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tcj_02 1983-10 01.02

1. s07 9c 704 9CS6 1 sco EB SED DFS sce 99 01200 na FLAG BRI sco3e CNTR I CNTR wees BR2 es BRS oe FIFO Y FIFO CouTz score 24 28 v assF ese 628 5 esoe Courz PNTR HERE SCLEAR T1 INTERRUPT FLAG I TEST THE FLAG 1SPEAKER FLAG OFF STOGGLE SPEAKER INCREMENT COUNTER OME DECREMENT COUNTER TWO SREAD KEYBOARD INO KEY IS IT CONTROL CHARACTER t VES STORE THE CHARACTER IN THE IRING BUFFER SOUTPUT THE CHARACTER I CLEAR STROBE F ADVANCE THE CURSOR S KET Y FROM THE STACK I GET A FROM MEMORY ERE TURN DELETE KEY CHECK THE DOT AMD DASH FLAGS 1GET OUT IF THEY ARE SET BREAD THE PUSH BUTTON I INPUTS s SET THE DOT FLAG OEC FOR A DASH ISET THE DASH FLAG Morse Table P we 7C F4 b 2 7C F4 5 2 D8 B8 ag continued on p 22 Beginner s Column Part Two The Computer Hacker 19 ANYONE FOR A LITTLE KISS ELECTRONICS by Phil Wells Technical Editor Haw much electronics theory do you have to know to be able to design your own computer related projects A dozen or so basic concepts and formulas will get you started Beyond that one of the great things about this hobby is that you can dig into theory just as far or as little as you want It just helps a lot to be able to learn it hands on That s what this column is for As discussed in last month s KISS you will need at least a VOM Volt Ohm Milli
2. e 6 DSR cc Signal Gnd e 7 Signal Gnd AB Carrier Detect amp 8 DCD CF Reset lt e 11 Keyboard Inhibit lt e 18 Reverse Channel 19 SCA Data Terminal Ready e 20 DTR Print Inhibit e 21 Buzzer e 22 CE Paper out Ribbon end 23 Interrupt Break gt 25 Figure 13 NEC 7700 series Spinwriter Only used with keyboard version Figure 14 the Qume Sprint 5 serial port illustrates what 1 think is an excellent idea This looks like a fairly typical RS 232 C port but it has a useful twist A switch just behind the front panel of the printer but accessible without removing screws can be set to MODEM or NO MODEM In the NO MODEM position the CTS and DSR and presumably DCD inputs are ignored and either the RTS or DTR output line can be used for hardware nanasnaking in a uirect connection to a computer These lines will be turned off negative when the input buffer is within two characters of being full and will be turned back on only when the input 10 The Cumputer Hacker buffer drops to within ten characters of empty If the printer is connected to a modem the switch is set to the MODEM position and RTS DTR CTS DSR and DCD become standard RS 232 C control lines DTE DB25S Frame Gnd Trans Data Rcvd Data lt Request to Send lt Clear to Send lt Data Set Re
3. sFORCE A BRANCH BACK TO GET ITHE REST OF THE CHARACTER I SEND A DASH SFORCE A BRANCH BACK X IS NUMBER OF DOTS STURN RELAY ON S SET UP MASK FOR FLAG I SET SPEAKER BIT IN FLAG SMALT FOR ONE DOT TIME S TURN RELAY OFF SCLEAR SPEAKER BIT IN FLAG ADD A SPACE ISPACE COMPLETE S DASH IS THREE DOTS FCHARACTER SPACE I MORD SPACE SSTARTING INDEX FOR MESSAGES ISAVE Y ON THE STACK GET A CODE FROM THE KEYBOARD OUTPUT IT TO THE MONITOR IWAS IT A BACKSFACE IND S YES GET Y BACK IDELETE THE CHARACTER BY LOADING ITHE BUFFER WITH SPACE SFORCE A JUMP TO GET A NEW KEY IWAS IT A RETURN IVES END THE MESSAGE STORE CHARACTER FOR A MOMENT SGET Y Back IGET CHARACTER BACK 3STORE IT IN THE MESSAGE BUFFER 360 BACK FOR ANOTHER CHARACTER SSTORE INDEX FOR THE END OF JEACH MESSAGE tGET OUT IF Ye IGET ANOTHER MESSAGE UNLESS JWE HAVE THREE ALREADY S INPUT THE NEXT MESSAGE IALL THE MESSAGES ARE IN MEMORY CONTROL KEY CODE TO x sPICK UP THE STARTING INDEX SSAVE x JON THE STACK sFETCH THE MESSAGE I SEND A CHARACTER SSAVE Y SNEED TO SEND A CHARACTER FROM 1 THE RING BUFFER so YES IGET Y BACK GET x IFROM THE STACH SEND OF MESSAGE YES 50 QUIT IND GET ANOTHER CHARACTER TRIZEHROENT IS VRE BER CSA RSSVERRELIYARSS Listing 4 The INTERRUPT Routine CODY WEUN weMIIWREAse BEKRsacsses PITCS SE EES Se 3 Y og FC E4 8 FC E4 68 palal g
4. 0 00 cece cece eens Joan Thompson Production Assistant Judie Overbeek Technical Editor Lance Rose Technical Edttor ccc cccccnceceees Phil Wells The Computer Hacker ts published 12 times a year Annual subscription ts 24 in the U S 30 in Canada and 39 in other countries Entire contents copyright 1983 by The Computer Hacker Postmaster Send address changes to The Computer Hacker P O Box 1697 Kalispell MT 59908 1697 Address all editorial advertising and subscription inquiries to The Computer Hacker P O Boz 1697 Kalispell MT 59908 1697 2 The Computer Hacker FILE TRANSFER PROGRAMS FOR CP M by Lance Rose Technical Editor Bypassing Incompatible Diskette Formats Although CP M has by now established itself as the standard 8 bit microprocessor operating system this doesn t mean by a long shot that programs are easily transferrable from one computer running CPM to another The CFM operating system includes within it a section known as the Basic I O System or BIOS for short This part of the system is very machine dependent and is what adapts the other standars parts of CP M to the particular hardware it is being used on These machine dependent parameters include things like the port addresses for keyboard and printer the disk controller type and the size and format of the particular disk system involved For this reason a version of CP M with a BIOS written for say the Nort
5. Since you rarely have access to be on both equipments remember there can be a lot of cu and try in the cabling Output Transmitted Data Input Received Data Output Request to Send Input Clear to Send Input Data Set Ready Signai Ground input gt Carrier Detect Output Data Termina Reacy Input Ring Indicator Figure 7 IBM PC and XT Asynchronous Communications Acapter RS 232 C like interface Figure 8 shows the cable recommended by Apple to connect a Qume Sprint 5 or DEC LA120 printer to an Apple III computer This illustrates several common problems The Apple III s built in serial port is hard wired as DTE so is the printer The MODEM eliminator or null MODEM cable crosses some leads so that each device appears to the other as a DCE Note that DTR and DSR are paired but crossed over Also note that the DCD inputs are driven by RTS and that each device s CTS input is driven by its own RTS output Apple tli Qume Sprint 5 DEC LA120 Chassis Gnd Signal Gnd Trans Data Revd Data e a Chassis Gnd o e 7 Signal Gnd o gt e 3 Rcvd Data o _ lt e 2 Trans Data RTS CTS DCD DSR DTR e8 DCD 5 CTS RTS Oo d 020 DIR DSR HK DN RONNY N oO Figure 8 Modem eliminator cable between Apple lil and Qume Sprint 5 where both devices are DTE Figure 9 shows Apple s recommended cable for connecting an Apple III to the I
6. Southwestern Bell Telephone Company s Oklahoma tariffs call for the charging of an information Terminal Service rate for anyone connecting a computer to the telephone lines via a modem This rate is approximately 500 higher than the standard residential base rate Obviously this tariff dramatically affects the entire industry as it practically prohibits noncommercial modem use The Computer Hacker considers easy access of communication lines thru modems to be very important for micro users and we would like to publish a report of how this access is handled in different parts of the country If you have had an experience with modem connections that would be helpful to others in similar situations please write and describe your experience and the solutions you arrived at Include anything that you fee would be helpful to someone faced with a similar problem For your protection no names will be published only the State will be given 24 The Computer Hacker Rate 50 per word minimum charge 7 50 All classified ads must be paid in advance and will be published in the next available issue No checking copies or proofs are supplied WANTED Teletype KSR 35 manuals needed to restore old teletype machine Also need manuals for paper tape punch and reader The Computer Hacker P O Box 1697 Kalispell MT 59903 1697 FOR SALE SSM 10 4 board for S 100 bus Two serial two parallel ports 100 DEC LSI 11 minicomputer Rack mount K
7. individual character to the printer a slow process that occurs at printer speeds the entire output to be printed is sent toa disk file known as the spool file Since writing to even a floppy disk is much faster than writing to a printer this happens quite fast and the CPU is then free to perform some other task But wait a minute How does the output get from the disk file to the printer This is done with some special software built into the operating system This software knows when the spool file has something in it that needs to be sent to the printer When this condition exists the software allows interrupts from the printer to occur whenever the printer is ready to accept a character When each interrupt occurs the interrupt handling routine retrieves one character from the spool file actually from a buffer containing perhaps a sector at a time of the spool file and sends it out to the printer This takes very little time since the printer is already known to be ready and the CPU doesn t have to sit around waiting for this to happen The result is that the time used for printing can overlap with useful time for doing another job with the system This method of spooling is widely used on mainframes and minicomputers where there is usually ample disk space more often than not a hard disk or two and where the operating systems include the necessary software to handle the spooling process There is even a program available for CP
8. still send output to an RS 232 printer via a spooler with a continued on p 15 14 The Computer Hacker REVIEW OF FLOPPY DISK FORMATS by M Mosher W henever the subject of software exchange comes up as it often does the question arises of Why can t I just take my diskette from system A and put it into a drive on system B and have it work To answer this question a discussion of the differences in floppy disk format standards is in order What I ll do here is take the characteristics of a floppy disk one at time and point out the similarities and differences Size This one is pretty obvious If you try to put a 5 25 inch diskette into an 8 inch slot it s going to just flop around in there no pun intended Conversely you just aren t going to fit an 8 inch diskette into a 5 25 inch slot at all unless you use a pair of scissors and I m not even going to begin to address that issue To add to the variation Sony has recently introduced a 3 inch microfloppy drive which should be entering production very soon Number of Tracks Most manufacturers have pretty much standardized on this parameter though there are some variations Full size 8 inch floppies almost always have 77 tracks to the diskette minifloppies 5 25 inch have mostly had 35 tracks in the past but many are showing up now with 40 tracks Of course a floppy that has 40 tracks of data on it can t be read on a system whose hardware can only read 35 track
9. Signiticant Multipuer Tolerance figure Silver 02 10 Goid 01 5 Black 1 Brown 10 Red 100 Orange 1000 Yellow 10000 Green 100000 Biue 1000000 Violet Grey Wnite No Color CMnmMUnwn oo Figure 11 are offset towards one end of the body of the resistor To read the resistance value hold the resistor end nearest the bands toward your left then read the colors from left to right The first two colors are the more significant digit and less significant digit of the resistance value The third band is the multiplier or number of zeros to tack on after the two significant digits The fourth band indicates tolerance or how far from the indicated resistance the specific resistor might be Gold is 5 silver 10 and no band is 20 A 1000 ohm 5 tolerance resistor s real resistance can be anywhere from 950 to 1050 ohms Next Time Next month we ll design a power supply We ll go into how to select the components and calculate the required values for a five volt 1 5 amp experimeter s bench supply E continued from p 18 ASCII Table Any Computer Hacker Machinists Out There We want to contact anyone interested in using micros to con trol machine tools for personal use or on small business level Contact us if you re using micro for measurement or control with lathe milling machine or other machine tool We need to know what you are doing how you are doing it what problems you hav
