4 Red Pixel RAM Green description 16MHz 1 2 or 4 bits in video Blue processor Flash programmable Figure 5 Colour palette operation 14 The video processor contains piece of high speed 16MHz static RAM called a palette This memory can be programmed to define the relationship between the logical colour number produced by the RAM and the physical colour which will appear on the display Note that the information in the main RAM is unchanged by changing the palette it is its interpretation into physical colours which changes Modes 0 to 6 in the Microcomputer use software generated characters that is to say the character font to be produced on the screen is held in the memory mapped display area of the RAM so that graphics and or characters may be held The definition of these characters is stored in the operating system ROM from 0000 to C2FF DATA BUS IN SERIALISER CONTROL REGISTER Display 64 bit R enable mE TIMING 6845 clock R G B in MULTIPLEXER 16MHz CLOCK GENERATOR Cursor CURSOR CONTROL Cursor width Display invert 8MHz 2MHz 4 2 1 MH MHz out Figure 6 Video ULA block diagram 155 The speed of printing on the screen is much increased by the use of hardware scrolling There is a register in the CRTC which is used to define the start of screen address in the
30. 5 FLANGE HEA 14 882 712 No 4 x 7 16 15 882 986 NYLON WASHER I D 5mm 16 17 18 19 882 022 M3 x 8mm CHEESE HEAD 20 21 22 2 3 24 103 003 POWER SUPPLY UNIT 25 26 27 882 988 4 28 882 914 NUT FULL 29 882 343 x 5 8 PAN HD POS 30 8l PAN HD SUPERDRIVE ACCESS D POSIDRIV POST DRIVE BA INTERNAL TOOTH SHAKEPROOF WASHI REF STANDARD DRG 4 1 PANEL MOUNT SK2 2 2 LENGTH 5 BLACK SELF TAP 2 PLASTITE 5 3 USE WITH ITEM 24 DRIV E N N ASTEC SMPS MAIN CIRCUIT BOARD EM PART No IT 1 HO 1 OY WN 203000 103 500 A 520 180 500 100 500 560 500 680 500 820 500 101 500 151 500 471 500 681 500 821 500 102 500 122 500 152 500 222 500 272 500 332 RESISTOR 500 182 500 392 500 472 500 562 500 822 500 103 500 123 500 153 500 223 500 393 500 823 500 104 500 124 500 154 500 224 500 274 500 824 500 105 500 155 590 682 590 223 580 103 580 204 500 182 272 505 102 505 152 505 103 505 563 505 104 631 010 631M015 022 631 033 631 039 B DISC ECONET DESCRIPTION QTY REMARKS PRINTED CIRCUIT BOARD 1
31. IC99 ii On issues 2 and 3 the following modifications are needed On the component side of the main PCB Cut the track between IC3 pin 16 and the through hole 8mm to the west Cut the track between IC3 pin 17 and the through hole 10mm to the west On the solder side of the PCB Link the through hole 10mm to the west of IC3 pin 17 to IC3 pin 16 Link the through hole 8mm to the west of IC3 pin 16 to IC3 pin 17 These operations switch the signal lines to IC3 pins 16 and 17 Then also on the solder side of the PCB Cut the track between IC98 pins 13 and 14 Link IC98 pin 13 to PL14 pin 3 0 volts iii Issue 1 keyboard PCBs also need modifying as follows On the solder side of the PCB cut the track between pins 14 and 15 of the edgecard connector The pins are those furthest from the speaker ie further east On the solder side of the PCB link pin 14 of the edgecard connector to 0 volts This can be found on either of the capacitor legs nearer th centre of the PCB iv When the modifications are complete procede as follows Reconnect the keyboard to the main PCB Add the new connector for PL14 then with the computer turned off test for continuity between the following points 27 Edge connector pin number 1 61 71819110111112113114115 1C98 pin number 1131415 61 7110111 113114
32. Plate ceramic nF Disc ceramic oO 2 ONO TENUN u 01 4S N H C 5 58 16V Elec Plate ceramic Plate ceramic 5 1 Hj Hj Hj Plate ceramic m Hj late ceramic F 10V Elec Disc ceramic 4 Hj 00 4 OY O1 4 CO 10V Elec Plate ceramic 10V Tant Hs 4 r nj nj 3 gt Not used 47nF Disc ceramic 100nF Disc ceramic Not used 10nF Plate ceramic 100nF Disc ceramic 33 F Polyester F 10V Elec luF 35V Tant 10V Tant late ceramic 10V Tant pF Plate ceramic F Plate ceramic gt 4 Hj Hj FU 820pF Plate ceramic 4 Tur 10V Tant 33pF Plate ceramic 2n2F Plate ceramic 2n2F Plate ceramic 10nF Plate ceramic 220 Plate ceramic 33pF Plate ceramic 47 Plate ceramic Not used 10 Plate ceramic 47 Plate ceramic 10uF 10V Tant 270pF Plate ceramic 150pF Plate ceramic 47 Plate ceramic y 390pF Plate ceramic 100 Plate ceramic 47uF 10V Tant 100pF Plate ceramic 39pF Plate ceramic 10uF 10V Tant 470 Plate ceramic 220nF 4u7F 10V Tant OP gt 00 4 00 0 CO 01 N Y DN ds ds ds W
33. appear sav on two out of the three 5V Earth 12V Earth View from rear 0 Orange 0 Red of free plug 5V 12V 0 Yellow 0 Brown NC 5 0 0 cable Supplies 5V 1 25A 12V 1 25A 5 75mA Mating connector is AMP housing 1 350234 9 male pins 350 664 1 Figure 10 BBC auxiliary PSU outputs 24 4 Upgrading the In these instructions about how to add extra hardware to the for disc Econet speech etc some differences may occur depending on which hardware is already fitted This is made clear within each set of instructions In order to locate the positions of various of the selection links reference should be made to section 5 2 which gives the X Y coordinates of each link Dealers and service centres performing these upgrades must also conform to upgrade procedures and requirements as notified by their supplier and should refer to any available information updates for latest details 4 1 Modification A Convert from EPROM MOS to ROM MOS 1 Remove the four MOS EPROMs from their sockets IC52 1 88 1 100 and Ic101 11 Remove the BASIC ROM from the 1 51 socket and replace it in the IC52 socket 111 Insert the MOS ROM into the 1 51 socket iv Set the following link positions using MOLEX jumpers if fitted or tinned copper wire 518 North 519 East 520 North 521 2 x East West 522 North 532 West 533
34. assembly of case etc me Cassette package and a UHF TV t the unit at a later date The lid of the Microcomputer case may be removed after undoing four fixing screws two on the rear panel and two underneath When reassembling press Take care not to rear fixing screws onto the case 114 into which the rear the lid down at the rear whilst tightening the two lose the two spire clips pushed fixing screws locate NB Do not remove the lid with the mains power connected Inside the Microcomputer are three main sub assemblies keyboard and the main printed circuit board power supply unit To remove the keyboard undo the two or in some cases three screws and nuts holding it to the case bottom of the associated washers Unplug the 1 2 way loudspeaker connector from the 10 way serial ROM connector if fit take care to note the positions 7 way keyboard connector and the the main printed circuit board and ted The power supply unit is connected to the main circuit board by seven which may be unplugged Three screws on the push on connectors underside of the case are undone allowing the unit to be removed On that the same type of screw is used reassembly ensure The main printed circuit board can be removed after the two wires to SK2 composite video BNC socket have been disconnected Undo the four fixing screws five or seven screws on later i
35. component side of the PCB S34 to 38 10 A 10k resistor R172 was introduced between the analogue input on the 1MHz bus and 0 volts in order to reduce the input impedance and hence improve the signal to noise ratio See section 6 4 11 Link S39 was added in order to connect the 470pF capacitor C58 from the base of 07 to the emitter of 09 12 A 220nF capacitor 59 was added in series with 90 in order to AC couple the log amplifier on the cassette interface 13 A number of changes were made to the Econet control lines in order to speed up software control For details of how to bring earlier issue boards up to the current issue see the section on upgrading the Econet system section 4 modification E 14 Provision was made for mounting a right angled phono socket as an alternative to the free wired BNC socket normally used for video output 15 A 200k potentiometer VR2 was added in parallel with R32 in order to adjust the operating frequency of 1 99 for the appropriate pitch of the speech output 16 At some stage between issues 3 and 4 C34 the cassette output coupling capacitor was increased from 47nF to 220nF 5 3 3 Changes from issue 4 to issue 7 Issues 5 and 6 never went into production 1 R114 changed to its present value of 18 ohms 1W and C42 changed to 33pF 2 R75 went to its final value of 82k The reason for the change i
36. from single sided single density 100K to dual double sided double track density 800 Dot matrix or daisy wheel printers serial or parallel interface Teletext acquisition unit enabling Tele software to be downloaded into the BBC Computer as well as providing access to the normal Teletext services Pages may be grabbed and stored for later use 3 MHz 6502 second processor with 64K of RAM 280 second processor with 64K of RAM and a fully CP M compatible operating system IEEE interface Winchester 10 megabyte disc drive Prestel adaptor unit 1 2 4 Software Considerable attention has been paid to the overall design of both Systems and applications software A modular approach has been adopted specifically to ease the interfacing of various high level languages such as BASIC and Pascal to the operating system 1 2 5 Machine Operating System A 16K ROM is used for the MOS This software controls all input output devices using a well defined interface The MOS supports the following interrupts the full implementation only being available from MOS 1 2 onwards Event Timer 10ms used as an elapsed time clock 4 channel analogue to digital converter Vertical sync Keyboard and keyboard buffer Music tone generation and buffer Serial interface input and output buffers Parallel input output port 7 and hooks are provided to support other devices
37. its characteristic impedance of 180 ohms The cassette interface is a standard CUTS cassette interface It has two speeds 300 and 1200 baud controllable by software There is also motor control provided on pins 6 and 7 of the 7 way DIN plug The rating of the relay contacts is 24V at 1A DC and they should not under any circumstances be used to switch mains voltages no matter how small the current If you wish to get two BBC Microcomputers to send programs to each other on the cassette system then it is possible to do so by a direct connection provided a 1 5k resistor is connected between the signal line and ground 52 analogue input is on 15 way D type connector provides four A to D converter channels and two digital input lines which work on the internal 6522 VIA The pin connections are arranged so that the signals are divided into two sets intended for use with two games paddles each of which will have two A to D inputs and a voltage reference source as well as the ground and one of the digital input lines The conversion time for each channel is 10 milliseconds but you can choose to have as few or as many of the channels working as you wish by using the FX16 command Therefore if all four converters are required you need to allow 40 milliseconds to be sure of a successful conversion on any one channel However ADVAL 0 in BASIC or OSBYTE call 128 in machine cod
38. little RAM only 1 Kbyte and apart from providing the addressing for the RAM the only thing which the CRT controller has to do is to add the cursor information and sync signals In the other screen modes the information is stored RAM as actual bit patterns for every character that is written to the screen This is expensive in terms of memory usage between 8K and 20K in the different modes but it makes it extremely versatile especially when mixing graphics with text The addressing of the RAM for the different modes is performed by the 6845 CRT controller whilst the data is taken from the RAM and serialised by a custom designed circuit known as the video processor This data is not used directly to produce RGB information but can be thought of as a set of logical colour numbers which are passed to an area of high speed RAM within the video processor referred to as the colour palette This determines for each logical colour number which combination of red green and blue is produced and whether or not the colour is flashing The video processor is also responsible for selecting either the RGB signals coming from the Teletext chip or the signals coming from the palette and sending them out to the RGB buffers and the PAL encoder This RGB information is presented after buffering on the RGB connector To provide a UHF output the RGB signals are combined with the sync signals and fed into a UHF modulator
