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THE SAME ONE. Figure 9 gives a list of' ASCII code 0-31 and their meanings. Note the two marked XON and XOEF. These you may recognise. XON/XOFF protocol is useful where large amounts of continuous data are being transferred. The receiving machine can send an XOFF code when it wants the transmitting machine to hold off sending data, and send an XON when it wants to resume. This type of control is often used with dumb terminals to mainframes. Another method of transferring data is to send it in blocks of 2-255 characters (usually 128 chars). This removes the need for XON/X0FE control because as each block is sent. the two machines exchange control codes. This method of transfer also allows error checking to be carried out on the data. One of the most popular error check methods is the Cyclic Redundancy Check (CRC). We won't go into this here as there is no real need for such a complex check with hardwired RS232. CRC is really useful for Telecommunications via modems. Phone lines are noisy and data errors may easily result at high baud rates. The CRC error check allows the two machines at either end to check the data for errors, if an error is found, the receiving machine requests that the data is transmitted again. When sending data using the block method, the software collects data bytes into - Assuming packet the link is open, the transmitting s" machine (TM) sends a start-oftransmission o f code. The receiving machine (RM) replies with an acknowledge 1 2 code. The TM then sends a 8 start-of-block code followed by a data b packet y t followed by an end-of- block e s code. . The RM replies with an acknowledge code (assuming the data was received correctly) and the TM sends the next block. This continues unitt all data has been exchanged, whereupon the TM sends and end-oftransmission code and the link enters a wait-state. The above description does not conform to any standard but will work. If error checks are included, the RM could send a data-error code if the data had been corrupted whereupon the TM would re-transmit the same data packet. RS232 0000 LDA tt$02 0002 LDX 0;02 0004 LDY #$00 0006 JSR $FFBA 0009 L DA C$02 0008 I.DX #NAME 000F JSR $FTBD 0012 ,ISR $FTCO 0015 L.DX ti$02 0017 JSR $FFC6 001A JSR $FFE4 00ID TAY; Save it 001E JSR $FFCC 0021 TyA 0022 BEQ 002A 0024 LDA RXTABLE,Y 0027 JSR $FFD2 002A ,ISR SFEE4 002D BEQ 0015 0021; TAY 0030 LDA TXTABLE,Y 0033 PHA 0034 LDX #$02 0036 JSR SITC9 0039 PIA 003A JSR $FFD2 003D ,ISR SITCC 0040 SEC 0014 BCS 0015 The program given in Figure 10 should run on any of the CBM machines discussed and is written in Basic. Since the comms are being controlled by the software, the receive buffer will not overflow. It allows a user to transfer a data file from one machine to another. The link is 1200 baud, eight data bits and no parity. It should be run on both machines. The control codes are shown in Table 6. Note that I'm not using some of these as they should be used but, as I said earlier, as long as you stick to the same protocol on both machines, you'll be OK. Code 5 6 1 4 2 3 ASCII ENQ ACK SOH EOT STX ETX Meaning in Ibis program Attent/enquire Acknowledge Start transfer (transfer filename) End transfer Start block End block 10 SUB DLE Enter terminal mode (special seq.) Exit terminal mode (data link escape) YOUR COMMODORE january 1987 88 ; Logical channel 2 ; Device 2 Set I fn Name length (2 chars) NAME is address of char string Setnam Opern Make RS232 the input device Get a character Restore default device If char =0 then no char rec. Get PETSCH char from TXTABLE Send to screen Check keyboard No char, loop to check RS232 Index