10. a virtue and try to design accordingly I think you can see some of the reasons for my design here Not all would necessarily agree with everything I ve said but I can say that this design approach leads to a circuit that is fairly easy to build and troubleshoot and works well when complete In Part 2 I will present the hardware construction layout and schematics for the spooler along with some suggestions for a power supply and case to put it in Also I ll give a flowchart and listing for the spooler s operating program along with some additional comments on the software B Correction The September Computer Hacker contained an error in the RS 232 C article On page 4 in the section titled Hacker s View of the Mechanical Requirements the first requirement reads The DTE must provide a female connector The sentence should read The DTE must provide a male connector We regret any inconvenience this error may have caused Please don t hesitate to write if you find something which you believe to be an error 16 The Computer Hacker SENDING MORSE CODE WITH AN APPLE by Marvin L De Jong Introductien Using a computer to send Morse code is a clean well defined programming problem and it has always been one of my favorite real time control applications Receiving Morse code with the aid of a computer is a more difficult task especially if any serious attempt is made to approach the capability of a human
11. about hacking and need ideas and information from our readers We will use this information to prepare press releases and an information packet for the press In order to get the information needed The Computer Hacker is announcing a contest for Hackers We ll award some prizes perhaps logic probes or breadboarding kits for the responses which are most useful There will be two categories 1 What is a Hacker 2 A description of a useful hacker project such as interfacing a micro so that a disabled person can control their world The final result which will be submitted to the press will probably be blending of many responses WE NEED YOUR INPUT THE HACKER STANDARD INTERFACE In this issue we continue the series on the RS 232 C interface The first part explained the standard s definitions Part two covers the standart configurations and describes real world examples of the RS 232 C interface as used with microcomputers and recommenations for hacker standards A future article will cover the use of UART and USART integrated circuits with the RS 232 C serial interface It may seem that we are spending a lot of time on the RE 232 C interface but many hacker projects at least RE ER y Fi E kis The Computer Hacker 1 Om ERE Nm be SY NIN IN A I ee on og Sasa oft h the ones I get involved with require the transfer of data and a good understanding of the standard interfaces is vital We suggest that
12. be converted into a form that is representable by ASCII characters a HEX file before transmission It is then transmitted and reconverted into a binary file at the receiving end While this works it forces the transmission of two bytes of data for each byte of binary information that must be transmitted thus in effect cutting the transmission speed in half Using this method would limit the effective data transfer rate on a modem to around 15 bytes second As I mentioned above even 30 bytes second is irritatingly slow especially when transferring long programs not to mention the expense if this is happening long distance The alternative to this which I am using here is to make sure the serial port used is configured for 8 bit words and no parity bit Most any serial port can be configured this way with a little snooping in the user s manual In fact I have found that most hardware manufacturers use this is the default configuration for their serial data ports With this accomplished a binary byte can be transmitted as is and any error detecting can be left to the checksum rather than the parity bit The Programs The programs presented here are designed to work with each other in moving files between CP M machines The basic method of transfer is to have the receiving machine in control of the situation The transmitting machine waits until the receiving machine is ready before sending anything This allows for the case where the rece
13. current flow by varying the resistance Figures 4 and 5 illustrate the use of Ohm s Law Knowing any two of the three parameters we can calculate the unknown one Give it a try Voltage Drops Another way of looking at Ohm s Law shows that if we apply a voltage to a complex circuit the current which flows through each resistance produces a voltage drop across What is the current W R 5 2000 0025A 2 Sm What is the voltage V iR O x 4000 40V What 15 the resistance R V 1 5 005 1000 onms Figure 4 Using Ohm s Law the textbook way What is the current In r i this part of the crcunt Part of complex V A 5 2000 0025A 2 5 M What is the voltage V lA 01 x 4000 40V What is the resistance Measure and V Re V la 5 055 1000 ohms the resistance equal to the product of current and resistance see figure 6 More current produces a larger voltage drop This may make more sense if we measure the voltage across each of two resistors connected in series figure 7 This figure contains a wealth of information The total resistance of two resistors in series is the sum of the two resistance values The same current flows through both resistors The voltage drop across each resistor equals the current through it times its resistance We have only one current Source Figure 6 Onm s Law says current flowing through resistor produces a voltage drop across
14. file if present LXI E TFCB MYI c 22 CALL Bros smake new file IWR aA LX1 C DCFER Point at error message J2 ABCRT Print and reboot Mv c 38 5end acme NULs first MVI E BH CALL XMTBYT FLUSH Flueh buffer if full LAR c R ACY 160 look for next record Nz NULS FLUSH C DATBUF 6tart at beginning of buffer Ik SIODAT Clear serial data port FLUSH DATPTR LDA RPTCTR AD cP RETRY x E GIH FLUSH2 More to go Jz READY iFirst time for this record ALE E Change request character to 62H L ORA A Test repeat counter FLUSH2 READY iTry again 19 not zero N H CATBUF ENCXMT D EOTMSG Print EOT message and exit DATPTA ABORT REALY XMTBYT 38end prompting byte FLUSH2 E READY C 11 Cc 26 Bros 1Get console status BDOS 16et DMA addrese A c TFCB READY 3 No key pressed c 21 1 BDOS Write record BDOS ct GIH a ENDXMT Terminate if control C pressed FLUSH3 IG00d write SIOSTA seed serial status port D DEFER Disk error IFLAG ABCRT READY2 1walt for character to be received FLUSKJ H 0060H SIOLAT Get received character D iPoint to mext record TFH sMask off bit 7 for ASCl codes 03H FLUSH1 60 write it af present CLOSE Flush buffer and close file FLUSH Plush buffer of data 64H E TFCB ENDXMT End transmission 16 1H BLOS Close file RCVREC Receive next record D LOFMSG 2H c 9 REALY2 Ignore other cheracters Bros Print EOF message RCVREC B 664 rByte count for cord BOOT Reboot 0 8 sInatsalize checksum S10STA sReceive byte t
15. gt 3 RX Data Signal Gnd 7 e 7 Signal Gnd DSR 6 e11 Buffer Busy Full pg 20 DTR Figure 10 Both ends are DTE Note use of pin 11 unassigned by the standard as Buffer Busy Full signa from printer Figure 11 shows Okidata s suggested connecting cable between an 82A printer and a Radio Shack Model II s built in serial port Notice that the printer s busy signal 11 is connected to the computer s CTS 5 input with the DTR DSR pair crossed over Compare this with figure 9 Radio Shack Model II Okidata 82A Frame Gnd 1 _e i Frame Gnd TX Data 2 o gt e 3 RX Data CTS 5 o 11 Butter Busy Full Signal Gnd 7 o _e 7 Signal Gnd DSR 6 o 20 DIR OTR 20e gt e 6 DSR Figure 11 Two way handshake for one way data Not really the printer s DSR requires external pull up Figure 12 is from an Apple Serial Interface card manual purporting to show how to connect this card to a printer Notice that the printer s control outputs are fed back to its inputs This is because the Apple serial card contains no driver circuitry or receivers either No handshaking is going on here the jumpers are needed to satisfy the printer s control input requirements If the printer s input buffer fills data will be lost so the BAUD rate must be set low enough that the printer can stay ahead of the comp
16. lines are very rare on low cost and some not so low cost equipment When the DTR DSR pair is used the procedure is for a DTE to set DTR high when it is prepared to listen and to check that DSR from the DCE is high before talking When the RTS CTS pair is used the procedure is for a DTE to set RTS high when it wants to send and to make sure the DCE has responded with CTS high before sending The Data Carrier Detect Data Carrier Detected control line often causes complications when a DTE computer is connect ed to a DTE printer since there is no complimentary line as there is with the DTR DSR and RTS CTS pairs Comput er or terminal serial ports often include a DCD input because most MODEMs use a DCD output to signal that they are receiving a carrier essential to ommunication between MODEMs Again when you want to connect two computers both DTE using a MODEM eliminator or null MODEM cable as shown in figure 6 s Several printers use the Secondary Request to Send SCA also called Reverse Channel Request to Send line to signal the host computer that the printer wants to transmit a message usually status or an error message back to the computer Although SCA is officially pin 19 at least one device calls pin 11 SCA pin 11 is officially unassigned NOTATION INTERCHANGE VOLTAGE Binary State 310 15 3to 15 1 0 Signat Condition Mark Space Control Function Off On Table 2 Some Real Exam
17. physical printing speed Since a number of handshaking conventions are in existence the program used to run the spooler contained in a ROM must be able to determine or be preset to use a particular handshake convention In the case of printers using the RS 232 serial interface standard many use the DTR line pin 20 to indicate a printer busy condition The main advantage of hardware spooling is that no changes to the software or operating system are necessary As far as the computer is concerned it is simply sending data to a very fast printer with a very large storage buffer All that needs to be done is to reconfigure the serial port hardware for a faster baud rate than if it were communicating with the printer directly Another advantage is that the method is not limited to any particular hardware or operating system Any computer that has for example an RS 232 interface can output to a hardware spooler instead of a printer The same would hold true if a Centronics interface were being used Of course there are some minor disadvantages to this spooling method The only serious one is that there may be printing jobs that exceed the spooler memory size If this happens then there is no appreciable speedup in printing since the computer must wait for the spooler to send some of the text to the printer before filling up its the spooler s memory again This may occur a number of times before the last portion of the data to be printed is fin
