Build a F.4,Sf Cassette fnterface
Contents
1. straddle tuning but improved per formance may be obtained by selecting a second R26 which will tune the high filter to 2295 The cassette read cable may then be attached to the short wave receiver and the microprocessor programmed as a radiotele type video terminal which can replace the noisy Teletype machine Of course a cassette interface specifically designed for this 170 Hz shift at 100 WPM will give superior performance under marginal condi tions The cassette interface may be used as a stand alone radioteletype terminal unit and audio frequency shift keying if desired and works quite nicely in this application Software would suggest using software for your cassette read and write timings Sample 8080 software is included as listing 1 Timings at locations lt 0 gt 116 lt O gt 133 lt 0 gt 241 and lt 0 gt 260 are based on an 8080 system with a 500 ns T time and no wait states Slower systems will require proportionately decreased loop timings A UART could be used instead of the software UART system shown However several disadvantages arise First a slightly greater cost and complexity More impor tant however is a degradation in total If you want a microcomputer with all of these standard features 8080 MPU The one with growing soft ware support e 1024 Byte ROM With maximum ca pacity of 4K Bytes e 1024 Byte RAM With maximum capacity of 2K Complete with2. Meas ure the output at pin 6 of 1C34 A stable condition no oscillation should be seen Connect the oscilloscope to the cathode of G1 again Adjust the balance potentiometer R42 so that the output voltage is a negative level Slowly turn the potentio meter until the output voltage jumps to a positive level and leave the setting at this point Disconnect the temporary jumper from the input connector and reconnect the audio oscillator Perform step 7 again The cross over threshold should be close to 2550 now If all proceeds well at this point the cassette interface is ready to receive data Connect the oscilloscope to pin 4 of the 566 voltage controlled oscillator 1C33 A triangular wave output should be seen Connect a temporary jumper between the TTL input going to DS1 and 5 V Connect a frequency counter to pin 3 of the VCO 1C33 Adjust potentiometer R41 for a resultant output frequency of 2125 Hz Remove the jumper from 5 V and connect the jumper from DS1 s input to ground This time adjust R40 for 2975 Hz output Remove the jumpers and you are ready for final tune in the driving circuit Connect the cassette interface to the driving output port and program the driving processor to send a TTL high level 1 output to the cassette interface Adjust R41 to 2125 Hz Then have the processor send a O level This time adjust R40 for 2975 Hz output The cassette interface is now ready
3. for use than gt 2 V so that valid TTL levels are not exceeded An offset adjusting poten tiometer allows the output to be placed ina Mark Hold condition when no tone input is being detected The cassette recording section figure 2 uses a single integrated circuit a 566 voltage controlled oscillator 1C33 A logic level from the computer s output port controls the resultant audio frequency output to the cassette recorder microphone input A high input 1 produces a 2125 Hz output and a low input 0 results in 2975 Hz The output wave shape is a symmetrical trian gular wave Should the user object to using a triangular wave a more nearly sine wave can be obtained by connecting a pair of back to back 1N914 diodes between ground and the output side of the coupling capacitor C5 Exact values and high quality com ponents will result in a trouble free voltage controlled oscillator The 47 K R17 resistor in series with the output is a typical value to be used when coupling to the low level low impedence external microphone inputs of most cassette recorders Using the AUX input of your cassette recorder generally gives better results Construction The cassette interface is available as a part of a printed circuit board kit from the Digital Group The printed circuit board is shared by a television display circuit to be described in the next article in this series A kit of the cassette inter