39. of the program so that to the receiving computer it is as if it were being typed in from the keyboard If you have a number of transfers to do then these commands could be programmed onto a single key on each machine When the program has been sent down you simply have to press BREAK on the receiving computer type OLD RETURN and then the program is ready for use If you are doing a BBC to BBC link over a short distance and want to use the full Speed of the interface then you will have to connect the hand shake lines as well as the data lines The data out from one computer should be connected to the data in of the other computer and vice versa and in a similar way for the control lines the RTS on one should be connected to the CTS on the other and vice versa If you are working over a longer distance and want to use only three cables data in data out and ground and are prepared to work at a slower speed without any handshaking then you have to loop back the RTS to the CTS on each of the computers so that each is permanently enabled for sending If you do not do so the 85423 output buffer fills up and printing stops after a number of characters have been sent to the Screen If you want to use the RS423 interface over a long distance at high Speed using the hand shake lines it might be necessary to terminate the receivers by making links 523 and S24 This terminates the line with
40. screen memory Thus in order to scroll the screen it is only necessary to increment this register by the number of characters per line and then write to the memory address where the last screen data was The number of address lines from the CRTC used to address the screen memory has to be sufficient to cater for the biggest screen 20 Kbytes Thus 14 address lines have to be used which means that when using the hardware scrolling technique the picture scrolls around in 32 Kbytes Consider a scroll of 8 Kbytes in a 20 Kbyte screen The original start of screen for the 20 Kbyte mode was amp 3000 After an 8 Kbyte scroll the current start of screen address is amp 5000 with the end of the screen as seen by the CRTC at amp 5000 plus 20 Kbytes which comes to amp 9FFF as illustated below amp 7FFF T Bottom of screen Screen Straight wrap around in 32K Top of screen Bottom of screen 8K Top of screen amp 3000 HIMEM Add amp 3000 if address gt amp 7FFF amp 0000 Before scrolling After an 8K scroll igure 7 Memory map to show addition of CRTC addresses 16 Since there is only 32 Kbytes of RAM this would mean that instead of accessing addresses amp 8000 to amp 9FFF you would be accessing locations amp 0000 to amp 1FFF Therefore when the address produced by the CRTC is greater than amp 7FFF ie 12 1 you have to add to the address from the CRTC a number which w
41. select lines of ROMs 52 88 100 and 101 in order to allow the use of extra ROM sockets on an external PCB Implemented from issue 4 39 CLOSED adds colour burst signal to the black and white video signal to produce PAL encoded video on the BNC socket OPEN Black and white video on BNC socket Implemented from issue 4 34 5 2 Table of link options The following table gives positions on the circuit board a list of selection links showing their circuit diagram grid reference interfaces are also given mm E N from SW corner and on the 5 main circuit diagram The links made in production on a standard model B without disc or Econet P plugable link T track W wire link closed 0 open NS E and W refer to orientation of tracks or plugs Some links have been omitted on later issue boards whilst others have been added LINK Circuit Options position diagram Model B 2 1082 5 Not used on issues 2 and 3 tted after issue 4 Z 2 16112 7 3 2 17314 14 Not fi 4 12 12 9 2 64 LOS 14 9 6 26 205 15 9 H z 7 30 65 4 9 8 32 4 215 249 TG 9 35 128 3 10 W C 10 45 15 1 8 El 78 210 137210 12 97 70 9 8 W 0 13 100 67 9 8 W 0 LOL 53 DS Tu I5 107 597 7 9 DRO 16 108 90 7 9 T C 17 108 52 7 10 TE 18
42. then fed to both receive clock and the monostable circuit IC87 is transmit clock inputs on the 6854 In the presence of a network clock permanently triggered and thus providing a data carrier detect signal for the data link controller chip Once the network clock is removed the monostabl immediately drops out and the data carrier is no longer detected The Econet is a broadcast system on which a number of stations may attempt to transmit their data over transmitting stations should detect software is included in the Econet the network at any given time In this case a situation called a collision can occur and then the the collision and back off before trying again to transmit over the network Collision arbitration System and is based on the station ID number Collisions on the network data lines result in the differential signal on the two data wires being reduced and this condition is detected by IC95 which When there is a good differential da is another dual comparator circuit ta signal on the network one output of IC95 or the other will be low in which case the output of IC91 pin 6 will be high indicating no colli on the network and the network sion When there are no collisions clock is detected by the clock monostable the data link controller is clear to send data over the network When there is a collision on the network bo
43. to be produced These signals NAND gates in order to produce the colour subcarrier signal which is added to the luminance output from Q8 by the buffer Q9 In order for the receiving the colour information a reference colour burst has to be provided at the beginning of each line A burst gate pulse of approximately 5uS television to interpret immediately after the horizontal sync pulse for each line is produced at pin 4 of 1 41 and it is timed by C45 and R109 This burst gate bcarrier signal which the television of that line The PAL signal may be allows through a standard colour su uses as its reference for the rest added to the video connector with the addition of a 470 pF capacitor between the emitter of Q9 and the base of Q7 This is provided as a link selectable option on later issues of the PCB issue 0 21 and 22 increase the luminance 4 on In modulated PAL diodes D2 of the darker colours eg blue in order to make coloured text displays more readable 3 5 Cassette RS423 serial processor For both the cassette and RS423 interfaces a 6850 asynchronous communications interface adaptor ACIA ICA is used to buffer and serialise or deserialise the da specifically designed for the switching to select ither RS423 circuit to synthesise a sinewave ta The serial processor IC7 Microcomputer contains two programma
44. used to select which of the four devices is being addressed by the address lines A12 and A13 Mixtures of these two cases are allowed for for instance two pairs of 8 Kbyte ROMs one pair or the other being selected by the ROM select latch and then the ROM to be used in each pair being selected by the 2 4 line address decoder Address decoding for the ROMs is by IC21 which decodes memory addresses 48000 to C000 and 4 000 to amp FFFF Locations from O amp 7FFF are assigned to the dynamic RAM and this is decoded by feeding A15 into pin 4 of IC21 11 the rest of the hardware is mapped within locations FC00 to amp FEFF This is decoded by IC22 whilst ICs 20 and 25 are used to mask off the ROM over this range of addresses ICs 24 and 26 decode the individual devices within this range some of which are read or write only IC23 detects when a slow 1 MHz device is being addressed and it calls for the 6502 to execute a slow clock cycle Note that in early versions of the BBC Microcomputer the operating System was contained within 4 EPROMs in IC positions 52 88 100 and 101 while the BASIC interpreter was located in IC51 This arrangement is abnormal and has been phased out Refer to the link selection survey 5 1 for more detailed information on this 10 FFFF FF00 TUBE 1 FEEO IO devices FECO Ext IO page 00 0 00 FE60 FE40 FE50 C000 FE20 1 Paged
45. 07 97 16 108 90 7 9 17 108 52 7 10 I8 T1057 52 7 8 19 102 102 7 8 20 233 55 9 8 21 122 65 9 8 22 127 70 9 8 23 177 215 13 43 24 181 195 3 3 25 215 185 26 221 68 0 1 27 226 95 2d 28 237 144 2 6 29 237 146 2 6 30 284 20 8 8 31 27041709 4 3 3222955 65 9 9 33 295 67 10 9 34 200 65 9 8 From issue 4 onwards 35 245 20 9 8 From issue 4 onwards 36 260 20 9 8 From issue 4 onwards 34280 20 9 8 From issue 4 onwards 38 300 15 9 8 From issue 4 onwards 39 2557215 9 8 From issue 4 onwards 63 64 9 Appendices 65 V IPPON 10 91 103 26 4446 0000 0 SVO WVU 491 IAAL 000 1 SVO M9T 5 5 552 66102 004d UAS 5754 HLLASSVO YOLVINGOW 0989 AYVMAYVYH 6879 ca Bx BE 73489 LYINOD4 LYINOOH 45 OL t block diagram ircui 1C 9 9 2 Assembly drawing 9 3 Case lower assemblv drawing 10 3 R93 R95R97 ru 505 ESI b maii lt 091 mau lt i 2 lt D ect nai 1 lt lt nw v 692 6014 Sou 9 lt 0 e 901u
46. 2 system CPU CR CRT CRTC CSYNC TS lt H J H 1 QH ID station IDC connectors underneath the computer IEEE488 A parallel interface usually associated with automatically controlled test instruments ACKnowledge line on the printer port Asynchronous Communications Interface Adaptor serial to parallel and parallel to serial converter 6850 Analogue to Digital Converter Advanced Data Link Controller Econet control IC 68B54 Attack Decay Sustain Release defining the envelope of American Standard Code for Information Interchange binary representing alphanumeric characters Beginners All purpose Symbolic Instruction Code British Broadcasting Corporation Bayonet Neill Concelman the type of bayonet connector the video output Control lines associated with the PA port on a VIA Column Address Strobe control line for the dynamic RAM Refers to the area of RAM selected by the CASO line Refers to the area of RAM selected by the CAS1 line Control lines associated with the PB port on a VIA Control Progam for Microcomputers 480 based operating Central Processor Unit 6502 Capacitor Resistor network Cathode Ray Tube Cathode Ray Tube Controller IC 6845 Composite SYNChronisation pulse from the Clear To Send control input on the 85423 port An American st
47. 21 vs 17 VSPINT 5 4 TES TMS6100 5v 5 1698 55 AB Rov VSPROY yee 1 144146 4 3 2 5720 07 2 a Ov c33 ov R3 5 POWER UP RESET 4038 11 55 Yl ov VREF 5 8 2 Stor ailh 307 pur De 5 gt 1 112 1 a 5 4 10 RSI 14 WE MI BMHZ 12 1618 9v Rh 76489 9 15 5 i ov 7 3 Hour 02 Busy Ico Rit 3 RI RIQIOK ROY x 6 2 of OSCILLATOR wi 25234 C VIAB R7 100K CT 8208 R92 8208 07 Far 2 e Cv 2v 33pF PRINTER 06 3 7 CS 2 RS R4 2 202 Qv INTE M 3 js 2K 107 OY 6 5 3 2 1 CONNECTOR Dt B 812220 N 819 04 4 3 R 1229 C40 5 S E 3 _ 741532 EI R r2 9 UK R 5 6 1043 22 17 8 3 Part or 1 8 2 147504 0 3 q a 215 ov 5 ca 4 2 10 ng 2 pn 5 144148 1 aly 7650 vipeo 242 1008 SIBL e e 16715 ov us D M 12 n aj PROC T ov vege p RB RI 13 x 5 7 5 PRE In 5 5 14324 ET n Lea TK A PBS Mj a 4 Ter 284 3 3 3 41 81585 2 A6 USERIIZ paz 12 3 3 CONNECTOR pp IU D 1 2 14 15 1 E X RIW 2 44 t PEZ 181682 2 deua mov
48. ARBON FILM 1 R75 RESISTOR 100K 1 4W 10 CARBON FILM 4 R5 7 11 77 RESISTOR 120K 1 4W 10 CARBON FILM 2 R132 133 RESISTOR 150K 1 4W 10 CARBON FILM 2 78 82 RESISTOR 220K 1 4W 10 CARBON FILM 2 R12 86 RESISTOR 270K 1 4W 10 CARBON FILM 1 R159 RESISTOR 820K 1 4W 10 CARBON FILM 1 R79 RESISTOR 1M 1 4W 10 CARBON 3 R13 20 21 RESISTOR 1M5 1 4W 10 CARBON 3 R36 44 64 RESISTOR PACK S I P 6K8 x 8 1 RP1 RESISTOR PACK S I P 22K x 8 1 RP2 POTENTIOMETER 10K VERT MIN PRESET 1 VR1 POTENTIOMETER 200K VERT MIN PRESET 1 VR2 RESISTOR 1K8 2K7 1 4W 105 CARBON FILM 1 R109 S 0 T ON BATCH BASIS RESISTOR 1K 1 4W 2 CARBON 1 R48 RESISTOR 1K5 1 4W 2 CARBON 1 R47 RESISTOR 10K 1 4W 2 CARBON 4 834 35 45 51 RESISTOR 56K 1 4W 2 CARBON 4 859 60 62 63 RESISTOR 100 1 4W 2 CARBON 4 838 39 40 41 CAPACITOR 10pF PLATE CERAMIC 1 e12 CAPACITOR 15722pF PLATE CERAMIC 1 C51 S O I ON BATCH BASIS CAPACITOR 33pF PLATE CERAMIC 2 C37 42 CAPACITOR 39pF PLATE CERAMIC 1 C56 82 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 631 047 CAPACITOR 47 PLA
49. Drawing Main PCB Layout Main PCB Circuit Diagram Issue 9 issue 7 board as full foldout Keyboard Circuit Diagram Power Supply Circuit Diagram Parts list for B disc Econet Glossary of abreviations WARNING THE COMPUTER MUST EARTHED IMPORTANT The wires in the mains lead for the computer are coloured in accordance with the following code GREEN amp YELLOW EARTH BLUE NEUTRAL BROWN LIVE As the colours of the wires may not correspond with the coloured markings identifying the terminals in your plug proceed as follows The wire which is coloured green and yellow must be connected to the terminal in the plug which is marked by the letter E or by the safety earth symbol or coloured either green or green and yellow The wire which is coloured blue must be connected to the terminal which is marked by the letter N or coloured black The wire which is coloured brown must be connected to the terminal which is marked by the letter L or coloured red If the socket outlet available is not suitable for the plug supplied the plug should be cut off and the appropriate plug fitted and wired as previously noted The moulded plug which was cut off must be disposed of as it would be a potential shock hazard if it were to be plugged in with the cut off end of mains cord exposed The moulded plug must be used with the f