18. the control lines are not used things go all right until the receiving device s input buffer overflows Figure 5 shows a very common three wire interface between a computer and a printer Note that the leads between pins 2 and 3 cross over since both the computer and printer are set up internally as DTE THis works if the printer can print faster than the computer sends data For example a Tally 1805 printer printing at 200 characters per second can stay ahead of a computer sending at 1200 BAUD about 120 characters per second But if an 80 character per second printer or a higher BAUD rate is used large chunks of text will not be printed when the printer s input buffer overflows data is simply lost Some printers will sound a warning buzzer turn on an indicator light and stop printing when an input buffer overflow occurs The Computer Hacker 7 Computer DTE Printer DTE Fe 7 Sig Gnd Sig Gna TXD gnidd 3 RXG RXD 3 2 TXO Figure 5 Simple 3 wire printer cable where the computer is DTE Notice in Table 1 there are two circuits for the DTE printer to send control signals to the DCE RTS and DTR The Request to Send line when high tells the DCE that the DTE wants to send data the DCE usually responds with a Pin RS 232 C CCITT Mneumonic AA 101 GND BA 103 TxD BB 104 RxD Received Data CA 105 RTS Request to Send CB 106 CTS Clear to Send CC 107 DSR Data Set Ready AB 102 GN
19. the resistor equai to times A l V Ryay Rygg Ri Rp VR IR VRz IR 2000 008 Hag 1000 2000 3000 ohms we V Rigyy 9 3000 003A ER 003x 1000 3V ER 003 x 2000 6V Figure 7 Voitage drops across resistors in series current value three milliamperes but two voltages If we take the most negative point in the circuit as a reference from which to make all voltage measurements usually called ground then point A 9 volts and point B 6 volts Do you see the reason for the expression There is a three volt drop across R1 and a six volt drop across R2 A common convention is to use the most negative point in a complex circuit as a reference then the most positive point has the highest potential or voltage The voltage drops across series resistances until we reach zero or ground While we re on the subject of conventions there is sometimes confusion about the direction of current flow through a circuit There are both negative and positive charges and carriers of these charges The two types of charges move in opposite directions when forced through a conductor by a voltage In figure 8 electrons move from the battery s negative terminal through the resistor and into the positive terminal This is called electron flow We will indicate the direction of current flow as conventional current in which current flows from more positive to less positive potential It doesn t rea
20. D Signal Ground CF 109 DCD Revd Line Signal Det 120 SRTS Secondary RTS 108 2 OTR Data Terminal Ready 125 RI Ring Indicator Description Protective Ground Transmitted Data 1 2 3 4 5 6 7 8 N N N O wo Clear to Send When the DTE is ready to receive data printer s buffer is empty paper and ribbon are 0 K it raises the Data Terminal Ready line high The simples and most common way to add a hardware handshake line to the three wire interface sh wn in figure 5 is to add a DTR DSR line as in figure 6 Notice also that the DCE uses five control lines to signa conditions to the DTE primary channel only These are Clea to Send Data Set Ready Data Carrier Detect Signal Qualit Detector and Ring Indicator The last three only make sens in the context of a modem but you ll find that many RS 232 DTE ports will not send unless CTS DSR and DCD are a high which is actually what the standard says Given these sets of control lines we find some common config urations which are different designer s attempts to find a reasonably cost effective simplification of the formal standard We find that the Data Terminal Ready Data Set Ready and the Request To Send Clear To Send pairs are the most often implemented lines with Data Carrier Detect appearing less Computer DTE Printer OTE SIG GND TxD RxD DSR Figure 6 Handshake line added to 3 wire cable amp The Comepuets r Hiet frequently The remaining
21. D11 F processor with KEV11 harware math chip DLV11 serial card DRV11 parallel card Total of 48K RAM Paper Tape 0 5 995 Write Lance Rose c o The Computer Hacker Box 1697 Kalispell MT 59903 1697 Authors Wanted We are interested in publishing specialized well written booklets for the serious computer user There is often need for information which is too long or too specialized for a magazine and too short for a major book In order to publish this information in a magazine it is shortened and re written for a broad general audience Or it is puffed up to fill a book Neither of these approaches filis the need of the hacker We will publish booklets of approx 10 000 to 60 000 words in addition to our magazine if you have manuscript which is too long for a magazine and too short for a major book contact The Computer Hacker Please query by letter with an outline and a self addressed stamped envelope before sending your manuscript Advertise Where The Action Is The Computer Hacker is THE place to advertise products for those who build interface and control with microprocessors Send for our advertising media kit P O Box 1697 Kalispell Mt 59903 1697 Advertising Department The Computer Hacker E
22. DS 440 445 460 and 560 line of printers These printers do not provide software handshaking Their serial interfaces are receive only They are also unusual in Apple I IDS 440 445 460 560 Chassis Gnd 1 Not Used Trans Data 2 np 3 Rcvd Data DSR o nama DTR DCO 8 Signal Gnd 7 e 7 Signal Gnd Figure 9 Cabie between the Appie 1i s built in serial interface and IDS printers Data goes one way only so the handshake is one way only The IDS DB 25P includes a paraliel interface that the circuit board mounted connector is a male DB 25 and contains a parallel interface as well as the RS 232 C pins These printers signal an input buffer full condition with the Data Terminal Ready line this is the most commonly used line for this purpose Figure 10 shows an Apple III to Okidata 82A printer connection The Okidata printer manual calls pin 11 SCA or BUFFER BUSY FULL This pin is offically unassigned in the RS 232 C standard which calls pin 19 SCA or Secondary Request to Send The excellent Mannesmann Tally 1805 printer also provides a READY BUSY signal on pin 11 but covers the spec by providing the same signal on pin 19 Notice that the Okidata printer is using its own DTR output to drive its DSR input This satisfies the printer s requirement for an external pull up on its DSR line Apple Il Okidata 82A Frame Gnd 1 e eo 1 frame Gnd TX Data 2 o
23. Detector See figure 3 There is no handshaking involved here except that if either DSR or DCD is false the DTE knows the DCE will not transmit data Interface types A E define primary channel only interfaces the rest include a secondary channel OTE terminal DCE modem SIGGND 7 7 SIGGND RxD 3 3 RxD DSR 6 lt e 5 DSR DCD 8 DCD Figure 3 Standard interface type C Simplex receive anly The simplest full duplex two way simultaneously configuration is interface type E figure 4 This requires six wires Signal Ground Transmitted Data Received Data Clear to Send Data Set Ready and Received Line Signal Detector DCE DTE SIGGND 7 7 SIG GND TxD 2 2 TxD RxD 3 e 3 RxD CTS 5 5 CTS DSR 6 6 DSR DCO 3 _ lt _ _ 8 DCD Figure 4 Standard interface type E Full duplex The remaining standard configurations are much more complex than needed for most simple tasks In the real world of low cost microcomputer equipment we seldom see any of these standard types The Real Micro World The remaining discussion concerns communication between a computer and printer or between two computers etc not between a computer and a modem The purpose of the control lines is to ensure that nobody tries to send data unless someone is ready to receive If
24. Duplex Two way transmission but only one way ata time Full Duplex Two way simultaneous transmission Often mistaken for Echo or Echoplex An echo is when the characters you type on your keyboard are not immediately displayed on your screen instead they are sent to the remote computer which echoes or returns them to your terminal which then displays them This is a simple but very good means of error checking for character at a time transmissions If your terminal sends your typed characters directly to your screen and the remote system is echoing you will see a double of every character you type If your terminal software requires a remote echo but the remote computer is not set up to provide an echo you will see nothing on your screen when you type in this case your modem may provide a local echo if you switch it to Half Duplex Switched Service Additional control circuits are required if the link includes switched communication circuits Generally this means that if you have a dedicated non Part Two switched set of wires connecting the two Data Sets you do not need complete handshaking between the DTE and DCE However if you are connected to the PSTN Public Switched Telephone Network or to some other arrangement where the data sets might not always be connected you are required to include the additional control interchange circuits The simplest standard type requires four wires figure 1 Signal Ground Tran
25. M which will perform this process albeit in a somewhat simplified manner called DESPOOL available from Digital Research Its use in microcomputers has been limited by the lack of true interrupt driven operating systems as well as lack of disk space in many systems thus denying the user place to temporarily store large files to print The hardware spooling method is something that has become popular only in the last year or so in the microcomputer area In this method the output is sent to a separate hardware print spooler which is most often simply a box containing a chunk of memory and a microprocessor The communication to this device is performed at very high serial data transfer speeds 9600 or 19200 baud The spooler performs two simultaneous functions First whenever a character is received from the computer it is input and stored temporarily in the spooler s RAM for later printing Second whenever the printer is ready and there is something in the RAM that should be printed the spooler outputs this to the printer In addition due to the fact that the memory of the spooler may be exceeded by the size of the printing job the spooler must handshake with the computer and let it know when to stop sending characters Similarly the spooler must be able to handshake in the other direction with the printer to keep from overflowing the printer s buffer in the case where the data transmission rate to the printer exceeds the actual
26. ONTROL CHARACTER WO iCLEAR CONTROL FLAG FREAD KEYBOARD AGAIN ICLEAR STROBE tESCAPE IPREVENTS INTERRUPTS WHILE IN BASIC SCONTROL A B 0K C ISEND THE MESSAGE FPREVENT INTERRUPTS WHILE LOADING MESSAGES ALLOWS INTERRUPTS AGAIN OEC DOT FLAG NO DOT ISEND A DOT AND SFACE ISHIFT THE CHARACTER REGISTER SCLEAR THE DOT FLAG SCLEAR THE COUNTER IET SPACE FLAG SFORCE A JUMP BACK TO BRE CHECK THE DASH FLAG I SEND DASH SCLEAR DASH FLAG JUME UP THEN BACA TO BRE iCHECK ON SPACES INOT TIME FOR CHARACTER HAS ONE DOT TIME FASSED INO I VES OUTPUT CHARACTER OF SFACE GET CHARACTER SLODK UP ASCII REPRESENTATION ISET BIT SEVEN OUTPUT IT RESET CHARACTER REGISTER tCHECA SPACE FLAG AGAIN sCLEAR CNTRI 1 OUTPUT A SPACE SCLEAR SPACE FLAG PEPE EG EL EET EEE 827333 m ICLOSE THE LOOP JANPUDUN End assemtly 230 bytes Errore 9 continued I8 he Conpater Hacker CEILS Seer e eR Tee BRET ROR SBR EEE N fk FATATATESAARASREGTTIIRS g o CSOTRSRERSSEIII RSIVISE End ass sbly 186 bytes Errore LDNPUDUN Listing 3 The SUBROUTINES I START COUNTING DOWN SIN INTERRUPT ROUTINE TO READ RING BUFFER IGET A CHARACTER HUPDATE POINTER TO RING BUFFER JASCIT TO x REGISTER ITO LOOK UP MORSE CODE I ZERO IS A WORD SPACE I STORE CHARACTER SHIFT IT INTO CARRY ZERO MEANS CHARACTER IS SENT 1CARRY SET IMPLIES DASH tS OTHERWISE SEND A DOT