4. 01 GETDATA IN 1 Read port 1 bit O again lt 0 gt 125 346 001 ANI 1 Mask ali but bit O again lt O gt 127 202 ADD D Sum old bits with new bit lt 0 gt 130 017 r RRC Rotate new and old into next position lt 0 gt 131 127 MOV D A Save result back in D lt 0 gt 132 006 200 MVI B 200 Time delay between bits lt 0 gt 134 005 WDATA DCR B Decrement data wait count lt 0 gt 135 302 134 lt 0 gt JNZ WDATA If not done then keep waiting lt 0 gt 140 035 DCR E Decrement data count loaded at 0 103 lt 0 gt 141 302 123 lt 0 gt JNZ GETDATA If not done then repeat for next bit lt O gt 144 162 MOV M D Save received data in memory lt 0 gt 145 043 INX H Point to next available location lt O gt 146 174 MOV A H Move high order address to A for end check v lt 0 gt 147 376 yyy CPI yyy Has high order address reached end lt 0 gt 151 302 103 lt 0 gt JNZ STARTBYT If not then reiterate for next byte lt 0 gt 154 166 HLT End input Notes e Input is assumed to be wired to bit O of port 1 from output of 1C38 pin 6 via resistor R38 and shunted by diode DG1 e Loading proceeds from split octal address xxx xxx to address yyy 000 Enter this program by jumping to location lt 0 gt 100 after setting up constants of address e indicates a timing constant for the software UART inputs v indicates the end of transfer comparison mentioned in text lt 0 gt indicates an arbitrary page location for this program to be repla
5. 1 R24 R25 R22 R23 2125 2975 Hz 1100 Baud 6 8k 68 k 938 47k 47k 1200 2400 Hz 300 Baud Simple 6 8k 68 k 4173 4 7k 47k 1200 2400 Hz 300 Baud Correct 12k 120 k 1668 5 6 k 56 k 2125 2295 100 Baud Simple 6 8 k 68 k 938 4 7k 47k 2125 2295 100 Baud Correct 36 k 360 k 156 27k 270 k Low Pass Filter vco R26 C13 C12 C14 R12 R15 697 0056 uF 01 015 2 7k 1 3k 1162 0056 uF 01 015 470k 2 7k 906 015 uF 033 047 470k 2 7k 1301 0056 uF 01 015 47k 27k 179 056 uF Al 15 47k 27k means that the value so indicated is the typical calculated value The precise value is dependent on component tolerance Table 1 Theoretical values of components for alternate frequencies This table gives values of components to be used with the circuits of figures 1 and 2 in order to make this cassette interface work with several alternate specifications See the text for a definition of the various comments at the left of the table Potential Troubles Knowing about potential problem areas is a first step to minimization of their effects Troubles seem to break down into six classes e Cassette recorders and the cassettes used A marriage betweenyour 1000 microprocessor and junior s 20 cassette recorder which has been using 30 cassettes for the last five years will not produce happy offspring have been using a Superscope C 104 for the past year and can report no failures except for defective cassette tapes The C 104 has several attract
6. 2 005 WOUTLOOP DCR B Decrement delay count lt 0 gt 243 302 242 lt 0 gt JNZ WOUTLOOP If time left then reiterate lt 0 gt 246 037 RAR Rotate new bit into position lt O gt 247 015 DCR C Decrement output bit count lt 0 gt 250 302 236 lt 0 gt JNZ WNEXBIT If data left then reiterate lt 0 gt 253 076 001 MVI A 001 Stop bit state defined lt 0 gt 255 323 001 QUT 1 then sent out to port lt 0 gt 257 006 377 MV B 377 Stop bit value set lt 0 gt 261 005 WIBDELAY DCR B Decrement stop bit counter lt O gt 262 302 261 lt 0 gt JNZ WIBDELAY If time left then reiterate lt 0 gt 265 043 INX H Increment memory address lt O gt 266 174 MOV A H Move high order address to A for end check V lt 0 gt 267 376 yyy CPI yyy Has high order address reached end lt 0 gt 271 302 231 lt 0 gt JNZ BYTEOUT If not then continue output process lt 0 gt 274 166 HLT End output Note e Output is assumed to be wired from bit O of port 1 to DS1 in figure 2 See notes to listing 1 for listing conventions 52 system flexibility The software UART allows the timing constants to be dynami cally modified if desired by detecting the variations in the stop bit timing thereby compensating for wow and flutter Digital integration of the incoming data bits is possible by setting a register to octal 200 at the beginning of each bit time During the bit time repeated sampling either adds or subtracts from the register depending on whethe