50. ED CIRCUIT 741500 742 002 INTEGRATED CIRCUIT 741502 741 004 INTEGRATED CIRCUIT 74504 742 004 INTEGRATED CIRCUIT 741504 742 010 INTEGRATED CIRCUIT 741510 742 020 INTEGRATED CIRCUIT 741520 742 030 INTEGRATED CIRCUIT 741530 742 032 INTEGRATED CIRCUIT 741532 742 051 INTEGRATED CIRCUIT 741551 741 074 INTEGRATED CIRCUIT 74574 742 074 INTEGRATED CIRCUIT 741574 742 086 INTEGRATED CIRCUIT 741586 742 138 INTEGRATED CIRCUIT 7415138 741 139 INTEGRATED CIRCUIT 745139 742 139 INTEGRATED CIRCUIT 7415139 742 163 INTEGRATED CIRCUIT 7415163 742 244 INTEGRATED CIRCUIT 7415244 742 245 INTEGRATED CIRCUIT 74L8245 742 259 INTEGRATED CIRCUIT 7415259 742 273 INTEGRATED CIRCUIT 7415273 83 1 7 FP PRONNISPNRENCRO HNH o IS O1 ES ES ES PO I ES S ES CO 50 C53 55 C49 cal 48 52 32 95 43 46 58 13 Cl 57 29 0324 33 10 23 40 45 C25 A C20 C6 9 15 21 24 C38 39 C59 C27 C28 60 EZ 18 30 47 57 5 9 36 54 14 16 26 C34 11 1 2 16 17 18 Di 2 6 748 13 15 19 22 211 01 6 9 11 07 8 010 55 94 95 93 4 91927 37 89 98 99 100 101 ICI 3 69 78 27 79 80 40 IC21 49 50 77 1641 1643 IC33 37 IC30 31 34 44 97 38 47 48 24 45 20 26 IC42 76 IC70 71 96 1 14 72 1632 1015
51. O GS NOS WO ORAN Operating system BASIC ROM External VIA ADC convertor ps5 Disc controller 13 8 ADLC Econet Not fi ted Not Speech ROM fi ted Speech generator Sideways ROM Sideways ROM SV 8 2 Transistors 0 Circult Diagram BC239 BC239 BC239 BC239 BC239 BC239 BC309 BC309 BC239 10 2N3906 11 239 LO CO N F N H lt S ww O1 O1 8 3 Diodes Cireuit Diagram 6 8 4 6 ot fitted ot fitted N N 7 8 8 6 6 6 5 7 Not fitted 8 Not fitted issue 7 onwards 8 8 OY O1 CO ND ES 2 gt J OY O1 4S 2 Not fitted issue 7 onwards Not fitted issue 7 onwards 5 3 3 3 lt HAHA N N N CO CO 3 3 ds WWW CO 4 WW CO 8 4 Capacitors C Type Circuit Diagram 2n2F Plate ceramic 4 6 4 5 F Plate ceramic 4 6 1 lt 2 1 lt nF Disc ceramic F
52. Room error These faults being related to relative timing can sometimes be cured by changing the 6502 processor or the 74L8245 4 and are more often noticed where the RAM in CAS 1 is a different type from the RAM in CAS 0 when an A to B upgrade has been done In particular it seems to be that the Fujitsu RAMs do not mix well with the Mostek or Hitachi RAMs 43 6 5 Test programs and sample waveforms 6 5 1 Test program The following program allows you to test the chip select lines of any of the devices on the computer It sets up a machine code loop which accesses the address which you specify as hexadecimal number Since it is a closed loop the only way to escape is to use the break key which is programmed to re enter the BASIC program To escape enter a zero address If you are accessing a slow device its chip select line should go low for a full microsecond but with a fast device it will only be low for 500 nanoseconds In either case the waveform should be high for 3 microseconds 10 KEY10 OLD M RUN M 20 CLS 30 DIM CODE 20 40 PS CODE 50 INPUT ADDRESS M 60 MS EVAL amp MS 70 IF 0 END 80 SEI 90 SEC 100 again 110 LDA 5 120 BCS again 130 140 CALL CODE The following is a reproduction of a photograph showing the waveform on pin 23 of the ADC chip 1 73 when the above test program is running with address amp FECO selecte
53. TE CERAMIC 631 100 CAPACITOR 100pF PLATE CERAMIC 631 150 CAPACITOR 150pF PLATE CERAMIC 631 220 CAPACITOR 220pF PLATE CERAMIC 631 270 CAPACITOR 270pF PLATE CERAMIC 630 039 CAPACITOR 390pF PLATE CERAMIC 630 082 CAPACITOR 820pF PLATE CERAMIC 632 047 CAPACITOR 47pF PLATE CERAMIC 633 047 CAPACITOR 470pF PLATE CERAMIC 629 001 CAPACITOR 1nF PLATE CERAMIC 629 002 CAPACITOR 2n2F PLATE CERAMIC 634 004 CAPACITOR 4u7F PLATE CERAMIC 629 010 CAPACITOR 10nF PLATE CERAMIC 650 333 CAPACITOR 33nF POLYESTER 680 001 CAPACITOR DECOUPLER 33 47 nF 628 470 CAPACITOR 47nF DISC CERAMIC 628 101 CAPACITOR 100nF DISC CERAMIC 634 002 CAPACITOR 2n2F PLATE CERAMIC 651 224 CAPACITOR 220nF 613 100 CAPACITOR luF 35V TANTALUM 610 005 CAPACITOR 4u7F 10V TANTALUM 635 047 CAPACITOR 4u7F 16V RADIAL ELEC 610 010 CAPACITOR 10uF 10V TANTALUM 635 100 CAPACITOR 10uF 16V RADIAL ELEC 610 047 CAPACITOR 47uF 10V TANTALUM 621 470 CAPACITOR 47uF 10V AXIAL ELEC 651 204 224 CAPACITOR 200 220nF 699 001 TRIMMING CAPACITOR 2 22pF 860 005 CHOKE 33 uH 820 160 CRYSTAL 16 MHz 820 177 CRYSTAL 17 7345 MHz 794 002 DIODE IN 4002 794 148 DIODE IN 4148 810 001 RELAY 5V 780 239 TRANSISTOR BC 239 780 309 TRANSISTOR BC309 783 906 TRANSISTOR 2N3906 800 114 I C SOCKET D I L 14 PIN 800 120 TC SOCKET 20 PIN 800 128 SOCKET D I L 28 800 140 SOCKET D I L 40 740 038 INTEGRATED CIRCUIT 7438 741 000 INTEGRATED CIRCUIT 74500 742 000 INTEGRAT
54. andard for frequency shift keying ie using wo different tones to represent logic levels Connectors such as the cassette socket RGB socket etc Dynamic RAM Erasable Programmable Read Only Memory Final Inspection Tester Floppy Disc Controller Integrated Circuit IDentity refers to the unique number of given Econet 8271 Insulation Displacement Connectors parallel cable Input Output Interrupt ReQuest control line on the 6502 processor PCB link MAO 13 Memory Access control lines out of the CRTC MOS MPU NMI PA PAL PB PCB PET PL PSU Machine Operating System Microprocessor Unit Non Maskable Interrupt control line on the 6502 processor Port A One of the two ports of a VIA Phase Alternation Line coding method used for combining Separate colour information into a single signal Port B The other port of a VIA Printed Circuit Board Progressive Establishment Tester Header plug Power Supply Unit 01 etc Transistor numbers 85 OWERTY 0 2 RAS RC RGB ROM RS423C RTS 5 SOT SW Taw TT VDU VIA VR 280 1MHz 1MHzE 2MHzF These are the upper left keys on the keyboard ie refers to the standard keyboard layout Row Address lines from the CRTC to access the RAM Random Access read write Memory Row Address Strobe Control line for the DRAM Y Resistor Capacitor network Red Green Blue indiv
55. anging the relative timing bring the timing back into a working condition Therefore having discovered a device which apparently has a temperature fault before de soldering it it is well worth temperature cycling the associated components 38 6 3 Fault Isolation Having checked that the apparent fault is not a problem with the program the first thing to do is to isolate the problem to a particular area of the computer For example if the problem is in loading and saving programs with a cassette recorder then attention should be focussed on the cassette interface itself However this is not as easy as it sounds in some cases because of the links between various sections of the circuit It would probably be worthwhile reading through most of the circuit description given in this book in order to try to gain an understanding of the operation of the computer as a whole before trying to deal with one apparently isolated section of the computer s hardware The simplest fault to check for is malfunctioning of the power supply Voltages can be measured at the terminals on the PCB where the power leads are attached but it is worth checking particularly with the older linear type power supplies that the 5 volts is available on each of the three pairs of connectors It is also worth checking that the 5 volts is present because although the processor memory and VDU will all
56. arefully see section 4 Also it is worth checking the soldering very carefully particularly around the area of the IDC connectors This is because it is easy to get solder bridges over th tracks which are fed in between the pins on this connector The worst place seems to be in the area of the tube connector If there are problems associated with the PAL or cassette circuitry it is worth checking very carefully whether the correct resistor values have been used Since there are so many resistors so closely packed together it is very easy to get resistors in the wrong places To check this it is best to remove the circuit board from the case entirely and use a strong light source in order to view the resistor s colour codes carefully This is well worth doing as it can save a lot of time looking for faults which are basically simple but which would be difficult to diagnose Our service centres tell us that there is a series of rather obscure faults which they have detected which is associated with timing problems with the RAM One symptom is twinkling characters in mode 7 but not in the other modes of graphics and another is that when playing Acornsoft s Defender not the later version of Planetoids Some very strange effects occur as the game continues Also there is a program of 3D Noughts and Crosses from Beebug which produces a strange fault stopping inexplicably at one particular line and giving a No
57. ble baud rate generators a cassette data clock separator or cassette operations and also a to be fed out to the cassette recorder 42 divides the 16 MHz clock signal by 13 1 23 MHz and transistor 03 from the serial pr this signal is divided further by 1024 within the serial processor to produce the 1200 Hz cassette signal Automatic motor control of an audio cassette recorder is achieved by using a small relay driven by a ocessor The signal coming from the cassett recorder is buffered filtered and shaped by a three stage amplifier IC35 The RS423 data request to send output RTS and cl in and data out lear to send input signals and the CTS signals are interfaced by ICs 74 and 75 which translate between TTL and standard RS423 232 signal levels 45V and memory mapped at 6 10 specifies 5V The control register which is clock bits 0 2 and the receive th frequencies clock bits 3 5 for the transmit used by the 6850 IC4 The switching between the cassette and RS423 inputs and outputs is also determined by the control register bit 6 and so is the motor control bit7 R75 and C28 provide the necessary timing elements for delay between receiving the high tone run in signal and asserting the data carrier detect signal to the ACIA The value of resistor needed is affected by the output impedance of t
58. ch run at 110 baud you have to change the position of link S28 This is explained in detail in section 7 3 One important point to note is that on issue 1 printed circuit boards the pin connections on the printer port are not quite the same as later issues Pin 19 is the ACK line and pin 26 is not open circuit as it should be for certain printers If you wish to use this interface for example with the Seikosha GP80A you will encounter problems since pin 26 is used as a reset line Another problem which may occur on the early issue boards is that there is no pull up resistor on the CA2 line thus leaving the collector of transistor 011 open circuit 54 7 3 Hardware Hints and Tips Here are a number of miscellaneous hints and tips which have come from various sources RS423 at 110 baud In order to get the RS423 in terface then change the position of the PCB between the centre pin and to work at 110 baud all that has to be done is to use the FX8 1 command in order to set 75 baud and link S28 This link is made by a track on the west pin This link has to be broken and a solder link made from the centre pin to the east pin This has the effect of speeding up all of the baud rates both send and receive by 44 and also makes the cassette port run fast by the same amount so a single pole doubl le throw switch could be wired to S28 in order to select the normal speed or the fast sp
59. ch should affect the addressable latch are strobed into it by a flip flop IC31 which is triggered from the 1 MHz clock signal Port A of this VIA is a slow data bus which connects to the keyboard the Speech system chip and the sound generator 3 7 Keyboard The keyboard circuit Section 9 5 connects via PL 13 A 1 MHz clock signal is fed to a 7415163 binary counter the outputs of which are decoded by a 7445 decoder driver circuit These outputs drive the rows of the keyboard matrix each row being driven in turn If any key is depressed the 741530 gate will produce an output when that row is strobed and this will interrupt the computer through line CA 2 of IC3 On this interrupt the computer will enter the key reading software In order to discover which key was pressed the microprocessor loads directly into the 74L8163 the address of each key matrix row allowing it to interrogate each row in turn Also the microprocessor loads into a 7415251 data selector the address of each specific key on that row ie column addresses In this way the microprocessor can interrogate each individual key in turn until it discovers which one was depressed and causing the interrupt Once read the keyboard assumes its free running mode 3 8 Sound speech serial ROM interfaces The speech system devic used is a TMS 5220 1 99 which on instructions from the Microprocessor will either produce at its aud
60. ching them to 5 volts or 0 volts This can be done crudely by bending up the three pins so that they 4 t engage in the IC socket and soldering on to them three leads terminating in crocodile clips These can then be used to select the address by clipping on to the 5V and OV rails being careful not to let them short out For a system that is to be used regularly for fault finding it is wise to use either a DIL switch or better still a thumbwheel switch properly mounted The three routines given are Routine 0 Provides a chip select pulse for each memory mapped device around the board in turn The pin numbers at which each pulse should appear are given in the progran Routine 1 This sets up the teletext mode of graphics by programming the 6845 and the video processor appropriately Then codes 0 to 255 are stored in the first four pages of video RAM If there is a RAM fault then the display will not be the succession of ASCII character which you would expect and by careful thought about which characters are in error you should be able to diagnose where the problem lies The pattern is periodically re written so that intermittent faults will show up and you can try temperature exercising any suspect chips Routine 2 This is similar to the previous routine but by incrementing a location on the screen it checks the combination of reading and writing