27. THE COMPUTER JOURNAL For Those Who Interface Build and Apply Micros 2 50 Issue Number 2 File Transfer Programs for CP M Part Two of a Series The RS 232 C Serial Interface Part One Build a Hardware Print Spooler A Review of Floppy Disk Formats Sending Morse Code With an Apple J page 16 Beginner s Column Part Two Anyone For a Little KISS Electronics WHAT IS A HACKER The September fifth issue of Newsweek contains a six page article Beware Hackers at Play with a cover picture and the headline Computer Capers Several months ago Wall Street Journal also ran a front page article about how Hackers break into computer systems I consider myself a Hacker but I have no interest in breaking into computers it takes more time than I have available to try to figure out what s going on in my own computer I object to the fact that the press has defined hacking as breaking into computer systems The press is giving all hackers a bad name because of the trespasses of a very few who call themselves Hackers When I mention computer hacking people ask How many computers have you broken into today I tell them that this is not hacking but when they ask what hacking is I have hard time trying to explain it And so I put the question to our readers What is Hacking What is it that Hackers do We at The Computer Hacker would like to generate some good press
28. a simple keyer interface if you wish to use the program in the keyer mode The circuit to key the transmitter is shown in figure 1 The optional keyer circuitry is shown in Figure 2 As far as a source of interrupts is concerned we used a John Beil Engineering 6522 board in slot seven The interrupts occur at a 1500 Hz rate For those who are inclined to build circuits a less expensive source of interrupts is a 555 timer multivibrating at 1500 Hz and connected to a 74LS121 one shot wired to produce a 10 microsecond logic zero pulse on the IRQ line The IRQ line can be accessed on a peripheral card connector The program initializes the John Bell 6522 card for proper operation but the program is transparent to the source of interrupts It is important that they occur at a 1500 Hz rate The program assumes the John Bell card is in slot seven the 6522 labelled U1 is used and a jumper is added to the card to connect the IRQ on the 6522 to the IRQ line in the Apple Holes on the card are provided for this jumper Figure 1 A circuit to key a transmitter from the game i o connector Program Operation Load all the programs in the listings Type in RUN and press the RETURN key The computer responds by requesting the code speed Enter this and then press return The screen will go blank and you can start typing Type some letters numbers and punctuation marks You should hear Morse code coming from the speaker If there is a problem chec
29. ady lt Signal Gnd lt Data Carrier Detect lt Revd line signa detector Data Termina Ready lt 20 Output Software Handshake Input Data to Print Output Input Must be High Input Must be High input Must be High Output High Ready Low Busy Figure 14 Qume Sprint 5 inputs 5 6 and 8 are tgnored when the Sprint 5 Is switched to No Modem Recommendations for Hacker Projects We will find three and possibly four serial interface Situations in our construction projects simplex send only simplex receive only full duplex and possibly half duplex Recall that half duplex means two way communication but only one direction at a time Full duplex means two ways at the same time Most serial interfaces today use an integrated circuit called a UART or USART This device provides most of the circuit functions needed to convert a microprocessor s parallel data to serial to buffer the received and transmitted data and to control at least a few of the interface iines USARTs These devices will be covered in detail in a future article For now be confident that one of these devices a few support chips and a little software makes it easy for us to implement any of the three or four configurations needed The limitation is that these chips usually provide at most three or four of the RS 232 C Control Interchange functions so we need to decide which ones to use and keep these to minimu
30. ally sent to the spooler During the time that the spooler is emptying its memory to the printer the computer is still waiting to send more output to the spooler and is thus prevented from beginning another task Of course since it is simple to provide a hardware spooler with up to 64K of RAM this should not be too much of a limiting factor except in the case of enormous printouts In fact in most applications much less than 64K of RAM can be used with a savings in expense Most commercial spoolers on the market today start at 16K versions and go up to 64K in 16K steps With a suitable design a spooler can be built with as little as 1K or 2K RAM at a much lower cost So in fact the main disadvantage of a hardware spooler namely the limited RAM can actually be an advantage if most printing jobs are relatively small allowing the construction of a smaller less expensive piece of hardware If it were absolutely necessary to design a hardware spooler with a capability of more than 64K it would be possible to base it on one of the new 16 bit microprocessors that can address at least a megabyte of RAM The cost of the microprocessor would not be too much more than that of say a Z80 but the additional cost of RAM would be substantial With all this in mind I will present a design for a hardware print spooler that should be adequate to handle most printing jobs and allow simultaneous printing and computer use by the The Computer Hacker 13 ope
31. ammeter and some small tools and parts I m using a Radio Shack 22 204 multitester and a Radio Shack 22 191 digital multimeter These are not the best but are widely available very low cost and have worked well for me for several years Electronies at our level is all about what happens when we push electrons through circuits We will talk about simplifications of the real world make calculations based on idealized components then construct real circuits and make measurements to test our simplified models What we care about is being able to put together a project that does something useful or interesting You should understand from the beginning that real components won t always match our simplified models that real measuring devices have built in sources of errors and that most of the time a measured value that comes close to our calculated value is a success Don t expect a 4700 Ohm resistor to measure exactly 4700 Ohms and don t waste time trying to get 5 000 volts when we need 4 8 to 5 2 volts Getting Started Ohm s Law We can easily measure current voltage and resistance These are most beautifully related by Ohm s Law figure 1 This formula says that if we connect a one ohm resistor across an ideal one volt battery one ampere of current will flow through the resistor figure 2 The battery supplies electrons each carrying one negative electrical charge A battery is a chemical device which produces a potential differen
32. are possible Although in practice the situation isn t this bad there are at present maybe a dozen different diskette formats in popular use So the next time you wonder why your Apple diskette won t work in a Radio Shack or S 100 machine just realize that it only has one chance in 360 of doing so something like 1 sof one percent E KES ESSENS SEG SE SE SER TISSE SETE EEE EEEE ESS ESN ER Build a Print Spooler continued from p 13 Centronics input and an RS 232 output Other conversions would be possible too 4 Serial Communications IC Having chosen the Z80 for a microprocessor there are several choices for a serial I O chip One is the Z80 SIO However it is an expensive chip and is so flexible as to be confusing to the average user The Intel 8251 is cheaper and not as complicated but still requires some understanding The variety of UARTs available are the least expensive require no software initialization and are adequate for the task here They are readily available from a number of sources for 4 and up Needless to say the UART was chosen here 5 Support Circuitry The choices here may not be so clear cut Since live in an area where it s not possible to walk or drive down to the corner chip shop for something I may need I tend to design most circuits around common easily available chips Most chips used in this circuit are available if absolutely necessary at Radio Shack In addition I believe simplicity to be
33. asynchronous techniques and handshaking protocols Part three of this series will describe methods for actually transmitting information over the TxD and RxD interchange circuits and part four will present some of the integrated circuit chips needed to build a working serial interface 12 The Computer Hacker BUILD A HARDWARE PRINT SPOOLER Part One Background and Design by Lance Rose Technical Editor M ost users of microcomputer systems would probably agree that printing hard copy is the slowest process occurring in their systems Due to its highly mechanical nature the printer simply can t keep up with the flow of data coming from electronic circuitry where processes occur in milli or microseconds If you re like me you ve probably spent hours just watching your printer chug through a long program listing or print an endless series of statements or reports With few exceptions there simply isn t anything to do except watch the printer during these long outputs Since this isn t by any means a new or unique problem with computer systems ways have been devised to keep the printer busy but still allow the user operater to continue to do something useful with the computer while the printing process is going on This is done by a method known as spooling There are two general types of spooling used which I will call software spooling and hardware spooling In the software version instead of the computer sending each
34. being using a modern communications receiver The latter problem is not associated with the computer or the program but rather with the analog circuitry that converts the tones into logic levels In this article we will confine ourselves to the problem of sending Morse code a task for which a machine can easily outperform a human being Program Features 1 Morse code can be sent from the Apple keyboard at rates selected from the keyboard from 8 to 100 wpm words per minute 2 A ring buffer allows the typist to type up to 225 characters ahead of the one being sent 3 Three messages totalling 256 characters may be stored and sent with commands from the keyboard Characters from the keyboard may be inserted in these messages as they are being sent a desirable feature for contest operation 4 The computer can also be operated as an electronic keyer that operates at the speed eatered on the keyboard 5 In its keyer mode the program reads what is sent and prints it on the video monitor You can use this feature to monitor what you are sending provided you send it correctly 6 The Apple J speaker provides a sidetone making the program useful for code practice 7 The game i o connector is used to interface the computer tothe transmitter with simple components Hardware The hardware required to use the program consists of a simple interface between the game i o connector and the transmitter a 1500 Hz source of interrupts and