7. Why Wait Build a ZAST Cassette Interface Dr Robert Suding Research Director for Digital Group Inc PO Box 6528 Denver CO 80206 This cassette interface does not have a 30 speed tolerance The design requires 12 V and 5 V to run A good quality recorder must be used along with excellent quality tapes Careful adjustments are required So why use it Well it works It s dependable And it s fast In contrast the proposed BYTE standard cassette interface runs at 300 Baud A Teletype paper tape reads 110 Baud have 24K on my system How long would it take me to completely toad my system not including any Bootstrap Loader operations Teletype 110 Baud 40 minutes 58 seconds Proposed BYTE standard 300 Baud 15 minutes 1 second The system to be shown in this article has been running for almost a year at 1100 Baud with an upper limit of 1750 Baud with critical tuning Suding system 1100 Baud 4 minutes 6 seconds Past issues of BYTE have included several articles on cassette interface proposals and 46 circuits would suggest re reading these articles You will find one common element Slow f you get the impression that I m impatient you re right I l bet you are too Imagine reading 300 Baud for 15 minutes to discover a noise pulse had destroyed data requiring re reading Ugh Thus the proposed standard of the BYTE Kansas City conference in 1975 has a major disadvantage Th
8. ance on personal checks l Delivery 30 60 days ARO H ae ii gt 4376 Ridge Gate Drive Duluth Georgia 30136 intelligent Systems Corp Telephone 404 4495961 53 i
9. card connectors Comprehensive User s Manual plus Intel 8080 User s Manual Completely factory assembled and tested not Bytes a kit e TTY Serial I O Optional ac e EIA Serial I O cessories Key 3 parallel I O s board video e ASCII Baudot terminal com patibility with TTY machines or video units Monitor having load dump display insert and go functions display audio y cassette modem interface power supply ROM programmer and attractive cabinetry plus more options to follow The HAL MCEM 8080 375 then let us send you our card HAL Communications Corp has been a leader in digital communi cations for over half a decade The MCEM 8080 microcomputer shows just how far this leadership has taken us and how far it can take you in your applications That s why we d like to send you our card one PC board that we feel is the best valued most complete HAL Communications Corp Box 365 807 E Green Street Urbana Illinois 61801 Telephone 217 367 7373 microcomputer you can buy For details on the MCEM 8080 write today We ll also include compre hensive information on the HAL DS 3000 KSR microprocessor based terminal the terminal that gives you multi code compati bility flexibility for future changes editing anda convenient large video display format 51 Listing 1 Stand Alone Suding Cassette Input Pro gram This program is a self contained data trans fer r
10. ced by a real memory page number when actually loading the program at byte 100 of some page Split Octal Address Octal Code Label Op Operand Commentary lt 0 gt 200 041 xxx xxx MEMTOCAS LXI H xxx xxx Load starting address in HL pair lt 0 gt 203 076 001 MVI A 1 Start port output in high state lt 0 gt 205 323 001 OUT 1 Send initial state out lt O gt 207 026 012 MVi D 012 Outer leader delay count lt 0 gt 211 006 377 LEADERS5S MVI B 377 Outer leader delay loop return lt 0 gt 213 016 377 LEADER5X MVI C 377 Middle leader delay loop return lt 0 gt 215 015 LEADERSY DCR C inner leader delay loop return lt 0 gt 216 302 215 lt 0 gt JNZ LEADERSY If inner loop not done then reiterate lt O gt 221 005 DCR B Middle leader delay count lt O gt 222 302 213 lt 0 gt JNZ LEADERSX If middle loop not done then reiterate lt 0 gt 225 025 DCR D Outer leader detay count lt O gt 226 302 211 lt 0 gt JNZ LEADERSS If outer loop not done then reiterate Upon reaching this point 5 seconds of mark high state have gt been output to the cassette interface lt 0 gt 231 016 011 BYTEOUT MVI C 011 Define output bit count decimal 9 lt 0 gt 233 257 XRA A Clear carry start bit level is 0 lt 0 gt 234 176 MOV AM Move current byte to A lt 0 gt 235 027 RAL Rotate bit into position carry 0 first lt 0 gt 236 323 001 WNEXBIT OUT 1 Send current LSB to output port lt 0 gt 240 006 200 MVI B 200 Time delay between bits lt 0 gt 24