61. d Scope parameters are 1V cm lus cm 44 The following shows 24 of the disc controller chip 1 78 selected by using address amp FE80 Scope parameters are 1V cm lus cm The following shows two traces while the test program is running top trace is 2MHz clock pin 37 Scope parameters are 2V cm and bottom trace is CASO pin 7 1 45 100ns cm 45 following shows two more traces while the test program is running top trace is 2MHz pin 37 and bottom trace is 51 pin 5 IC45 Scope parameters are 2V cm 100ns cm 2 pee HEHEHE EE The following shows two more traces while the test program is running top trace is 2MHz pin 37 and bottom trace is RAS pin 12 IC43 Scope parameters 2V cm 100ns cm 46 6 5 2 Test ROM The listing shown below is the object code for a ROM which could prove extremely useful for fault finding an apparently dead machine especially if you do not have a PET There are three routines given but you could extend the idea for up to 8 different routines if you wanted to do so The idea is that a 2764 ROM is put in place of the operating system ROM and the routine which the system starts on power up or break is determined by taking the address lines that would normally be connected to 10 All and Al2 pins 21 23 and 2 of the 2764 respectively and have some means of atta
62. d only when either FRED or JIM is accessed pages amp FCOO and amp FDOO 3 14 Power supply The power supply unit produces 5 volts at 3 75 amps and 5 volts at 100 milliamps for use on the main circuit board Some auxiliary power for accessories is also available on an external connector and this includes 12 volts at 1 25 amps but the amount of power avalable depends on what hardware is connected internally Econet disc interface sideways ROMs etc The power supply connects to the main circuit board by seven push on connectors with the 5 volts being fed to three different points across the main circuit board These points are all connected together electrically However by distributing the power in this way the need for very large copper tracks to distribute power around the board is avoided Most computers in production will have a switched mode power supply the circuit diagram for which is given However it is not recommended that attempts should be made to repair this power supply instead it should be treated as a module to be exchanged This is because of the stringent safety regulations relating to such units A small number of early computers have a linear power supply unit with a conventional mains transformer and regulator circuit These also should be treated as modules to be exchanged rather than serviced though it should be noted that the three outputs are from separate regulators thus it is possible for power to
63. dinates in millimetres corner of the PCB 8 1 Integrated circuits NB Some ICs which contain more than one circuit will than one place on the circuit diagram IC Type 6502A 6845 6522 6850 5050 5C094 2C199 11595 11595 11595 11595 11595 CO CO CO 11595 CO NRO 00 10S O1 4S CO PO S 7415245 7415273 LM555 LM324 76489 LM386 7416139 741500 741530 741530 7415138 741520 7415139 7438 741551 741532 741574 741534 7415259 741504 741 674 LM324 741510 741504 741586 741528 74500 741502 7415163 74504 3 PCB Circuit Position Diagram 160 8 5 10 6 5 9 5 2 spa or 4 2 2 N O N A A 4 py DL O1 1 O1 O 1 3 OF Y CO CO CO W Hs Oy J ND lt 14 14 gt 00 o lt 14 14 gt m O 00 U Oy OF pa UN 01 00 1 1 AO N gt oO 56 1 5 their positions on the
64. e IMS 6100 This facility for accessing serial ROMs is also used to provide an external serial ROM facility on the keyboard See sections and 28281 Moving down to the SW corner we have the disc controller interface based on 8271 floppy disc controller This is responsible for sending out the command signals for a floppy disk drive and for reading and writing the data from and to the disk drive See section 21401 The next devic is VIA B referred to as the external VIA which is used to provide interfaces for a printer and user applications It also has two timers which are available to the user for his own applications programs See section 3 11 The last two sections of the circuit board are the 1 MHz extension bus and the TUBE These provide two different ways of accessing various external devices The 1 MHz bus is available for more general use but works at the slower speed of 1 MHz whilst the TUBE works at the full 2 MHz but is only intended for use with second processors See section 3 13 3 Detailed Circuit Description 3 1 Processor clock circuitry reset circuitry The microprocessor is a 6502A and runs at either 1 or 2 MHz Most processing is done at 2 MHz including accesses to the RAM and ROM but the processor slows down to 1 MHz when addressing slow devices viz the 1 MHz extension bus the ADC the two VIA s the 6845 CRT controller the ACIA and the serial processor Clock signals for
65. e can be used to see which channel has just converted and OSBYTE call 17 allows you to force a particular ADC channel to convert out of turn The resolution is software selectable between 8 and 12 bits resolution using OSBYTE 190 However in the 12 bit resolution mode the true resolution is somewhat less than 12 bits It is probably more realistic to think in terms of a 9 or 10 bit accuracy Also provided on this connector is an input to the light pen strobe on the 6845 CRT controller The hardware involved in setting up a light pen system is quite simple The light pen circuit has to produce a positive going 5 volt pulse with a duration of greater than 100 ns However the software involved is quite complicated if you want to do more than identify character blocks This is because of the way in which the 6845 has been extended beyond its normal memory range in order to provide bit mapped graphics The final connector which is provided on the back of the computer is only available when the unit has been upgraded with an Econet interface This is a standard 5 pin 180 degree DIN plug to provide the necessary data and clock signals for the Econet interface Underneath the computer is a set of standard IDC connectors First of all there is a 34 way connector for the disc drive s This connector carries the standard connections for a 5 25 disc drive The next two are a 26 way and a 20 way IDC connec
66. e whole of the circuit Information is also given about how to upgrade the various models of microcomputer and the purpose of the various links on the circuit board Some details are also given of the ways in which the circuit has changed in its evolution from issue 1 up to issue 7 There is some guidance about servicing and fault finding further information about interfacing and a few suggestions about possible applications Finally there is a section of hardware hints and tips which is a compilation of ideas from various sources 1 2 Technical Specification Microcomputer is supplied with two levels of hardware provision designated model A and model B the former being fully upgradable to the latter 1 2 1 Model A Specification fast powerful self contained computer system generating high resolution colour graphics and capable of synthesising 3 part music 1 channel of noise The computer is contained in a rigid injection moulded thermoplastic case The following are contained within the computer thus ensuring the minimum of connecting wires 73 key full travel QWERTY keyboard including 10 user definable function keys The keyboard has two key rollover and auto repeat Internal power supply is fully encased and manufactured to BS 415 Class 1 The internal loudspeaker is driven from a 4 channel sound synthesis circuit with full ADSR envelope control A colour television signal for connection t
67. eed for the 110 The actual speed produced is teletype printers Disabling Break link from the keyboard PCB at the back of the main PCB marked 108 333 but this is near enough for most For certain applications particularly when the computer is being used by very young children it is most frustrating when the user presses the break key It is easy enough to disable the escape key within a program but the break key is mare of a problem You could use KEY10 OLD M RUN M but even this is not very satisfactory The alternative is to disable the break key electrically which can be done by removing a You could then wire it up to a miniature ON OFF toggle switch which can be mounted on the back of the case by drilling a suitable hole or leave it open circuit and use the contacts RST SW to provide an alternative break key The disadvantage of doing this though is that if you are using a disc system you may want to be able to do a SHIFT break in order to boot the disc The position of the link to be removed is shown KEYBOARD CONNECTOR below 74L8251 Figure 13 Disabling the BREAK key 55 Break link here 8 Component location tables The following lists of components should enable you to loca te any component on the main circuit diagram by its X and Y grid reference see grid numbers on main PCB circuit diagram links defined by X and Y coor
68. er ADLC This is a sophisticated serial communications device allowing the sending and receiving of data at a variety of speeds between as many as 254 computers The data transfer is synchronised by a clock signal fed to all the computers as a differential signal on one pair of cables whilst the data itself uses another pair of cables Data is both transmitted and received on the same pair of cables but obviously only one computer at a time is able to broadcast onto the data highway See section 3 12 There are two 6522 versatile interface adaptors on the one on the model A the first being used mainly for internal control and the second for external interfacing VIA A is used both for control of internal hardware and also for generating interrupts from various devices such as the ADC and the keyboard Of its two internal timers the first is used for generating regular interrupts at one centisecond intervals and the second is used occasionally by the operating systen See section 3 6 Of the two ports on this VIA PA is used to provide a slow 1 MHz data bus for the sound and speech chips and also for the keyboard whilst PB is used to provide control lines for various functions throughout the circuit board The sound is produced by a four channel sound generator chip SN76489 whilst the speech is produced by a TMS 5220 which can get its data either from RAM through VIA A or from a serial ROM th
69. er computers For example it is possible to communicate with a mainframe computer either directly within a building on a wired link or by using the telephone network to a computer in another building or even another country This would of course require the use of an acoustic coupler There are various levels at which this link could be used Firstly the computer could be used as a dumb terminal which would simply be capable of sending characters typed on the keyboard to the mainframe computer and receiving characters from the mainframe and printing them on the screen The following program will allow you to do so 50 10 REM Dumb Terminal Program 20 REM Only works on 05 1 0 amp following 30 REM Works even if Tube fitted 40 REM 50 OSASCI amp FFE3 60 OSBYTE amp FFF4 70 OSWRCH amp FFEE 80 CLS 110 DIM CODE 50 120 FOR J 0 TO 2 STEP2 130 P CODE 140 OPT J 150 85423 160 LDA 4691 170 LDX 41 180 JSR OSBYTE character 85423 buffer 190 5 200 220 JSR OSWRCH or OSASCI for CRLF 230 keyboard 240 LDA 54691 250 LDX 0 260 JSR OSBYTE character in keyboard buffer 270 BCS RS423 280 TYA 290 JSR OSWRCH or OSASCI for CRLF 300 LDA amp 8A 310 LDX 2 320 JSR OSBYTE Put character in RS423 output buffer 330 JMP RS423 340 350 NEXT 360 FX 7 7 370 FX 8 7 380 FX 2 2 390 CLS 400 CALL CODE The next level would be its use as a sem
70. ers are held within it as ASCII bytes The SAA 5050 then translates these bytes into a standard Teletext Prestel format display A 6 MHz clock signal is required for the Teletext character generator 105 This signal is produced by knocking a reset flip flop two quarters of 1 40 backwards and forwards from the 8 MHz and 4 MHz clock signals The output of this flip flop is then itself inverted according to the state of the 2 MHz clock signal by an exclusive OR gate 1 4 of IC38 Glitches on this output are removed by R119 and C48 to produce the 6 MHz clock signal at Pin 8 of IC37 157 2MHz 4MHz 8MHz 1 40 4 1 40 Pin 6 6MHz D Y CA Figure 8 6 MHz clock generation The is still used to generat Teletext mode 7 needed henc to Logic 1 address start carry is generated on MA13 th top four address disable ICs 8 and 9 and also enable the data latch IC15 18 the RAM addresses even but using only 1K means that only 10 address lines are lines on the 811595 The Teletext mode is selected by setting the value of video registers 12 and 13 in the CRTC so high that This is used to enable ICs 10 11 are tied in the an ex 3 4 RGB PAL encoder UHF output The red green and blue logic signals produced by the _video proce