35. cal Editor Introduction The first part of this article discussed the electrical mechanical and functional specifications of the EIA RS 232 C serial interface standard Part two will briefly discuss the standard configurations defined in RS 232 C then describe some real world configurations and present several suggested hacker standards Standard Configurations The RS 232 C standard defines 13 Standard Interfaces called Interface Type A through M with a fourteenth catagory called Interface Type Z for specials defined by the manufacturer I think every RS 232 C compatible interface I ve ever seen in microcomputer equipment has been Type Z including those found on typical low cost modems The standard interface types are defined in terms of which interchange circuits are implemented All standard configurations include a number of circuits required for that type plus possibly some circuits required for switched service some required for synchronous service and some optional circuits Keep in mind that the standard defines a serial interface between a computer or terminal Data Terminal Equipment DTE and a MODEM or Data Set Data Communications Equipment DCE RS 232 C was not intended to define an interface between a computer and printer or directly between two computers Some often misused terms apply to the interface type descriptions Simplex One way only transmission Not reversible Half
36. ccessful first time Print again if record repeated Print CRLF 1G0 wait for next prompt to send 1Put in message Transmit character through serial port File name missing 9DH BAR File not found Transfer comple DH OAH Transfer terminated Record i s transmitted egains ODH GAH DATBLF Record tranamitted rRepeated record 1CRLF sequence Pointer to nent data byte to send 1Flag tor EOF reed tFlag for repeated record 1Data buffer Listing 2 Program to receive a CP M file Program to receive a CP M file through a serial port Accesses port directly bypessing the BDOS Assumes an B bit word length and no parity Version of 9 17 83 rou grou zou EOU ECU toU tou tou eocen 0005k OSCH om 6H am OT 126 4 1004 CP M reboot address CP m BDOS entry point sFOB for file to be transmitted 1810 status port 81C data port tinput fleg for rial port Output flag for serial port Buffer sise CP M records mumber of retries before quitting The Computer Hacker 5 If required place serial port initialization code here Print CRLF LEA TFCB Decrament repeat counter cpl Fa Check for filename Sh OPEN rUnsuccesetul read LX1 C FNMER Prant error message and reboot A RETRY Myl c 9 CALL BOOS HR OG MYI E 84H m ECCHT CALL XMTBYT 16end EOT character Ane JMP BOOT Return to CP M K LXI T TFCB MVI c 19 CALL DOs Delete old
37. ce or voltage The Voltage Current times Resistance V lRor E IR l V R Re V I Figure 1 Ohms Law Use the diagram as a memory ad if you don t like algebra potential difference represents an ability to do work The work is performed by moving charges from one side of the battery to the other through a conductor connecting the two terminals If there is no conductor no work is performed but the potential remains When the battery runs out of charged particles there will be no more potential difference and no more work The battery s voltage will be zero and it is said to be discharged We ve all seen this kind of action if only by forgetting to turn off our car s headlights To understand figure one we need to define some terms Charge is one of the basic properties of matter It is a measure of one of the ways in which two pieces of matter exert forces on each other gravity is a similar property A quantity of electric charge is measured in coulombs One coulomb of charge is about 6 24E18 6 24 times 10 to the eighteenth power electrons The charge on a single electron is 1 60E 19 coulomb The number of charges which flow between our battery terminals in one second is the current measured in amperes or milliamperes thousandths of an ampere One ampere of current is one coulomb of charge flowing in one second or about 6 240 000 000 000 000 000 electrons per second The amount of current which flo
38. d S1OCAT Get received character TFH sMask off bit 7 for ASCI codes 4H ENDXMT eik XATREC 2H REATY ignore other characters CATPTE D 0062H E Pornt at last record CATPTR c 60 back one record Check for filename Print error message and reboot Oper file iNO key pressed Prant ECT message and abort sRead serial status port End trenemission Transmit next record A RFTELG Save repeat flea CCUNT iincrease record message if new record C Decrease record count XMTRC More an bufter ECFFLG A RE ACFI LECFMSS More in file Listing 1 continued BOCT BDOS TFCB S1OSTA SIODAT IFLAG OF LAG BUFREC RETRY SSRRSETRRERE sg c 9 BLOG 034 XMIBYT BOOT H DATBUF DATPTR yo vomr gt gS Ea ee Fo B C E AM COUNT nm 8 COURT SIOSTA OFLAG XMTBYT ALE SIODAT Print EOF message i end LOF byte Point at beginning of data butter tinitialise record count to sero 5et DMA address Read next record 1EOF detected 7Poant at next record in buffer Increase record count Stop at end ot extent Bet EOF flag 1 end response to request for record 1Point at next record to tranemit 1Character count for record 1Checksum for record Get next byte to send Update pointer s ave in E tUpdate checksus 1More ir record 1 end checksum Send 4 NULS to replace any missed bytes 1 ave pointer to next record iPrint record message Skip again if su
39. e and what additional information would help you The Computer Hacker 23 Sa GRE PN le ho Size 23 Stepper Motors from Clifton Clifton Precision Litton Systems Inc has expanded its line of size 23 1 8 stepper motors to include models with up to 170 oz in holding torque and 120 oz in dynamic torque The motors operate with 200 steps per revolution The Clifton motors are compact they are available in lengths as short as 1 5 excluding shaft In addition they are quiet operating making them ideal for office environments They are well suited to applications such as carriage wheels for matrix and daisy wheel printers paper feed drives machine tool controls disk drive head positioners tape readers plotters robotic systems wherever precise positioning is required in an open loop system These size 23 stepper motors offer standard accuracy of 5 percent accuracies of up to 3 percent are available High reliability is an advantage of the Clifton motors achieved requirements through close tolerance construction For additional information on the Clifton size 23 stepper Special winding configurations mounting and other motors contact Clifton Precision P O Box 160 Murphy NC modifications can be produced to meet specific customer 28906 704 837 5115 TWX 510 935 1068 a Ma Bell Hits Modems With Tariff The September issue of The Computer Shopper P O Box F Titusville FL 32796 reported that
40. e joule per second one watt if it is in open air at about room temperature Unfortunately some types of resistors suffer permanent changes in resistance if you get them too hot even within their wattage ratings Most resistors run at their rated wattage get hot enough to burn your fingers especially high power ceramic resistors Carbon composition and some carbon film resistors can literally go up in flames if their wattage rating is exceeded Before you install a resistor in a circuit and turn on the power you must calculate the expected power in the resistor with any of the three formulas in figure 9 Then select a 4 watt 4 watt or larger size resistor Most microcomputer circuits use a five volt power supply and very low currents since the wattage needed is the product of voltage and current you can usually use a watt resistor Electron current Conventional current Figure 8 We will use conventional current flow from positive to negative Power in Watts voltage times current P VI or P El Pm VI R Pm Vi Vx W R V A P I R P Vie IR x IPR Figure 9 Formulas for calculating power in a resistor Resistors In Series And Parallel You often won t have exactly the right value of resistor called for by your calculations You can dummy up an equivalent resistance by combining resistors in combination as shown in figure 10 Adding a series resistor increases the total resistance Adding a re
41. e taken and used immediately without the need for preparing or formatting them Hard sectoring also is usually a little simpler than soft sectoring since all the circuitry has to do is detect a pulse rather than decode header information This is really a minor difference though A more important advantage is that without the need for sector headers more space on the track can be allocated to storing actual data Although soft sectoring has the additional overhead of sector headers with the need to format a diskette before using it formatting simply writes the sector headers onto each track it does have some advantages By detecting sectors by reading the header which contains among other things the track number and sector number the computer can verify that it is on the correct track and reading the correct sector This usually isn t done with hard sectoring and provides an additional protection against errors Also since the sector boundaries and thus sizes are recorded in the same way as data on the track the boundaries can be almost anywhere thus allowing a variety of sector sizes and number of sectors per track This flexibility can sometimes be useful Density This has to do with the amount of data that can be packed onto a given space on the diskette Originally all floppies used single density encoding methods also known as FM encoding In this the data pulses and clock pulses are combined and both recorded onto t
42. es of the bytes received one by one as they come in When the final data byte has been received the checksum is transmitted and examined by the receiving machine It then compares the checksum received with the one it has been calculating and if these are the same it is assumed that the block has been transferred correctly While an in depth explanation of error detecting and error correcting codes is not appropriate here suffice it to say that the probability that there will be two errors in transmission which cause equal and opposite results is so minute as to not be worth worrying about In a case where something like national security is involved more elaborate error detecting and correcting codes are available to cover this possibility but for our use they are not needed In addition they would slow down what is already a painfully slow method of moving data bet ween computers Choices of Data Format There are basically two ways that data can be represented during transmission from one computer to another The first of these is simple ASCII coding This works fine for text files but runs into a bit of a snag for machine language or executable programs ASCII is defined as a 7 bit code with a parity bit added as the 8th bit However binary files may have any combination of bit patterns making up the byte and can t afford to waste the 8th bit as a parity bit In the case of some existing file transfer programs a binary file must first
43. h Star Horizon would be of no use on a Morrow Micro Decision or other system with even a slightly different hardware configuration Since it is often necessary and desirable to move a program from one system to another a way must be found to overcome the differences in all these versions of what is the standard operating system One way that allows a speedy transfer is to simply have a version of CP M with a custom BIOS written for multiple disk systems This would be used in a machine with two or more disk controllers operating at the same time Each controller could have a drive connected to it and assigned a logical drive name for example an 8 inch single density floppy might be Drive A a North Star minidisk drive might be Drive B a TRS 80 CP M format disk drive might be Drive C and so on I think you can see right away that this isn t a very practical setup One would have to have all these types of disk controller active in the same machine at the same time a near impossibility considering the various schemes used for addressing disk controllers Also it s questionable whether any machine has enough slots for all the different types of hardware required and the BIOS would be quite long and involved Another way of moving files around which while slower is at least more practical is to simply transmit the file from one computer to another via some sort of interface This method has the advantage of not requiring the same disk s