11. ddresses The check marks in the listing V indicate the points where start and stop addresses may have to be modified The software may be adjusted to run at different data rates by changing the values at the addresses mark with an asterisk Note that the constants at lt O gt 133 and lt 0 gt 241 are the same The constant at lt O gt 116 is 50 greater and the constant at lt O gt 260 is twice the value of the constant at lt 0 gt 241 Summing It Up This cassette interface represents a simple but fast and dependable way to store pro grams and data for the serious hobbyist It does not seek to be all things to all users but a number of applications can be run using the same basic design The detail interface design has independence from other com ponents in the system allowing various processors to use the same cassette system with appropriate software UNBELIEVABLE The intecolor 8001 Kit A Complete 8 COLOR Intelligent CRT Terminal Kit 1 395 e Limited Graphics Mode Background Color Special Graphics Characters Games ISC WILL MAKE A BELIEVER OUT OF YOU no Intecolor 8001 kits at 1 395 plus 15 00 ship i ping charges each a Enclosed is my O cashiers check O money order O personal check 1 350 deposit kit for C O D shipment for NAME ADDRESS m STATE ZIP CITY e Allow 8 weeks clear
12. e use of a redundant Man chester format with a 1200 Hz low fre quency critically restricts the user to slower data rates A related disadvantage for those who use filters or phase lock loops as an input detection method is the fact that the Manchester code employs harmonically re lated frequencies this leads to design problems in detectors based upon frequency discrimination techniques The system shown in this article avoids the above pitfalls It uses the non harmoni cally related tones of 2125 Hz Mark and 2975 Hz Space The second harmonic of 2125 Hz occurs at 4250 Hz well down on the passband of a 2975 Hz detector Suffi cient space exists between the two frequen cies to allow for reasonable recorder speed discrepancies The higher frequencies in volved permit increasing the data rate Several approaches are possible in cassette interfacing as seen in past BYTE articles However their emphasis on wide cassette speed tolerance made them slower My 2975 Hz BANDPASS FILTER LIMITER ci 055 IN4148 R22 DS4 IN I4B FROM CASSETTE RECORDER OUTPUT oe Ji Si Ol cg ol 2125Hz i 3 POLE BANDPASS FILTER 12V LOW PASS FILTER Cle 5V R3 A OK gt R38 R36 t 470 5 10K 3 e ew 4 3 4 DGI 14 P i IN48 12V R37 R39 IK 100K 12V 12V R42 50K TO TTL READ OFFSET ADJUST INPUT PORT Figure 1 The schematic of the Suding cassette input interface as found in the Digital Group systems This inter
13. face amplifies and clips the cassette output with limiting amplifier 1C34 discriminates the two data frequencies see table 1 with bandpass filters followed by full wave detectors passes the detected signal through a 3 pole active low pass filter then converts the result to a TTL level which is read by a single bit input port One example of software see listing 1 to drive this input interface uses a programmed simulation of UART input algorithm an actual UART or ACIA device could be substituted if desired approach to out of specification cassette speed is put it in the specification or get a good recorder More of that later Theory of Operation The 1100 Baud Digital Group system uses the circuits of figures 1 and 2 The cassette receive circuitry detects the prerecorded frequency shift keying and produces a 1 or a 0 output as a result of a detected 2125 Hz or 2975 Hz tone at the input A 741 operational amplifier 1C34 is used asa clamped limiter which prevents variations in cassette amplitude from affecting the detec tion process The output of the limiter should be about 6 V peak to peak roughly a square wave with rounded edges of the incoming frequency constant in amplitude regardless of tape volume setting or minor tape dropout problems Two bandpass active filters IC35 then amplify a tone five times when actually tuned to their respective frequencies of 2975 Hz for the top filter a
14. face only is also available from the Digital Group for 30 which includes all parts and the printed circuit board The experienced builder can build the circuit in an evening or two by hand wiring components on standard 1 inch grid Vectorboard All the circuitry can be contained in an area of approximately 3 inch by 5 inch about 8 cm by 13 cm Be sure to use only high quality com ponents particularly in the active bandpass filters and voltage controlled oscillator Some strange frequency jump problems have been traced to surplus 566s which were temperature sensitive Lay out the receive circuit to avoid feedback paths from output to input particularly in the limiter active bandpass filters and slicer areas Different op amps could be used but may result in instability or degradation of final perfor mance due to suboptimization Modifying Your Cassette Recorder It is very helpful to listen to the data from the cassette so that the beginning of the data burst may be detected as well as 49 VIDEO TERMINAL for your ALTAIR Minnie T VIDEO TERMINAL INTERFACE Connects to standard TV monitor or modified receiver to display 16 lines of 64 or 32 characters in a 7 x 9 matrix Character set includes 128 upper and lower case ASCII characters and 64 graphic characters for plotting 48 x 64 128 with memory option array Text and graph ics may be mixed on the same screen 8 bit keyboard input port is provided Cha