71. f the computer For example the 6850 ACIA IC4 the serial processor IC7 the ADC Converter IC73 and the external 6522 IC69 Having removed these devices if the fault disappears then it may simply be a case of replacing them one by one until the fault reappears If the fault remains then if you have any spare ICs or another machine with which you could exchange ICs it would be worth replacing the internal VIA IC3 the 6502A 1 the 6845 IC2 and the video processor IC5 At this stage the next thing to try is to examine each of the individual address and data lines to see if one or more of these lines is permanently high or low If so look for short circuits solder bridges etc on that line It is worth checking these lines both on the 6502A itself and also 1051 the operating system ROM When looking around the board at various points with an oscilloscope try to find any waveforms which either have slack edges ie sloping rather than square or which have voltage levels which are not within the normal TTL range Logic 1 must be greater than 2 8V and logic 0 less than 0 8V though normally one would not expect to see voltages of less than about 3 4V or more than 0 4V Another very useful test with a model B is to move link S25 to the south position to see if the computer will operate in the 16K mode in which case it suggests a problem with the CAS 0 area of RAM Then if
72. function normally if the 5 volts is not present it is essential for cassette sound speech and RS423 interfaces If the power supply is NOT working then you should NOT attempt to repair it The reason is that in order to maintain the safety specification to which the computer was designed any repair to the power supply including replacement of power supply cable must be checked for earth continuity at a current of not less than 10 A and must undergo a Dielectric Withstand Test between both live and neutral to ground of 1500 V AC This requires specialist equipment and training and should not even be attempted by dealers unless they have the necessary equipment and expertise The worst kind of fault with a microprocessor system is that the processor is unable to fetch instructions from the ROM process them and then produce some sort of result which the operator can see or hear In the case of such faults the whole system appears completely dead and it is very difficult to locate the specific fault This kind of problem is made worse on this particular computer because of the technique used to refresh the dynamic RAM Not only must the processor fetch instructions from ROM and process them but also it must successfully program the CRT controller which in turn must begin to produce refresh addresses for the dynamic RAM before the system memory can operate Assuming then that the
73. hat pin on the serial processor which has been subject to a certain amount of variation Thus the value of R75 has changed through the evolution of the circuit 16 13MHz CLOCK BAUD RATE GENERATOR TXC Transmit clock DATA RECEIVE BUS BAUD RATE To from GENERATOR 6850 DES EIER ACIA HIGH TONE DCD Data RUN IN carrier detect CASSETTE RXC Receive clock RAW SEPARATOR CASSETTE To from DATA RXD Received CASSETTE data RECORDER OUTPUT dh DATA TXD Transmitted data RAD RTSI Request to send input TXD To from RS 423 RTSO DRIVERS CTSI CTSO Clear to send CTSI output Figure 9 Serial ULA block diagram 20 3 6 Internal One 6522 device 1 9 is devoted to internal system operation Port B drives an addressable latch IC32 which is used to provide read and write strobe signals for the speech interface the keyboard and the sound generator chip Also coming from this latch are control lines CO and Cl which provide the memory address addition for the CRT controller depending on the amount of RAM devoted to the display memory Pins 6 and 7 of the addressable latch drive the capitals lock and shift lock LEDs on the keyboard The rest of Port B on the internal system VIA is used to input the two fire button signals from the analogue to digital convertor interface and two response lines from the speech interface Each time the system VIA is written to any changes on Port B whi
74. i intelligent terminal which would enable you to use some of the processing of the BBC Microcomputer to deal with file handling so that the text could be prepared off line stored on disc and then spooled down to the mainframe when the link is made The third level then would be to use the graphics facilities of the BBC Microcomputer in addition to its ability to print text This produces the possibility of using the computer as a colour graphics terminal to a mainframe computer at a fraction of the cost All that is needed is for someone to write the appropriate terminal emulation software and put it in sideways ROM There are now a number of such packages commercially available 51 other way in which the 85423 can be used is to link two computers together One reason for doing this would be to enabl Software to be downloaded from a disc system to another computer which does not have a disc interface A 15K program can be downloaded using an RS423 link in approximately 20 seconds which is clearly faster than using a cassette to cassette link The only software involved in doing this is to type in on the receiving computer NEW RETURN FX2 1 RETURN This sets the RS423 as input instead of the keyboard and then on the sending computer you would type in FX3 7 RETURN LIST RETURN FX3 0 RETURN This enables the RS423 as the output and sends a listing
75. ich are specifically designed for the BBC Microcomputer which are known as the PET Progessive Establishment Tester and the FIT Final Inspection Tester Whilst the service agent or dealer might be expected to have these pieces of equipment the average user is unlikely to feel that it is worth purchasing them for the limited amount of fault finding he or she would be likely to do The purpose of the PET which is the more expensive of the two items is to take an apparently lifeless computer and attempt to find out where the fault lies The FIT on the other hand is somewhat simpler and its aim is not to isolate a known fault but to check whether an apparently working computer is in fact working in all respects Both the PET and the FIT are the subject of entirely separate documents produced by Acorn Computers PLC Two other very useful pieces of test equipment are a can of freezer spray and a hair dryer It is fairly common for faults in some of the ICs to be associated with temperature conditions Therefore if you have reason to suspect a particular component it is sometimes helpful to exercise the device by the use of these two items This can sometimes show up a fault quite clearly However this can be slightly misleading in cases where the fault is caused by a timing problem on some device This is because changing the temperature conditions of one device which may not itself be at fault may by ch
76. idual colour signals for the VDU Read Only Memory ROMSEL ROM SELect latch An internationally defined convention for serial transmission of data Ready To Send control output 85423 port Socket Select On Test South West Tinned Copper Wire Tranistor Transistor Logic a standard type of digital IC 74 series Ultra High Frequency signal for input toa aerial socket Uncommitted Logic Array semi custom IC Visual Display Unit Versatile Interface Adaptor 6522 Variable Resistor a commonly used 8 bit microprocessor 1 MegaHertz usually refers to the interface bus running at that speed Strobe to which the processor is synchronised when accessing slow devices such as 6522 VIA and 1MHz bus Strobe to which the processor is synchronised when accessing fast devices such as ROM and DRAMs 86
77. ill bring the actual address back up to the area of RAM which is currently being used for the screen ie above HIMEM Thus for numbers greater than amp 7FFF you simply add the number 83000 which brings the addresses back to the range amp 3000 to amp 4FFF as illustrated in the diagram above In the 20K modes you add 83000 12 in the 16K mode you add amp 4000 16 in the 10K mode you add 45800 22 and in the 8K mode you add 86000 24K This number to be added is defined by the control lines CO and Cl from the 7415259 IC32 and computed by some AND gates with the result being added to the higher CRTC refresh address lines by a 7415283 adder IC39 The CRTC address line 12 is used as a carry to determine whether zero or the number computed by the AND gates is added to the address lines Confusion may arise when looking at 1C 9 on the circuit diagram since it looks as if AAO to are being buffered to AO to A2 But if you look at the pin numbers and compare them with the other 81L895 s you will see that they are in fact buffered to the top three bits AA to MA4 to MA7 are buffered to AO to A3 Display mode 7 is a Teletext mode and to implement this an SAA 5050 IC5 Teletext character generator Read Only Memory is used 1 15 latches the information coming from the RAM prior to the SAA 5050 When using this mode only 1 of RAM is devoted to the display memory and the charact
78. in the Teletext mode ADDRESS BUS 0 15 6845 CRT ICs 53 68 controller IC 2 Multiplexed address Non Teletext control information Figure 3 Block diagram of CPU RAM and CRT controller 12 has CRTC 6502 microprocessor runs from a constant clock and 50 its requirements for memory access are predictable control of the RAM address lines is switched between the microprocessor and the CRTC Thus in each one microsecond period the microprocessor two RAM accesses and the CRTC has two RAM accesses Becaus Every 250 nanoseconds displ the sequential CRTC accesses 2MHzE RAM 0 6 RAS CAS 1 or 0 RAM DATA DATA BUS DQ VIDPROC Figure 4 DRAM operation 13 generates a sequence of addresses in order to refresh the VDU all the row address lines of the RAMs are constantly cycled The addressing methods have been designed so that in each screen mode dynamic RAMs are automatically refreshed by virtue of the The row address connected signal then drives al RAM are enabled by strobe to the 8 and 4 MHz clock signa signal is produced by a D type flip flop 18 half of IC44 This RAS S via R106 The two banks of eir column address strobes 1 of the dynamic RAM virtue of having th individual RAM CAS 1 is used CAS 1 the upper 16 circuit half of IC producing the CAS si selecting CAS 1 The pr
79. io output speech from its associated memory IC98 or from speech data fed to it directly from the Microcomputer s memory On later issue boards a variable resistor is provided VR 2 to adjust the clock frequency to give the best effect of the speech IC18 is a four channel sound generator chip which may be programmed to give varying frequency and varying attenuation on each channel The audio output of the speech system device is filtered by an operational amplifier circuit with a cut off frequency of 7 kHz This signal is then added to an amplified and level shifted signal from the sound generator by a virtual earth amplifier to which is also added an extra analogue input from the 1 MHz extension bus This summated audio signal is then finally filtered by an 8 kHz low pass filter 11 of these operations are done by a quad operational amplifier IC17 IC19 provides audio power amplification to drive speaker from PL15 A low level audio output is provided from PL16 for feeding the auxiliary input of an external power amplifier 21 3 9 to D convertors A four channel analogue to digital convertor facility is provided by IC73 This device connects straight to the Microcomputer s data bus and is a dual slope convertor with its voltage reference being provided by the three diodes 06 D7 and D8 Each time a conversion is completed the microprocessor is interrupted through of the internal VIA which res
80. ircuit board which can be cut soldered wire inks or shorting jumpers plugging on to the rows of pins This is followed by a tabular survey of the options selected in production on a standard model Microcomputer Option Select Links are as follows 1 Used only on issue 4 and succeeding boards to select printer strobe or direct output from CA2 2 OPEN enables ECONET NMI CLOSED disables ECONET NMI Do not fit this link with IC91 in place 3 Clock base frequency selection for ECONET Not used after issue 3 4 EAST selects 5 1 4 disc WEST selects 8 disc This changes the pin connection of the side select line on the disc interface 5 NORTH enables ECONET clock SOUTH disables ECONET clock Not used after issue 3 6 NORTH divides ECONET clock by 2 SOUTH divides ECONET clock by 4 Not used after issue 3 7 WEST applies 4 5v to pin 30 of disc controller 78 EAST applies Ov to pin 30 of disc controller Readable by software bit 0 of the result register of the 8271 Not used 8 CLOSED links disc head load signal to PL8 OPEN isolates disc head load signal from PL8 9 CLOSED disables DISC NMI OPEN enables DISC NMI Do not fit 1C78 with this link closed Due to PCB faults various different modifications are necessary with different issue boards in order to use the disc interface S section 4 modification D 3T 10