44. h it was constant That is more voltage caused more current to flow The ratio of voltage to current is a measure of the electrical resistance of the conducting material One ohm is the electrical resistance when a potential difference of one volt causes a current flow of one ampere This is what we now know as Ohm s Law Getting Practical How much resistance does a resistor or other conducter offer Connect the resistor to a battery measure the voltage across the resistor and the current through it see figure 3 then calculate the resistance through it with Ohm s Law R V I Then measure the resistor s resistance with your ohmmeter You will find some error because the milliammeter itself has some resistance so less current flows when the meter is in the circuit in series with the resistor Additional error stems from the meter s limited accuracy and from less than perfect measuring technique Try different resistors but don t try resistor values much lower than 500 ohms they ll get too hot and a 9 volt transistor radio type battery can t supply enough current 009 amperes Ammeter 9 von battery Figure 3 Try out Ohm s Law if you have different batteres or a variabie output power supply try different voltages Keep R greater than 500 onms Ohm s Law says that if we increase the voltage more current will flow If we increase the resistance less current will flow Usually we have a fixed voltage source and we control the
45. he diskette surface When read back the clock and data are separated by appropriate circuity and the latter passed on to the CPU To keep up nwith the demand for larger databases and such ather encoding methods have been developed to pack more information into the same space on the diskette In double density encoding known also by the term MFM the data is written onto the diskette without any clock pulses This in effect allows each pulse on the track to be a data bit rather than alternating data with clock The only problem here is that when the data is read back in the clock pulses must be The Computer Hacker 15 resurrected from the data How this is done is beyond the scope of this article but suffice it to say it can be done but with some difficulty relative to single density encoding This makes the timing requirements and disk rotational stability more critical in double density but with development it has become quite reliable and many disk systems sold today are capable of recording in both single and double density In Summary I think you can see by now that there are a lot of variables involved in diskette formats If we take the three possible sizes three different values of tracks per diskette two possibilities for number of sides two values for type of sectoring perhaps five different values for number of sectors per track and two different densities we have something like 3x3x2x2x 5x2 360 different formats that
46. he items in parentheses the drive and filetype are optional The quotes are not entered If the filename is omitted an error message results and the program reboots After waiting a few seconds for the computer to open the file and load the buffer the receiving operator types RX drive filename filetype His computer then erases any old file by that name opens a new file and signals the transmitting computer to begin At this point the computers may be left alone until the process is complete barring any unrecoverable error conditions In practice even though the receiving computer is calling the shots I have found that it doesn t seem to matter who actually types his carriage return first the sending or the receiving party The handshake link is established satisfactorily either way and the transfer proceeds normally So don t worry too much about counting to five or whatever before hitting return Multiple File Transfers In order to keep the complexity of the programs down it was necessary to limit the transfer to a single file for each execution of the program This is not really a problem in the case of long files since one would want to check on the progress of the transfer periodically and re executing the program for the next long file wouldn t be burden In the case of a large number of small files I have found that the best procedure to accomplish this is the SUBMIT utility of CP M Making up submit file such a
47. ime now and found them to be very satisfactory for our purposes The listings are included here for those who want to key them in If you would rather avoid the effort and errors involved with keyboarding the data send 15 to The Computer Hacker for an 8 inch single density floppy disk containing a CP M copy of the source files Listing 1 Program to transmit a CP M file Program to transmit e CP M file througr a serial port Accesses port directly bypassing the BECS Assumes an bit word length and no parity Version of 9 2 83 BCOT EOU BEDBK BDOS EQU pop Sk TFCB EQu 5CH SIOSTA EQU 87H SIODAT EOU 86k IFLAG EQU 2H OFLAG Fou BIK BUFREC EQU 128 CP M reboct address CP M BECS entry point FCB for file to be transmitted SIC status port ISI data port lrz r flag for serial port Output flag for serial port Bufter size CP M records ORC 188K If required place serial port initialization code here LDA TFCBe1 CPI i JNZ OPEN LXI D FNMER MVI c 9 CALL BDCS MVI E 4H CALL XMTBYT Send ECT character JMP BOOT Return to CP M LXI E TFCB MVI c 1s CALL BECS INR A JINZ FOUNL File present LXI T FNFER Point at error message ABCRT Print and reboot A TFCB 3Z 1Set next record byte to rerc C A Set record count in butfer to zero 5 Save reccro count 11 BDOS 1Get console status B A REALY 1 e C 1 BLOS B Bih REACY1 E EOTMSG ABCRT SIOSTA FLAG REALY Wait for character to be receive
48. iving machine may have a slow disk system and a large file is being transferred that can t be buffered in memory in its entirety The receiving machine must pause to dump its buffer to the disk and during this period the transmitting machine must wait to insure that it will not be transmitting when the receiving machine is busy with its disk work Upon dumping the buffer the receiving machine can signal that it is ready to begin accepting data again and the transmitting machine can start sending at that time An additional feature present is a certain amount of error correcting The term correcting is a bit of a misnomer because it is accomplished here by simply retransmitting a garbled record until it is received correctly There is a two way communication between the machines full duplex so that they can decide when a record has been correctly received The number of retries for a badly transmitted record is 4 here but can be altered to any other value to suit the user s purpose Aborting a transfer is also possible since the program polls the console device using a BDOS call and if the operator types a control C the transfer is terminated and a message so stating is printed on the screen As each record is transmitted and received a message is printed on the screen so that the operator can monitor the process If a record must be transmitted the word again is appended to the message When the entire file has been finally tra
49. k your disassembled version of the program against the listings Also make sure that the MORSE TABLE and the ASCII TABLE are loaded Asssuming that everything is running correctly you can practice sending at the keyboard The reverse arrow key allows you to delete characters entered in the buffer provided they have not yet been sent Try typing ahead then delete some characters with the reverse arrow key To change code speeds simply press the ESC key and the program will return to the BASIC routine to allow you to enter a new speed To load messages press CTRL L Type in message A For example message A might be CQ CQ CQ DE KOEI KOEI K When message A is complete type RETURN Now enter message B followed by RETURN and then enter message C followed by RETURN Now you are back in the code sending mode To send message A B OR C simply type CTRL A CTRL B or CTRL C ANy message may be interrupted from the keyboard but you must be alert It will help to insert an extra space or two in the message where you with to interrupt it To use the program as a keyer you must construct the circuit in Figure 2 and make the connections to the game i o connector Try this and see how you like the keyer operation Note that what you send is what you see The program converts your characters from Morse to alphanumeric characters on the video monitor PB1 pin 3 Figure 2 Circuit diagram of the keyer Pin numbers refer to the Apple J game i o co
50. l order firms These are conveniently jumpered with small two pin female blocks or with wire wrap wire These jumper blocks allow you to configure the hardware as either DTE or DCE The dashed lines indicate the most standard connections Note that you can get away with driving more than one line e g CTS DSR and DCD from one MC1489A even though this might not exactly meet the RS 232 C specs DTE DCE Reve Data Busy Ready Reay Busy Figure 15b Simplex receive oniy with OTR Ready Busy Pin numbe s in 11 are tor DCE Figure 16 is a suggestion for a transmit only interface Eliminate the jumpering if you have a dedicated situation this arrangement makes it possible to accidentally short multiple line driver outputs together e g CTS DSR and DCD outputs from a DCE The DTR and SCA ready busy lines are provided in case this port must be configured as DCE to drive a printer which signals buffer full on pin 20 or 19 Unfortunately a fair number of printers use the SCA Secondary Request to Send as a handshake line The full or half duplex controlled by software general Looks Like DTE DCE Transmitted Data ok 2 1x0 Data Out 1 3 RXD Request to Send gs 4 RIS o K 2 or Sh Om 5 crs o DSR Signal Gnd Ready Busy Figure 16 Simplex send only Preferred connection dashed line It would be a good idea to provide a separate driver and receiver f
51. lly matter which is used as long as we are consistent Power To avoid having resistors go up in flame you need to know how to calculate power dissipation When a source of voltage pushes a current through a resistor work is done energy is used and heat is produced How hot a resistor gets depends on its size and composition but is proportional to the rate at which work is done moving charges through it One joule of work is done moving one coulomb of charge through a potential difference of one volt Power is defined as the rate of doing work in joules per second We more commonly express electrical power in watts One watt of power rate of doing work is defined as one joule of work per second Power in watts is calculated as volts times amperes Power watts joules second volts x coulombs second volts x amps P Vxl Figure 9 shows various ways of calculating power found by using Ohm s Law and substition Use these formulas on the earlier examples to find out the power in the resistors The Computer Hacker 21 Resistors are manufactured in a wide variety of types and sizes Their specifications are in ohms resistance accuracy or tolerance temperature stability ohms per degree Celcius and power dissipation rating The last parameter indicates how fast the resistor can get rid of the heat caused by a current moving through it A one watt resistor for example can safely handle the heat from on