15. ive features Besides the usual conveniences such as index counter cuing etc it has a variable readback speed control dandy for out of spec cassettes from friends Inside another special motor speed control potentiometer is located near the speaker which allows precisely setting the record write speed Quality control seems good overall and the list price of 120 cheaper at discount stores is worth the invest ment Don t waste your money on cheap cassettes Sony Low Noise C 45s have been generally good Some 2 4 Data Certified Cassettes are superior but not needed Microprocessor caused problems Some microprocessor designs will not work directly with this interface system This interface was designed to be connected directly to a single bit IO port with the processor handling all of the bit timings through timing loops If your processor must Periodically catch its breath for such things as dynamic memory refreshing you may be unable to directly use the Software UART system What a shamel However a hardware UART will permit using the system even with a system of this nature Cabling problems It is possible to connect your cassette recorders with the read and write cables reversed Enough crosstalk from the write line to the read limiter existed to give the appearance of data being read but so many errors resulted that the programming would not run e Tuning problems Circuit tuning is the most common proble
16. m Carefully tune the active filters Cassette Crashes Cassette damage is frequent 50 on tapes which have always worked before but now mysteriously fail The most common cause of this is removing a cassette from the recorder without completely rewinding The exposed oxide then gets damaged and is no longer usable e Miscellaneous circuit problems Defective level output from cassette read limiter 1 None at all Check for 12 V to I1C34 and 1C34 2 Too high output level Diodes DS4 and DS5 open or one is reversed Bandpass active filters don t filter 1 Off frequency 2 Bad 5558 3 Check for shorts or out of tolerance con densers C8 C9 C10 or C11 Disk ceramics are a no no in tuned circuits 4 Resistors improperly wired or inserted Fult wave detector does not work as described 1 Diodes open reversed or shorted 2 Defective IC36 Low pass active filter fails to work 1 Shorted or out of tolerance condensers 2 Defective 1C37 Output slicer 1G38 fails to produce TTL levels 1 Reversed open or not Germanium diode at DG1 2 Too heavily loaded output This circuit should drive no more than one TTL load standard for most 1O ports VCO won t oscillate 1 Defective 566 1C33 2 Shorted condenser C6 VCO has parasitic oscillation high frequency 1 C7 not connected 2 Defective 566 3 C6 is open producing a very high fre quency VCO won t tune to frequency or stay the
17. nd 2125 Hz for the lower filter The further off the tuned frequency the tone is the less amplification the filter will produce The gain bandwidth and tuned frequency are set by the three resistors and two condensers in each filter Each filter may be exactly tuned to fre quency by carefully setting the variable resistance value which may be either a potentiometer or selected fixed values Full wave active detectors produce rectified full wave pulses at the summing junction pin 5 of IC37 The 2975 Hz tones are rectified to a positive voltage and the 2125 Hz tones are rectified to a negative voltage As received tones depart from either exact frequency a value less positive or 47 Sa Ri4 1 6K SPACE ADJ 2975 Hz TO CASSETTE R40 C5_ giz MICROPHONE SK Ro a 033 ayk INPUT OR AUX J2 OSI IN4148 Abe TYPICAL pz RIG 5V ZENER 220 172W FROM TTL OUTPUT PORT 12V Figure 2 The schematic of the Suding cassette output interface as found in the Digital Group systems The output interface is a simple audio frequency shift keyer made up of a 566 voltage controlled oscillator with two frequency States controlled by a single TTL data line The TTL level which drives the output modulator is a single bit derived from an output port The software see listing 2 to drive this output interface is shown as a programmed simulation of a UART output algorithm an actual UART or ACIA device could be substitu