81. it provides two pages 2 x 256 bytes of memory locations mapped between amp FCOO0 to amp FDFF By using one of these locations amp FCFF as a paging register it is possible to extend the memory addressing capability to a full 64K bytes The final connector is a 40 way IDC which provides an interface known as the Tube intended for use with a second processor Although the hardware on the BBC microcomputer side is very simple the hardware on the second processor is extremely complex and it really requires a ULA to incorporate all the hardware necessary to handle the protocol It is therefore suggested that any interfacing to the Tube should be done only using products from Acorn Computers Ltd 7 2 Interfacing to various printers It is possible to interface to a wide variety of printers using the two interfaces provided First of all for serial printers the 5423 connector can be used and secondly for parallel printers the Centronics interface can be used It is also possible to link up to various other non standard printers such as the IEEE 488 printers used for the Commodore Pet computers This is more difficult and requires connection not only to the Printer Port but also the User Port to provide extra control lines and also requires extra software within the machine It is possible to write your own printer driver routine which works through the operating system In order to link up to teletype printers whi
82. ldering equipment The most common reason for the cassette system becoming inoperative is a damaged LM324 Another problem is with the clock input to the serial processor and it is worth checking that this is the correct frequency ie 1 23 MHz 812 ns periodic time Another problem which sometimes occurs is that the value of R75 needs to be changed The optimum value is different for different issues of the serial processor ULA because of the variation in impedence of pin 15 to earth This affects the timing between receiving a high tone lead in and asserting the data carrier detect on the ACIA For ICs numbered 2C199E and 2C199E 3 R75 should be 100x or 56k as required for consistent loading of data For 2C199E 7 R75 should be 82k One simple problem but unfortunately fairly common is that the pins of ICs tend to get bent as they are pushed into the sockets If you have isolated the fault to a particular area then this is something to look out for It is also not unknown for the IC socket itself to have a pin bent underneath This may not have been noticed by quality control if the IC socket was empty at the time of production eg speech IC socket If you wish to get two BBC Microcomputers to send programs to each other on the cassette system then it is possible to do so by a direct connection provided a 1 5k resistor is connected between the signal line and ground With the later issue boards
83. loured background and text plus pixel graphics all to the Teletext standard The non Teletext display modes modes 0 to 6 provide user definable characters in addition to the standard upper and lower case alpha numeric font In these modes graphics may be freely mixed with text Text characters can be positioned not only on for example a 40 x 32 grid but at any intermediate position Separate or overlapping text and graphic windows can be easily user defined over any area of the display Each of these windows may be filled separately and the text window scrolls independently of the rest of the screen The Model A is able to support the following modes Mode 4 320 x 256 2 colour graphics and 40 x 32 text 10K Mode 5 160 x 256 4 colour graphics and 20 x 32 text 10K Mode 6 40 x 25 2 colour text only 8K Mode 7 40 x 25 Teletext display 1K All graphics access is transparent see section 2 2 resulting in a fast snow free display Extensive support is provided in the Machine Operating System for the graphics facilities and this is reflected in the BASIC interpreter These facilities include the ability to draw lines very rapidly and to fill large areas of colour In addition very rapid changes of areas of colour can be effected by the use of a colour palette The Model A BBC Microcomputer can be expanded at any time to the Model B In addition or as a
84. machine appears totally dead even though the power supply unit is apparently working and that you do not have access to a PET then here are a number of things you could check i Check that the reset line on the 6502A pin 40 is high and only goes low when BREAK is pressed ii Check that the line is not permanently in either a low or high state Pin 4 of the 6502 39 111 Check for the presence of the various clock signals for example the clock input and output on the 6502A pins 37 and 3 and the 1 2 4 and 8 MHz signals on pins 4 5 6 and 7 of the video processor IC6 iv A very useful pin to check is pin 7 on the 6502A This is the sync pin and although it is not actually used in the circuit it gives an indication of whether or not the 6502A is fetching any instructions this is permanently high or low then the 6502A is totally stalled v Check that the read write line pin 34 of the 6502A is working normally and also check that the same signal having been inverted and re inverted is available at pin 10 of IC33 vi Check for the horizontal and vertical sync signals coming from the CRTC pins 39 and 40 of IC2 which will reveal whether or not the CRTC has been successfully programmed at system reset you do detect something abnormal in one of these tests then the next stage would be to remove from the board any devices in IC sockets which are unnecessary to the basic operation o
85. menting this interface is only provided with operating systems 1 2 onwards An 8 bit input output port with 2 control bits is also provided Four analogue input channels are provided Each channel has an input voltage range of 0 1 8V The conversion time for each channel is 10 milliseconds These analogue inputs can be used not only as inputs for games paddles or joysticks but also in laboratory control situations The resolution of the ADC chip is 12 bits but its conversion is such that only 9 or 10 bits are significant However with suitable averaging this can be extended to the full 12 bits accuracy A 1 MHz buffered extension bus is provided for connection to a variety of external hardware such as a Teletext acquisition unit IEEE 488 interface Winchester disc drive etc 1 2 3 Expansion following expansion options are available some of which be fitted internally at purchase but all of which could be fitted by Dealers at a later date Floppy disc interface fitted as an option at purchase Econet network interface fitted as an option at purchase Voice synthesis circuit with cartridge ROM pack interface Various alternative high level languages in ROM External options which plug directly into the machine include Games paddles Cassette Recorder Black and White and colour monitors and televisions 5 1 4 disc drives ranging
86. n value of R75 was to control the data carrier detect delay time to avoid loosing the first bit of the first byte of the first block when recording data 3 The diodes and resistors on the ROM select circuitry which can be used to produce 1MHz operation were omitted 4 Links S18 and S19 are made with tinned copper wire 5 When the video processor ULA was replaced by the first set of custom designed ICs a modification was necessary 526 was left unconnected and a wire link was made from the TTX VDU line pin 17 of IC2 to the invert input of the videoprocessor pin 27 of 1C6 Later versions of the custom IC made this modification unnecessary All other changes from issue 4 to issue 7 were cosmetic changes including some thickening up of the tracks to improve the power supply distribution 3 7 6 Servicing Fault finding 6 1 Introduction Before starting it should be realised that attempt at repair by any person other than a registered dealer or service agent will void the warranty 6 2 Test Equipment The very minimum test equipment required in order to trace even th simplest fault is a digital multi meter and an oscilloscope or possibly a logic probe It is difficult in a book such as this to do more than give few general guide lines as to the sort of problems to look for and a few techniques which might be used Acorn Computers Ltd supply two pieces of test equipment wh
87. n alternative other facilities such as the Econet may be fitted within the computer system 1 2 2 Model Specification Model Microcomputer is enhanced version of the Model Microcomputer with the following differences 32K Random Access Memory RAM This enables the following extra graphics modes to be used Mode 0 640 x 256 2 colour graphics and 80 x 32 text 20K Mode 1 320 x 256 4 colour graphics and 40 x 32 text 20K Mode 2 160 x 256 16 colour graphics and 20 x 32 text 20K Mode 3 80 x 25 2 colour text only 16K The installed RAM is divided between the high resolution graphics display the user s program and Machine Operating System variables If higher resolutions are required with large programs then the second processor option may be fitted 6 pin DIN connector provides separate RGB and sync outputs at TTL levels RGB are all high true and sync is link selectable as high or low true pulse duration 4 7 microseconds Serial interface to RS423 standard The new standard has been designed to be inter operabl with RS232C equipment but offers a considerably enhanced specification for example in maximum length of cable and maximum data transfer rates Baud rates are software selectable between 75 baud and 9600 baud The interface provides not only two way data transfer but also two way hand shaking using RTS and CTS lines The software for imple
88. o a normal domestic television aerial socket is available through a phono connector This signal is 625 line 5082 interlaced fully encoded PAL and is modulated on UHF channel 36 A BNC connector supplies a composite video output to drive a black and white or PAL colour monitor A standard audio cassette recorder can be used to record computer programs and data at 300 or 1200 baud using the Computer Users Tape Standard tones The cassette recorder is under full automatic motor control and is connected to the computer via a 7 pin DIN connector An interrupt driven elapsed time clock enables real time control and timing of user responses The unit uses a 2 MHz 6502A and includes 16K of Random Access Memory A 16K Read Only Memory ROM integrated circuit contains a Machine Operating System designed to interface easily to high level languages A further 16K Language ROM contains a fast BASIC interpreter The interpreter includes a 6502 assembler which enables BASIC statements to be freely mixed with 6502 assembly language Up to four 16K sideways ROMs may be plugged into the machine at any time These four ROMs are paged and may include Pascal word processing computer aided design software disc and Econet filing Systems or Teletext acquisition software The full colour Teletext display of 40 characters by 25 lines known as mode 7 has character rounding with double height flashing co
89. oards only PL9 pin 26 should be cut out of the header Care should be taken to ensure that the pin is cut right back so that no connection can be made to it vii On issue 1 2 circuit boards only a BC239 transistor should be added in place of link 51 as follows Cut the track between the centr and South pins of Sl on the solder side of the circuit board Cut the two tracks connected to the North pin of 51 on the solder side of the circuit board then reconnect the ends of these tracks leaving the North pin isolated Insert a BC239 transistor into the 51 position with the base in the South pin the emitter in the North pin and the collector in the centre pin Finally link the North pin of 51 to IC27 pin 7 with a short length of insulated wire viii On issue 1 or 2 circuit boards only add a 4k7 ohm resistor R162 between th xisting two holes located approximately 5 mm East of 70 pins 11 and 13 as shown below IC70 Figure 11 Adding R162 on issues 1 and 2 of PCB 26 ix On issue 1 circuit boards only disconnect the LPSTB signal between 69 pin 18 and PL10 pin 2 by cutting the track on the solder side of the circuit board which is connected to IC69 pin 18 x Test using a FIT and if available a PET 4 3 Modification C Add Speech Option i The following components are required off Integrated Circuit TMS6100 IC98 off Integrated Circuit TMS5220