52. m We need signal ground transmit data or receive data or both and some way for a receiving device to indicate a busy eady status For a design where we don t know what will be on the other end of the cable we should provide more of the control lines We can save effort by using a jumper arrangement to configure the port To avoid confusion over lead names Table 1 gives the RS 232 C names the common use mnemonic names the CCITT names and the descriptive names of the interchange circuits we will be using Table 2 should remind you that an ON condition on a control line is a positive voltage A simple receive only design recommendation is shown in figure 15a I m going to assume we are making Data Terminal Equipment if your design is for a DCE connect the pins shown in parentheses Data is received on pin three the RS 232 C line receiver inverting meets the termination requirements The interface signals that it is busy by putting a negative voltage on the DTR line Looks Like DTEIDCE l Reve Data lt p 3 Axo 2 TXD Data in RS 232 C Line Receiver MC 1488 I Busy Ready Do 20 DIR Pr 16 DSR Ped Eir RS 232 C Line Driver MC1489Ai i l K 7 4 Logic Gna Figure 158 Pin numbers n 1 are for DCE Figure 15b is a suggestion for making this receive only design more flexible The jumper blocks can be the dual rows of pins spaced 0 1 inch apart available from Radio Shack and many mai
53. most hacker projects be built as separate stand alone devices using a standard interface instead of being built to operate only with a specific computer The problem with a computer specific peripheral is that the device will probably not work with another computer Computer technology is changing very rapidly and most of us will eventually get a different or an additional computer which will not be able to use the same computer specific peripherals There is also the possibility that you will want to lend or sell your project to someone with a different computer The decision to use either a computer specific or a standard interface design is not always clear cut There are advantages and disadvantages to both approaches and you ll have to decide on a case by case basis When you do design something which is not meant to be computer specific you should use a standard interface if at all possible In future issues we will continue coverage of the RS 232 C interface and will also cover other standard interfaces suitable for the hacker We are interested in feedback from the field so please tell us about your experiences with interfaces both the good and the bad If you would like to write an article perhaps something on A D and D A or the factors to conside when deciding between using a computer specifi versus a standard interface design send us an outline B Editor Publisher 0 0 0 0 cc ccc ec ceaee Art Carlson Art Director
54. nnector My circuit worked very well without the more or less standard pull up resistors 22 kohm The listings have extensive comments enough to make the program understandable During each interrupt the keyboard is tested to see if a character has been entered If a dot or dash is being sent the speaker is togged to produce a 750 Hz tone Two counters are incremented or decremented to keep track of the number of 1500 Hz pulses that have occured The pushbutton inputs PBO and PB1 are tested to see if the keyer is being used Various bits in a register called FLAG are set or cleared depending on which events occur key down PB1 at logic zero speaker to be toggled etc This memory location is then analyzed by the main program so that it can take the appropriate action Once the flow of the action is appreciated the comments labels and subroutine names should make the operation understandable Listing 1 The BASIC Driver Routine IL IST S REM MORSE DRIVER ROUTINE 1 POKE 1 76 POKE 11 09 POKE 12 17 2 PRINT AT WHAT SPEED WOULD YOU LIKE TO SEND JO PRINT TYPE A NUMBER BETWEEN B AND 199 THEN PRESS RETURN 46 INPUT SPEED S DOT 1096 SPEED og POKE 6 DOT 78 yY USR 0 Be GOTO 29 vo END Listing 2 The MAIN PROGRAM The Con pater Herker 17 PREVENT INTERRUFTS S TURN RELAY OFF TENABLE INTERRUPTS EROM T1 CLEAR SCREEN SET UF THE INTERRUPT VECTOR CLEAR VARIOUS REGISTERS ALLON INTERRUPTS SC
55. nsferred the message Transfer Complete is printed and the program reboots Procedure for Transferring a File The procedure to follow in using these programs once they have been entered into the machine is quite simple First of all the user must identify the port numbers and input and output flag bits for the serial port concerned This information is almost always available as part of the user documentation for the system These values must then be inserted into the source listing for the programs and the programs assembled If the connection is a hardwired cable it must connect the serial interface on the first machine to the serial interface on the second In most cases a cable may be needed that reverses The Computer Hacker 3 pins 2 and 3 of the DB25 connector on one end since it will probably be the case that both machines are wired to connect directly to printers and will use pin 2 for received data If the connection is being made via a phone line and modems each machine should have a cable suitable for connecting it to a modem Most computers are wired as DCE Data Communications Equipment i e they emulate a modem and will probably need a crossover cable to connect to an actual modem but this is not universal so consult your user manual on this Since the receiving program is in control of the process the transmitting computer should begin first with the operator typing TX drive filename filetype where t
56. o 1024 bytes per sector this introduces incompatibilities A second aspect of this is in the form the sectoring may take either hard or soft In hard sectoring the beginning of each sector on the diskette is marked by a small hole punched in the diskette near the large center hole As the disk rotates these small holes pass under a light source with a photodetector on the opposite side of the diskette A short electrical pulse is generated by the photodetector as each hole exposes the light source This indicates to the computer that the beginning of a sector is present One additional hole called the index hole is punched midway between two of the sectors A pulse coming halfway between two sectors tells the computer that the next sector pulse will be the first one on the track In soft sectoring quite a different method is used Only a single hole is punched in the diskette the index hole It tells the disk controller that the track begins immediately Here however the beginning of each sector and the boundaries between them are actually written onto the track as information The computer finds a sector by reading the track continuously until it comes to a header a short piece of coded information that indicates the start of the desired sector It can then begin to read the actual data contained in that sector Both types of sectoring have their advantages and disadvantages In hard sectoring the diskettes can usually b
57. or the OTR and DSR lines purpose DTE interface in figure 17 assumes you are using a USART which provides two output and four input control signals These are inexpensive and readily available today For most applications one output and one input control will work which ones you select depends on what parts are Figure 17 Full duplex general purpose DTE can be jumpered to DCE configuration The Computer Hacker 11 available to you The most commonly omitted input is the Ring Indicator since this is only used with a direct connect modem Figure 18 is a suggested layout for use with USARTs having only two control inputs and two control outputs Given the variety of configurations of both DCE and DTE with which your general purpose DTE interface may have to work it may be simpler to forget the jumpering and build custom cables such as those shown earlier in this article Figure 18 Full duplex generat purpose jumperabie as either DTE or DCE for use with USARTS with only two control inputs Parts one and two of this series have examined the RS 232 C standard from the viewpoint of the microcomputer user The standard was written long before the invention o the micro so it takes a bit of shoe horning to make it fit our needs It is often mistakenly thought that this standard describes methods of encoding data to be transmitted Not so several other standards cover the ASCII code start bits stop bits parity synchronous and
58. ples Let s start with an example of what passes for a complete set of leads in the micro world Figure 7 shows the signal leads implemented in the IBM Asynchronous Communications Adapter for the IBM PC and XT models This is about as complete as you are likely to find in personal computers Machines designed for use in master slave multi station systems will have more control leads but that is a different world This is an RS 232 C like interface permanently configured as Data Terminal Equipment Connecting this to a Hayes SmartModem 300 is simplicity itself a straight thru cable works fine but watch out IBM put a current loop interface on pins 9 11 18 and 25 of its DB 25P connector You need to be sure the device you re connecting to doesn t use these pins if you use a 25 wire cable I just used a 9 wire cable Note that IBM is one of the few to actually use a male connector on the DTE as specified by RS 232 C Now if only they hadn t stuck on that current loop and if only they had used a Centronics style connector on their parallel printer port instead of a DB 25S It is not possible to know exactly how the control lines on this port behave just by examining the documentation accompanying the adapter card since this card is highly software controlled To know which lines are used for handshaking which lines generate interrupts and which lines are ignored you need to examine both the hardware and the software
59. rator Let me address each major point of the design separately 1 Microprocessor Although the program executed by whatever microprocessor is chosen will be relatively simple in order to allow for upgrades the microprocessor should have a capable architecture It should also be a low cost device and be in wide use The Z80 fulfills these criteria and is widely available in different versions for as little as 5 2 Memory Here we have the choice of static vs dynamic RAM Each has its advantages Dynamic memories are less expensive for the same storage capability and take up less board space for a full 64K They are however more sensitive to noise on the power supply lines require in most cases 3 supply voltages and are somewhat less reliable than their static counterparts Static memories are easier to design with more immune to noise and operate from a single supply One other factor is that most inexpensive dynamic RAMs are available in a 16K or 64K x 1 bit architecture whereas static RAMs are available in 1 bit 4 bit and 8 bit widths The choice I have made here is the 6116 2K x 8 bit CMOS static RAM chip Its architecture allows any size spooler to be built from 2K up to 62K I m allowing 2K for the program ROM It has a low power consumption is quite reliable and is easy to design with Cost is somewhat more than dynamic RAM for a full 64K version but due to the fact that the dynamic RAMs need all the timing and control circ
60. rom serial port DATPTR Prepare to store data IFLAG RCVRC1 RCVBYT Get a byte RCVBYT M A Store in data buffer SIODAT H D DA Update checksum Erosta 1Trenemit byte to serial port B TR ROVAC1 Continue for 128 bytes AE et pa 1Get checksum 10DAT PSW RCVRC2 DATPTR Save new record pointer D RECMSG c 9 BDOS sPrint received record message RPTCTR RETRY RCVRC3 Pirst try D AGAIN C9 BDOS Print agein D CRLF RPTCTR Reset counter for next record m a COUNT Put in message BUFCHA DATPTR D DATBUP BUFREC 1 28 D Compare to see it empty tReset data pointer File name missing 0H AH S Disk or directory full DH AH Transter complete DH Transter terminated 0H GAH 7H Record i rRecord received receiveds ageing rPepeeted record OOH BAH sCRLF sequence DATBUF sPointer to nert storage location in buffer RETRY Counter for repeated record SSRTESRRSSSS 528 Date buffer g 0 A Challenge to FORTH Advocates Our readers are involved with interfacing and control and are interested in hearing more about FORTH Here is your chance to convince them of the advantages of FORTH Submit your outline or articles SASE appreciated for prompt consideration Author s guide available Write to The Computer Hacker P O Box 1697 Kalispell MT 59903 1697 6 The Computer Hacker THE RS 232 C SERIAL INTERFACE by Phil Wells Techni