18. outine which will transfer a block of data from cassette to split octal memory locations xxx xxx through yyy 000 This program assumes that MEMTOCAS see listing 2 was used to create the tape being read A more gener ally useful input facility would be modelled on this program and linked to a system monitor as a sub routine Listing 2 Stand Alone Suding Cassette Output Program This program is a self contained data trans fer routine which will transfer a block of data from split octal memory locations xxx xxx through yyy 000 onto cassette tape after a five second leader output delay This program assumes that CASTOMEM see listing 1 will be used to read the tape being created A more generally useful output fa cility would be modelled on this program and linked to a system monitor as a subroutine Split Octal Address Octal Code Label Op Operand Commentary lt O gt 100 041 xxx xxx CASTOMEM LXi H xxx xxx Load starting address in HL pair lt 0 gt 103 021 010 000 STARTBYT LX D 000 000 Load E clear D lt 0 gt 106 333 001 SYNCHLOO IN 1 Port 1 bit O read for input lt 0 gt 110 346 001 ANI 1 Mask all but bit 0 lt 0 gt 112 302 106 lt 0 gt JNZ SYNCHLOO If not start bit then reiterate loop lt 0 gt 115 006 300 MVI B 300 Time delay to middle of first data bit lt 0 gt 117 005 WSYNCH DCR B Decrement synch wait count lt 0 gt 120 302 117 lt 0 gt JNZ WSYNCH if not done then keep waiting lt 0 gt 123 333 0
19. r 1 or 0 and a branch minus instruction system effectively eliminates re ceive problems This digital integration detection is utilized by the Digital Group Z 80 cassette read software Versions of this software UART sys tem have been written for 8008 8080 Z 80 6502 and 6800 All work satisfactorily Operation This cassette system is utilized by first turning on the cassette recorder and waiting until the lower tone 5 second leader tone is heard At this point restart the system to the beginning address of the Cassette to Memory software Cassette writing is accomplished by re starting the system to the beginning of the Complete Means 8080 CPU 25 Line x 80 Character Line 4Kx8 RAM PROM Software e Sockets for UV Erasable PROM 19 Shadow Mask Color CR Tube RS232 1 0 Sockets for 64 Special Graphics Selectable Baud Rates to 9600 Baud Single Package 8 Color Monitor ASCII Set e Keyboard Bell Manual And you also get the Intecolor 8001 9 Sector Convergence System for ease of set up 3 5 minutes and stability Additional Options Available Roll e Additional RAM to 32K 48 Line x 80 Characters Line Light Pen Memory to Cassette programming Be sure to set the appropriate start and stop ad dresses prior to beginning the read or write operations The monitor programs in the various Digital Group systems automatically set the start and stop a
20. racters are stored in the on board memory and may be read or written by the com puter Cursor control text editing and graphics software is included Fully compatible with Altair and IMSAI Sockets included VT 32 32 character line 185 00 kit VTI 64 64 character line 210 00 kit 285 00 assd a ANALOG INTERFACE Complete interface tor a CRT graphics display or X Y plotter Provides 8 channels of sottware controlied A D conversion 1 or 2 channels ot analog output with 10 bit resolution 0 10v or 5v out 6 bits of latched digital output and 8 analog comparators Software included for A D conversion by successive approximation and track ing Sockets included ADA 1 1 analog output 145 00 kit ADA 2 2 analog outputs 195 00 kit 255 00 assd SEE THESE AND OTHER PRODUCTS AT THE FOLLOWING COMPUTER STORES THE COMPUTER STORE 213 478 3168 Los Angeles CA THE COMPUTER MART 714 633 1222 Orange CA COMPUTERS amp STUFF 415 278 4720 San Lorenzo CA COMPUTERS amp STUFF 801 377 1717 Provo Utah COMPUTER SYSTEMS CENTER 404 231 1691 Atlanta GA BYTE SHOP 415 969 5464 Mountain View CA DEALER INQUIRIES INVITED All prices and specifications subjaci to change without nolice Pricas are USA only Calit residents add 6 sales tax Add 5 shipping handling and insurance POLYMORPHIC SYSTEMS 737 S Kellogg Goleta CA 93017 805 967 2351 Bank Amercard and Master Charge Accepted Low Filter High Filter R2