90. ocessor and CRT addres Using this microsecond horizontal active display area microseconds y availabl technique for refreshi line having a period of 64 microseconds e in model A computers with only one bank of In the model B CAS 0 controls the lower 16K and The second bank of RAMs 15 selected by 741551 28 which controls the 745139 half of IC45 gnals When 14 is high the B input is low thus other half of IC45 is used to select between the s lines two bytes of information are available per ng the raster scanned video display With each a 40 microseconds 2 bytes x 8 bits x 40 is usual Thus 640 bits of information per horizontal line are produced from the memory mapped display At the end of each 250 nanosecond CRTC access period single bit stream of the video processor according to the display mode in oper la IC6 tches the byte from the RAM and ation serialises the byte into a or two bit streams of 4 bits or four bit 8 bits streams of 2 bits In this way display modes varying from 640 pixels in 2 logical colours to 160 pixels in 16 logical colours can be produced Memory timing VDU access CPU access VDU access CPU access G Colour options 1 Palette 8 8 pixels 1 bit pixel 2 colours 640 X 256 4 pixels 2 bits pixel 4 colours
91. own TUBE 641 Character A 0 write INC STA STA STA INY BNE LDA STA JMP amp 7 01 Change character on screen amp 7D00 Y amp 7E00 Y amp 7F00 Y write amp 42 Character B amp FE20 N Write to Vidproc write ENDPROC The following is a reproduction of a photograph showing the waveform on pin 34 of the 6502 R W of Scope parameters are 1 2us cm 49 7 Interfacing Survey 7 1 Purpose of each interface Since there are so many different interface connections on the BBC microcomputer it may be a help to look at each in turn and talk about possible applications for each Working from left to right on the back of the computer we start with the UHF output which provides a PAL colour TV signal for use with a normal colour television Next is a video output on a BNC connector which is intended to be used with a black and white video monitor However it is possible to introduce the colour burst information onto this signal in order to produce a PAL composite video signal On circuit boards issue 4 onwards it is possible to introduce this signal by adding a simple link S39 On previous issue boards it is necessary to introduce a 470pF capacitor from the emitter of 09 to the base of 07 This capacitor would have to be soldered directly on to the circuit board The third connector which is provided for video output is a 6 way 240 degree DIN plug This
92. ponds by reading the value and storing it in a memory location 3 10 Disc interface IC78 is a floppy disc controller circuit which is used to interface to one or two single or double sided 5 1 4 inch floppy disc drives Logic signals from the controller to the disc drive are buffered by two open collector driver packages IC79 and 80 The incoming signal from the disc drive is first conditioned by monostable IC87 producing a pulse train with each pulse of fixed width These pulses are then fed to the data separation circuits ICs 81 and 682 These form a digital monostable 1C86 divides the 8 MHz clock signal down to 31 25 kHz ICs 83 84 and 85 are then used to detect index pulses coming in from the drive which show that the drive is ready for a read or write operation 3 11 Printer user port interfaces 1069 is a versatile interface adaptor Port A is used to provide a centronics standard parallel printer interface with an octal buffer IC70 to improve on the current driving capabilities of the data lines Control line CA2 is used as the strobe line having been buffered by part of IC27 011 It is asserted low for approximately 5uS to signal that the data is ready This circuit has been changed on the various issues of the PCB as explained in section 4 4 Port B is left uncommitted and is free for user applications as either input or output For full details of what can be done with the user por
93. provides the red green blue and sync signals needed for an RGB monitor The sync signal is a 5 volts negative going pulse of 4 705 duration but it can be changed to positive going by changing link 531 Also provided on this connector are a 0 volt and a 5 volt supply but these should not be used for providing more than a few milliamps to external circuits The next connector is a serial port of the RS423 standard This is a standard which has superior drive capabilities to the RS232 interface and is run in this case between 5 volt and 5 volt levels The speed is software selectable at 75 150 300 1200 2400 4800 or 9600 baud There is a higher speed of 19200 baud but this is not guaranteed to be error free It is also possible to get the interface to work at 110 baud but this requires a modification which would also change the speed of the cassette interface See section 7 3 on hardware hints and tips for more information The control signals provided are the normal CTS and RTS lines the RTS output also working on 5 volt and 5 volt NB When making up a connector for the RSA23 note that connections as shown on the circuit diagram refer to the socket For the plug connections refer to page 504 of the User Guide which gives the connections as seen from outside the case Apart from using the 85423 interface to run a serial printer it is also possible to use it to communicate with oth
94. r components wer changed from a 741500 to a in various ways The clock s were changed in order to improve removed certain component circuit performance and the t was altered in order to improv talk 22k resistor R174 was introduced from pin 20 of IC7 the serial ssor to 0 volts to ensure that if was held low ving added the 2k2 pull up resistor and put 011 in its own position 51 the shielding and to reduce IC74 was absent that the CTSI R170 to printer port strobe was reinstated uitry associated with 514 to S17 which change the 1MHz bus to 2MHz on the issue 3 PCB was incorrect This should be checked against the current circuit diagram if it is to be used 5 The position of D13 was changed to put it in parallel with the relay coil rather than across the collector and emitter of the transistor Q3 6 The connections from the speech cir changed VSPRDY and VSPINT were changed over to connect to PB7 and PB6 respe ctively 36 cuit IC99 to the VIA were 7 resistor 8171 was connected series with the line of the ADC 1C73 in order to prevent momentary output contention which may occur during power up 8 A Ak7 resistor R173 was connected between pin 7 of IC89 and 5v as the output is open collector 9 Resistors R104 8125 R142 R149 and R153 which were in series with the ROM chip select lines were replaced by copper links formed on the
95. s actually used for memory mapped input output The ROM is arranged in such a way that one group of 16K bytes forms a fixed part of the memory map 15 1 4K ROM for the operating system 3 4K of I O whilst the other 16K has been organised to give as much flexibility as possible There is a ROM select facility for accessing up to 16 different memory devices although only four sideways ROM SOCkets are available on the PCB It is expected that the normal way in which these four sockets will be used is to provide 2 MHz access to each of 4 chips which could be either 16K or 8K ROMs or EPROMs See section 3 2 The RAM is also divided into two sections of 16 Kbytes each of which contains eight 16K by 1 bit DRAM chips In the model A microcomputer only one bank of 16K is present whilst both are present in the model B This RAM has to be accessed by both the processor itself and also the CRT controller This is done by using a form of transparent access in which both the processor and the CRT controller can access the RAM at the full clock speed by interleaving the accesses on alternate phases of the system clock See section 3 3 The display is extremely versatile and uses two entirely different methods depending on screen mode Mode 7 uses Teletext hardware which produces RGB signals by having its own character generator and accepting data from the RAM as ASCII characters This means that it uses very
96. sed to Technical Enquiries ACORN Computers Limited Newmarket Road Cambridge 5 8PD All maintenance and service on the product mus nual t be carried out by ACORN Computers authorised dealers ACORN Computers can accept no liability whatsoever for any loss or damage caused by service or maintenance by unauthorised personnel This manual is intended only to assist the reader in the use of this product and therefore ACORN Computers shall not be liable for any loss or damage whatsoever arising from the use of an y information or particulars in or any error or omission in this manual or any incorrect use of the product This manual is for the sole use of ACORN Computers authorised dealers and must only be used by them in connec product described within tion with the SECTION 1 BBC Microcomputer Models A B Microcomputer Service Manual Contents I Introduction Try T Nature and purpose of the manual 2 Technical specification 2 1 Model A specification 2 2 Model B specification 1 2 3 Expansion 2 4 Software 2 5 Machine operating system 1 2 6 BASIC 3 Packaging 4 Mechanical assembly of the case etc 2 General Circuit Description 2 1 Introduction 2452 Hardware description 3 Detailed Circuit Descriptions Sul Processor clock circuitry reset circuitry 3 2 Memory address decoding 3 3 DRAMs CRT controller
97. ssor are buffered by transi stors 04 905 and 06 and fed out together with a composite sync signal to the RGB connector SK 3 The red green and blue lines are summed together by binary weighted resistors to feed 07 which produces a 1V composite video signal suitable for feeding to monochrome monitors on which the different colours will appear as different shades of grey Also available from outpu is modulated using a UM1233 for PAL domes which modulates the colour informa fre uency 010 is a 17 73 MHz oscillator circu the main printed circuit board is a UHF TV signal on channel 36 suitable for feeding to the aerial input of a domestic television This Colour is provided for tic televisions by a PAL phase alternating line encoder circuit tion on to the colour subcarrier it which is divided by a of line frequency in order to produc ring counter 1046 giving 2 output 4 e s at the colour subcarrier frequency Thus on IC46 pin 9 we have the U V signal A row of exclusive OR blue cyan magenta or yellow colo then drive resistors via a row of 433618 MHz One of these two outputs is switched by the horizontal the alternate phase on each TV line signal and on 1648 pin 11 the gates is used to select different phases of the U and V signals according to whether a red green ur is
98. ssue boards and remove the circuit board from the case by sliding it forwards and then lifting it from the rear 2 General Description of Hardware 2 1 Introduction This next section gives a general description of the hardware of the computer and reference is made to the functional block diagram section 9 1 which is laid out approximately as the components are situated on the printed circuit board General areas and component orientations are referred to by using compass points as shown on the block diagram When any reference needs to be made to the specific position of a component then X Y co ordinates will be used giving the distances in millimetres from the SW corner This is also shown on the block diagram A list is given in section 8 of this manual of all the integrated circuits transistors diodes capacitors resistors and selection links by number including their X Y co ordinates on the PCB and on the main circuit diagram As each section of the hardware is described reference is made to sections of the following chapter in which more detailed descriptions are given The heart of the hardware is the 6502 microprocessor and in this general description we shall move around he 6502 in an anti clockwise direction starting from the SE corner c the PCB 2 2 Hardware description The 6502 accesses an area of just less than 32 Kbytes of ROM 3 4 of this memory allocation i
99. such as Teletext acquisition 42 Prestel acquisition Econet file system Disk file system Byte transfer to second processor The majority of the operating system calls are vectored to enable the user to change them if required 1 2 6 5 The BASIC interpreter is extremely fast implementation with numerous powerful extensions Long variable names Integer floating point and string variables Multi dimension integer THEN REPEAT UNTIL 1 rocedures ocal variables assette loading u support for asy control of H HH M L Fu Effective error C F E B lti line integer floating point and string arrays Extensive support for string handling ELSE floating point and string functions recursion on all functions and procedures trapping and handling and saving of programs and data th xtensive colour graphics facilities the built in music generation circuits uilt in 6502 mnemonic assembler enabling BASIC and assembler to be mixed or pure assembly language programs to be produced 1 3 Packaging The BBC Microcomputer is supplied in a two part moulded polystyrene packing which is further packaged within a cardboard sleeve With the Microcomputer a User s Manual a Welco lead are also supplied The packaging should be kept intact in case it becomes necessary to transpor 1 4 Mechanical