61. s TX PROG1 TY1 TX PROG2 TY2 TX PROG3 TY3 TX PROG4 TY4 and calling it SEND SUB allows the whole thing to operate by typing in SUBMIT SEND The SUBMIT program then executes each line in turn until all files have been transferred The receiving end computer must of course have a similar file but with the letters RX in place of TX on each line Our procedure here is to first send the submit file with a manual command then have the receiving end operator edit it and replace the TX s with RX s This helps insure that the order of the programs being transferred will be the same on both ends The receiving computer operator can then type SUBMIT RECEIVE assuming he has named the file RECEIVE SUB We have used this procedure to transfer series of files that take an hour or more via modems and 4 The Computer Hacker except for checking the machine every ten minutes or so both operators can work on something else during the transfer process Summing Up These programs offer the hacker a good way to move files between CP M machines even with toatlly different disk systems The only requirement is that each have a serial port Transfer can be made with either a direct interface cable or a pair of modems Why is it I always want to make the plural of modems moda The programs are very tolerant of timing and contain the facility for retransmitting records containing transmission errors We have been using them in this area for quite some t
62. s from the diskette Yet another variation are floppy drives whose tracks are packed twice as closely on the diskette allowing 80 tracks on a minifloppy Number of Sides Early floppies used only one side of the diskette to record data on leaving the second side blank Many hobbyists saw this as a waste and took to punching another hole and write protect notch in the diskette jacket to be able to use the second side of the diskette as well most diskettes have a magnetic coating on the second side as well as the first To get at the second side however you have to remove the diskette from the drive turn it over and re insert it into the slot More recent drives have a second head to read the second side making it unnecessary to modify the diskette itself or turn it over to get at the data on the second side Something to watch out for here a double sided diskette made on a true double sided drive won t work on a single sided drive by just turning the diskette over and trying to read the second side The sense of rotation is opposite in each ease Think about it for a while Sectoring This takes a little explanation Within each track the data is subdivided into sectors a sector being simply a fraction of the total track One obvious variable is simply the number of sectors a track is divided into Various disk formats have anywhere from 8 to 32 sectors per track Since the sectors may be different sizes anywhere from 128 t
63. sistor in parallel decreases the total resisance Notice that putting two equal resistors in parallel gives you an equivalent resistor of 1 of each resistor Putting three in parallel divides by three 22 The Computer Hacker 100 ohms Resistors in Series ADO R Ryg R Ry Ry 300 onms 100 ohms Figura 10a Seres resistances sum to an equivalent resistance K 4 Ry a 1 Ry 17874 1 R3 100 three 100 ohm resistors Resistors in Parallel DIVIDE Figure 10b Paraile resistances divide equivalent parallel resistance is the reciprocal of the sum of the reciprocals Remember that the power dissipated in each resistor is the product of current through it times the voltage across it When you make up an equivalent resistor the power is spread among the individual resistors You can make a high wattage equivalent resistor out of a number of lower wattage ones by putting many higher valued resistors in parallel or lower valued ones in series For example 10 resistors of 100 ohms watt each in parallel is the equivalent of one resistor of 10 ohms with a power rating of five watts A series string of ten 10 ohm 2 watt resistors can handle 5 watts of power but will have a resistance of 100 ohms Resistor Color Code Carbon composition and carbon film resistors are marked with color bands as shown in figure 10 The four color bands and sig figure Multiplier tst sig figure tolerance Resistor Color Code Color
64. smitted Data Clear To Send and Data Set Ready Data Terminal Ready and Ring Indicator are required for switched service This configuration Type A is transmit only Simplex meaning one way only type interface DTE terminal DCE modem SIGGND 7 7 TxD 2 2 TxD CTS 5 5 CTS DSR 6 6 Figure 1 Standard interface type A Simplex transmit only The handshaking here is strictly one way Before transmitting the DTE must check for an on high level on the CTS and DSR lines DSR on means the DCE is connected to a communication channel is not in test talk or dial modes and has completed any answer tone and timing functions CTS on means the data set DCE is ready to transmit data to the communication channel Interface type B figure 2 is the same as type A with the addition of the Request to Send line by which the DTE can tell the DCE that it wants to transmit This allows the DCE to disconnect from the channel between transmissions Ring Indicator is required for switched service DTE DCE pE SIGGNO 7 7 SIGOND TxD 2 m p 2 TxD RTS ee RTS CTS Se 5 CTS DSR 6 lt 6 DSR Figure 2 interface type B includes RTS handshake line The other four wire interface is Type C a Simplex receive only interface using Signal Ground Received Data Data Set Ready and Received Line Signal Detector Data Carrier
65. uitry even for a small amount of actual memory a spooler with a small or moderate amount of memory should cost the same or less to build with static RAM than with dynamic I have estimated the crossover point at about 32K both in cost and in board layout space so that is the size I have chosen to present in this series of articles Although the EPROM type is not too important the 271 has virtually the same pinout as the 6116 RAM chip so th chip select logic is simplified if it is used 3 Interface There are a number of interface standards in use today RS 232 Centronics IEEE 488 to name just a few I chose the RS 232 interface to use in this design simply because most of the letter quality printers I work with use it and I m more familiar with it than any other It may be the most widely used standard but I m not aware of any statistics to that effect I m assuming a DTR handshaking protocol here that is pin 20 is used to signal a printer busy condition by going to a logic low state approximately 12 volts This will be used both by the printer to tell the spooler to stop sending and by the spooler to tell the computer likewise In the last part of this series of articles I will show how to modify the spooler to use a Centronics interface or software handshaking ETX ACK or X on X off That will also allow interface conversion to occur during the spooling process For example a computer with only a parallel Centronics interface could
66. uter Apple has replaced this older card with the Super Serial Card probably the best example of a universal in terms of flexibility serial interface card I ve ever seen Far superior to the Apple III s built in serial port The Computer Hacker 9 Apple Il Serial Card Printer Frame Gnd Revd Data Trans Data CTS RTS DSR Signal Gnd DCD L 29 DTR Frame Gnd Trans Data Revd Data oo N mw eB YO Ww 6 Signal al 7 Figure 12 Real world nearly useless interface Notice the control lines on the Apple Serial Interface Card are simply jumped together There ts no handshaking capability If the printer s input butter fills Gata will be lost The BAUD rate must be set so slow that the printer can always stay ahead Figure 13 is from the NEC 7700 series Spinwriter printers It illustrates the implementation of a complete DTE RS 232 C interface designed to connect to a modem with additional printer control functions on the two test lines and three unassigned lines The five additional functions are provided for use in direct connect no modem situations to provide more complete control of the printer A standard computer serial port would not be able to take advantage of these lines but they would be very useful in an OEM custom design Transmitted Data gt 2 TXD Received Data lt e 3 RXD BB Request to Send e 4 RTS CA Clear To Send e 5 CTS cB Data Set Ready _
67. ws through our resistor depends on the electrical force supplied by the battery There must be an imbalance of charge or a potentia difference between two points to sustain a current betweer them The potential of the battery is called it electromotive force or emf Electrical potential is definec in terms of work Two points are at a potential difference o one volt if one joule of work is required to move one coulomb of charge between them A joule is the amount of work performed when a force of one newton moves a point one meter one joule one newton meter of work or energy The resistor in our circuit is not a perfect conductor it current Constant R 1 ohm 1 volt battery Figure 2 Onm s Law circut Isn t t convenent that ohms amps and volts all equal one Resistance s defined as the ratio of vottage to current 20 The Cumpuier Hacker resists the flow of electrons to some degree The amount of opposition to electron flow is the measure of the resistor s resistance measured in ohms You can also look at a resistor as a conductor its conductance is the reciprocal of its resistance Conductance is measured in Mhos yes that s Ohms spelled backwords What George Simon Ohm 1787 1854 discovered was that if he connected the terminals of a battery actually a chemical wet cell together using various kinds of conductors the ratio of the voltage across the conductor to current throug
68. ystem in fact it doesn t require the same anything except the interface convention i e RS 232 Centronics Etc and that both machines have CP M running on them The connection between the two systems may be a simple interface cable or it may have a pair of modems and a telephone line between them thus allowing remote transfer of files The modem method is of course much slower since it is no problem at all to send files from one machine to another on an RS 232 cable at 9600 baud some 32 times faster than most modems are capable of Still it is sometimes impossible to place the computers physically side by side and modems may have to be used Transfer Conventions With transfers between machines using a hardwired cable it isn t usually necessary to add a checksum to insure data integrity it doesn t hurt however since interface reliability in the absence of a phone line should be quite good However the case of transfer via modems is much different Telephone line quality can vary from good to atrocious more often the latter than the former and some means must be adopted to make sure that what arrives at the receiving end is the same information that started out at the sending end This is where the checksum comes in For each block of data sent the sum of the bytes transmitted is calculated and at the end of transmission of the block this checksum is also transmitted At the receiving end the computer is adding up the valu

tcj_02 1983-10 01.02

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