21. re Out of tolerance or defective C6 You really didn t use a disk ceramic here did you Defective 566 Non TTL levels used to drive VCO Defective potentiometers R40 or R41 DS1 or DZ2 reversed or defective hearing the end of the data When the cassette read cable is plugged into most cassette recorders earphone output jack the speaker output is usually cut off However since a closed circuit jack is all that is involved a quick solution is to connect a jumper on the jack so that the speaker is not disconnected Even better use a 100 ohm 4 watt resistor instead of the jumper and the data howl won t be so loud A 10 ohm 14 watt resistor from the amplifier lead to jack to the jack frame will prevent potential damage to the output driving transistor s Alternative Frequencies and Applications The cassette interface design may be used with the proposed BYTE standard should you so desire Table 1 has appropriate component values calculated for two alter native possibilities the simple way less desirable and the right way The simple way permits using a switch on the bandpass active filters to select the frequency pairs The right way involves setting the circuit to the optimal values and using separate inter faces for each frequency pair Amateur radio ham _ radioteletype RTTY generally uses 2125 2295 Hz frequency shift keying for 170 Hz shift The existing cassette interface can be used by
22. sine wave output of 5 or so although the precise level is not at all critical Apply 5 and 12 voltages to the circuit Measure the output at pin 6 of the 741 limiter 1C34 with an oscilloscope The wave shape should be a rounded square wave of about 6 V peak to peak Set the audio oscillator to 2125 Hz Measure the output at pin 1 of the 5558 active bandpass filter Slowly turn R25 until the signal peaks Be sure that you are peaking at 2125 Hz not a harmonic Vary the oscil lator frequency a few decades to insure 2125 Hz is the tuned frequency Similarly set the oscillator to 2975 Hz and measure the output at pin 7 of the 5558 1C35 Slowly turn R26 until the signal peaks Vary the oscillator to insure a 2975 Hz peak Measure the detected voltages at pin 5 of IC37 When the oscillator approaches 2125 the voltage should go negative When ap proaching 2975 the voltage should go posi tive Trouble in this area would most likely be caused by reversed or defective diodes or shorts between adjacent lines Measure the voltage at the cathode bar end of the output clamping germanium diode the actual output voltage from going less G1 Sweeping the frequency between 2125 and 2975 Hz should result in a clean voltage jump somewhere between 2125 and 2975 Hz Measure the output swing to insure that it does not exceed 5 3 V Remove the audio oscillator and short input connector J1 temporarily to ground
23. ted if desired negative is produced until approximately midway 2550 Hz a summed voltage of 0 results A three pole lowpass active filter then removes the remaining traces of pulsating DC from the summed signal with almost no effect on the data pulses up to a speed of 1000 bits per second if lower data rates were to be utilized an improved signal to noise ratio could be obtained by multiplying the values of C12 C13 and C11 by the reciprocal of the data rate ratio Table 1 shows some component values for alter native frequency designs The final receiver section is a 741 opera tional amplifier 1C38 connected as a slicer This operational amplifier detects whether the voltage at its pin 2 is positive or negative with respect to the constant voltage at its pin 3 The output voltage will then swing either to nearly 12 V or to nearly 5 V Notice that this operational amplifier has 5 as its positive supply voltage pin 7 A forward biased germanium diode prevents Tune Up Notes The cassette interface must be carefully tuned to achieve proper performance Careless tuning has been the most frequent cause of cassette system failure 1 Plug in the six integrated circuits of the cassette interface 2 Connect a calibrated audio oscillator between the limiter input and ground A digital frequency counter driven by the audio oscillator is highly recommended The oscillator should cover the desired range of 2 3 kHz with a
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