100. t you should refer to the 6522 data sheet but basically apart from being used as a straightforward input output port PB7 can be used as a programmable pulse output using one of the timers PB6 can be used as an input to the other timer for pulse counting and and CB2 can be used for automatic hand shaking and in conjunction with the VIA s own shift register 22 3 12 Econet ICs 89 to 96 are concerned with Advanced Data Link Controller Circ the Econet interface 1089 is an uit type 6854 which handles the Econet protocol Data to be transmitted on to the network is fed from line driver circuit which produces the ADLC to the line drive circuit IC93 via an inverting Schmitt trigger circuit part of 1 91 Transmit data then goes through the a differential signal drive to the Econet cables Received data is detected and converted to a logic signal by one half of IC94 which i LM319 The received data is then fe circuit s a dual comparator circuit type d back to the data link controller An Econet installation has a single master clock station which provides the clock for the whole of the transmitted around the network as a it is used to clock the data in an circuits The network clock is dete network This clock signal is second differential line signal and d out of the data link controller cted using the other half of 94 and the detected clock signal is
101. th outputs of IC95 will go high and the clear to send c the computer is not connected to the ondition will cease Note that when network a collision like situation results in which case again the data link controller will not get a clear to send condition Up to 254 stations may be connected to each Econet with each station being indentified by a unique station identification number This station ID is programmed on the lin by the octal buffer IC96 The data ks S11 and the ID can then be read link controller circuit produces interrupts which are fed to the processor s NMI line These interrupts can be enabled and disabled under software control by using the address decoded signals INTOFF which is achieved by reading the station ID at amp FE18 and INTON which is generated by reading amp FE20 Writing amp FE20 loads the Video processor register 23 3 13 1 MHz bus The address and data lines AO A7 and DO D7 together with two page select lines are available as the 1 MHz extension bus to which various peripheral devices may be connected g Teletext interface All accesses to this bus will be at a 1 MHz processor speed although links are provided to increase this to 2 MHz if desired s the selection link survey The octal buffer IC71 and the octal transceiver IC72 are used to interface these signals to the internal data and address buses IC72 being enable
102. the microprocessor are produced by a 16 MHz crystal oscillator IC43 in conjunction with divider circuitry in part of the video processor 1 6 which produces 8 4 2 and 1 MHz signals The 1 MHz signal coming directly from the video processor is only used for the Teletext generator chip whilst a D type flip flop half of IC34 divides the 2 MHz clock signal in order to produce the system 1 MHz clock 1 MHzE A 2 MHz signal of suitable phase is produced at the output of another D type half of IC31 which remembers when a 1 MHz cycle has been requested At the appropriate time as governed by the 2 MHz clock one of the 2 MHz clock cycles is masked off by the D type half of IC34 and when this happens the D type that remembered that a request had been made is cleared Depending on the phase relationship between the 1 and 2 MHz clocks at the time of the request the delay on the 2 MHzE clock is different as illustrated by the diagrams below The following simple program will produce these conditions alternately so that they may be viewed with an oscilloscope PS 63000 SEI start STA amp FCOO STA amp FCOO JMP start CALL amp 3000 Case A EE 3 REF Li A au m M W REQUEST J Vos f gt 1 95 REF REQUEST W DEN LL SEN coast Figure 1 2 1 MHz stretching
103. the printer port Services Department of Acorn was moved from pin 23 to pin 4k7 resistor R162 was added to the ACK line to pull it up to 5 nk S1 was removed in order to put in a transistor inverter 011 n 26 of PL9 was left unconnected to S used on some printers as the reset avoid the problem that this line ges 1 4 were made retrospectively on most of the issue 1 and 2 it boards 5 Various modifications were made in the region of 59 and IC27 at us stages and so for the correct implementation see section 4 on vario the modification for adding 6 print Js Rl the disc interface 2k2 pull up resistor R170 was added to the strobe line of the er port pin 1 of PL9 09 became select on test SOT with a value between 1 8 and 2k7 in order to set the correct colour burst length 8 C51 also became SOT at a value between 15pF and 22pF in order to set the colour burst frequency to the correct value of 4 4336 MHz or 100 Hz 9 In order to improve the waveform of the 16MHz signal C42 was at one a fixed capacitor with a trimming capacitor parallel Therefore various issues of boards will have various different values for C42 Also the gate used 1 40 was 74500 stage 5 3 2 Changes from issue 3 to issue 4 value layou cross 2 11 3 line 4 The circ 1 The Econet circuitry was modified generator and terminato
104. tor which are used to provide connections to the two ports of the external 6522 VIA The 26 way connector links to port A and is arranged in a standard format for use with a Centronics type parallel printer The 8 port lines are buffered to provide better drive capabilities but it does mean that they can only be used for output If you want to use these lines to drive some other device you should work on the basis of each line being able to sink 10 mA at logic 0 or to source 400 uA at logic 1 Of the two control lines is available as an input with a single 4 7 pull up resistor on it whilst the CA2 line is available as an output the line being buffered by a single transistor 011 to improve the current sinking On boards from issue 4 onwards there is a selection link 51 which will enable this line to provide direct connection to CA2 so that it can be used as either input or output 53 20 way User Port is much simpler in that all connections go directly to the lines on the VIA PBO to PB7 and also control lines CBl and CB2 If you are using these lines for output you should consult the 6522 data sheet to establish the amount of drive current available The next connector another 34 way IDC provides an extremely versatile interface known as the 1MHz extension bus This interface is the subject of a separate Application Note and at this stage it is sufficient to say that
105. uration NORTH selects CAS 0 and 1 for 32K RAM configuration If removed altogether this selects CAS 0 only but this should only be used for testing purposes on a Model B 26 WEST selects normal video output EAST selects inverted video output 27 WEST selects 8 MHz clock for 5 1 4 disc EAST selects 16 MHz clock for 8 disc 28 WEST selects base baud rate 1200 baud EAST selects 1300 baud cassette rate If link 28 EAST position RS 423 baud rate is also changed by the Same factorr 13 12 29 EAST selects base baud rate 1200 baud WEST selects 1300 baud cassette rate If link 28 is in the WEST position RS 423 baud rates are also affected 300 increase the flexibility used for the addition of extra sideways ROM sockets This would be in connecton with other links S20 21 22 to enable a total of 16 sideways ROMS to be selected 31 WEST selects CSYNC to RGB video output EAST selects ve CSYNC to RGB video output 23 32 WEST selects 13 input to pin 26 of ROMs 1 52 IC88 FAST selects 5v input to pin 26 of ROMs IC52 and IC88 This enables 24 pin ROMs to be used in the 28 pin socket 33 WEST selects A 13 input to pin 26 of ROMs IC100 and IC101 FAST selects 5v input to pin 26 of ROMs IC100 and IC101 This enables 24 pin ROMs to be used in the 28 pin socket 34 38 These are used to provide contact with the ROM decoder IC20 and the chip
106. use and the fuse carrier firmly in place The fuse carrier is of the same basic colour as the coloured insert in the base of the plug Different manufacturers plugs and fuse carriers are not interchangeable In the event of loss of the fuse carrier the moulded plug MUST NOT be used Either replace the moulded plug with another conventional plug wired as previously described or obtain a replacement fuse carrier from an authorised BBC Microcomputer dealer In the event of the fuse blowing it should be replaced after clearing any faults with a 3 amp fuse that is ASTA approved to BS 1362 This computer was designed and manufactured to comply with BS 415 In order to ensure the continued safety of Acorn products power supplies should be returned to Acorn for repair Do not use the Microcomputer in conditions of extreme heat cold humidity or dust or in places subject to vibration Do not block ventilation under or behind the computer Ensure that no foreign objects are inserted through any openings in the Microcomputer Not necessarily the same shade of that colour 1 Introduction 1 1 Nature Purpose of the Manual The purpose of this manual is to provide technical and diagnostic information about the BBC Microcomputer After giving general information about the technical specification and the mechanical assembly of the BBC Microcomputer it gives a detailed description of the operation of th
107. utput from page FC enables FRED OPEN enables ROM output from page FC disables FRED If link 17 is open then link 16 must be closed and R73 must be fitted See comments on link 14 18 SOUTH forces slow access to IC100 ROM NORTH allows fast access to IC100 ROM To allow the use of 1MHz EPROMs 32 19 WEST forces slow access to ROMs 1 52 IC88 IC101 EAST allows fast access to ROMs 1 52 IC88 and IC101 Diodes D10 D11 and D12 be selectively added to slow down ROMs IC101 IC88 and IC52 respectively when link 19 is in WEST position but for any ICs to have slow access R55 must be added D10 11 amp 12 and R55 are not fitted from issue 7 onwards 20 SOUTH connects high ROM select bit to IC20 decoder from A 13 NORTH connects high ROM select bit to IC20 decoder from ROMSEL 1 21 2 x NORTH SOUTH selects blocks 8 to B in IC51 and blocks C to F in ICs 52 88 100 and 101 4 EPROMs for OS 2 x EAST WEST selects blocks C to F in IC51 and blocks 8 to B in ICs 52 88 100 and 101 OS in IC51 22 SOUTH connects low ROM select bit to IC20 decoder from A 12 NORTH connects low ROM select bit to IC20 decoder from ROMSEL 0 23 OPEN RS 423 receiver not terminated DATA CLOSED RS 423 receiver terminated DATA 24 OPEN RS 423 receiver not terminated CTS CLOSED RS 423 receiver terminated CTS 25 SOUTH selects CAS 1 only for 16K RAM config
108. video processor Teletext hardware 3 4 RGB PAL encoder UHF output 35 Cassette RS423 serial processor 36 Internal 3 7 Keyboard 3 8 Sound speech serial ROM interfaces 34 9 A to D converters 3 10 Disc interface 3 11 Printer user port interfaces 3 12 Econet 3 13 1 MHz bus 3 14 Power supply 4 Upgrading the PCB 4 1 Convert from EPROM MOS to ROM MOS 4 2 Convert model A to model B 4 3 Add speech option 4 4 Add 5 1 4 inch disc interface to model B 4 5 Add Econet interface to model A 4 6 Add 8 inch disc interface to model B 4 7 Partial upgrading Selection links amp Circuit changes 5 1 Selection link survey Sz Table of link options 53 Circuit modifications from Issue 1 to Issue 7 5 3 1 Changes from issue 2 to 3 5 3 2 Changes from issue 3 to 4 5 3 3 Changes from issue 4 to 7 CO CO OO OO Y XO O LO O O O I I WN I 1 WN H Servicing Fault finding Introduction Test equipment Fault isolation Most common faults Test programs sample waveforms Test program Test ROM Interfacing Survey Purpose of each interface Interfacing to various printers Hardware hints and tips Component location tables Integrated circuits Transistors Diodes Capacitors Resistors Links Appendices Circuit block diagram Assembly Drawing Case Lower Assembly
109. you remove S25 altogether it puts the machine again into the 16K mode but this time with the CAS 0 area enabled and the CAS 1 area inoperative 40 6 4 Most Common Faults In the following section we shall try to give some ideas which have been colllected from various people who have been doing a good deal of servicing and repair work on BBC Microcomputers There will be no particular order to the comments but reading through all of them should give some useful ideas about faults which are likely to occur A common reason for getting sound on vision effects is that the power leads have become intermittent To check whether they are giving a problem a quick flick with one finger is what the experts recommend If this causes the display to flicker then switch off the unit remove the power leads pull back the insulating sleeves solder along the area where the wire is crimped by the terminal and replace them being careful not to exchange the 0 and 5 volt connectors It is possible for the ROM sockets to develop bad contacts This is sometimes caused by heavy handed use of the butterfly carrier boards which were used at various stages to put two 8K eproms into one single 16K socket The only solution for this is to replace the ROM socket entirely and you would be well advised to use the best quality socket available This is not an easy task unless you are experienced in the use of deso

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