Get ASCII equivalent Save Make RS232 the default output : Output character ; Restore normal output Forced loop Possible Developments Table 5 The example programs I've given here do not show all of what may be achieved using the RS232 interface. It is possible (using a special version of the circuit described earlier) to have more than two machines running on a single 3-line RS232 commlink. This would allow a group of users (with special software written in machine code for maximum speed) to set up a mini-network. The possiblities are endless. I hope this article has helped to fuel your imagination. If you have any comments or ideas, please write to me c/o Your <strong>Commodore</strong> or leave a COURIER on Compunet ID SCl2. Table 6
RS232 Figure 9 ASCII Control Codes. 15 SI Shift in Code 0 2 3 4 5 6 7 8 9 10 11 12 13 14 ASCII NUL SOH STX ETX EOT ENQ ACK BEL, BS HT LF VT FE CR SO Function Null Start Heading Start text End text End transmission Enquire Acknowledge Rings terminal bell Backspace Horizontal tab Line feed Vertical tab Form feed Carriage return Shift out 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 DLE DC1 DC2 DC3 DC4 NAK SYN ETB CAN EM SUB ESC ES GS RS US Data link escape Device control 1 X O N Device control 2 Device control 3 XOF1 , Device control 4 Negative acknowledge Synchronous idle End transmission block Cancel End medium Special sequence Escape File separator Group separator Record separator Unit separator Fig 10 PROGRAM: TRANSLATE Cl E9 320 END 330 DATA 169,2,162,2,160,0,3 56 320 REM GOSUB MODE,FTX,TERM 00.EXIT 2,186,255,1E9,2,162,0,160.0, 39 330 GOTO 290 32,1B9,255,32,192 6F 340 READY. 79 340 DATA 255,162,2,32,198.25 59 350 BB Be 66 AF S A0-4864,NA-ADAR-24:ER-0 6 REM PLUS4 USERS USE BR-24: CR-S 10 DIM F(255),74(255) 20 FOR .P.0 TO 6TUJ.P.J:NEX 2A 61 5,32,228,255,166,32,204,259, 152,240,6,1BS,0 350 DATA 0,32,210,255,32,228 ,255,240,230,168,186,0,0,72. 162,2,32,201,255.104 360 DATA 32,210,295,32,204,2 74 BF 47 BO CB 79 360 REM EXIT PROGRAM 370 CLOSE2:END 360 390 400 REM MODE 410 REM ENO RECEIUED - WAIT ES 34 23 ES 30 M(10)-0:TM20)-9 40 FOR ..P-55 TO 50K..J 4J)..K:NEXT .3211 50 FOR T O BS:MJ)-J:NE XT 60 .7 FOR ..11.93 TO 218:KJ-129: ( 1 SS,S6,175,210. - 10PROGRAM: FIGURE 10 1 READY. 99 F7 83 99 FOR EITHER 420 REM SS (TERMINAL MODE R EOUEST) 430 REM SOH (FILE TRANSFER R EQUEST) 440 450 PRINT "CSE,SN.SO1 RECEIU 92 35 -70 T -, 80 FOR ..P"O TO 255 2F 100 OPEN 2,2,0.CHR%(24)+CHR5 (0) 37 4E0 GDSUB 900 REM SEND ACK F7 EA E.G Al Ra OF SB ES BC 14 FA 20 C2 30 72 SE 92 08 SO z ...; KT . ( 100 IF 1((>0 THEN F(K)..J,F. J(14 ).; K. , ) 5,) (J) -110 :( NEXT - 1 1G: 111 POKE AO,BR KT 112 POKE A0.1,CR 2, 8) . 113 AD•AD N .. E4(1 1- 120 R8-A0 X J 33) 2 130 FOR X-0 TO 255 140 POKE ADI-XJA(X) 150 T-18NEXT 160 AD..A0.255:T6-A0 170 FOR X••0 TO 295- 180 POKE ADI-X,T(X) ISO NEXT 200 AD..4411-1-256:R -210 RESTORE NAD 220 READ BY 230 IF BY-1 THEN POKE AD,B Y:AD-A04-1:GOTO 820 CS 42 BA 93 AC 4E 90 CA 84 30 SS 49 FD 110 REM +4 OPEN 2,2,0,CHRS(2 4)+CHRS(S) 120 130 REM 064 MAY NEED TO RUN AT 600 BAUD 140 REM I.E. OPEN 2,2,0,CHRI 123) ISO 4 160 CHR GOSUB 2390:REM SETUP TAR LES 170 S(0) 190 PRINT"(CLE) CHRS( 14) 190 200 REM 210 REM MAIN LOOP STARTS HER 220 REM 230 REM WAITS FOR R5232 ENO 70 Sa GS CF ES 6F CF BS 84 AE SO 02 1E 470 00SUB 1310: REM GET RECE IVE MODE 480 ON C COSUB 640,730 490 RETURN SOO 610 520 REM FTX 530 REM INITIATE FILE TRANSF ER 540 GDSUB 2300:REM FILENAME FROM USER 550 GDSUB 950 :REM SEND END 560 GOSUB 1150:REM WAIT FOR ACK 570 GOSUB 1050 :REM SEND SOH FILENAME SOO GOSUB 1150:REM WAIT FOR ACK 590 PRINT "ESEJILE TRANSFER BEGINS.." A4 AC e40 HR-INT(R81256):LR-RB-HR 4, ass 250 HP.INT(TB/256):LTTB-BT* GE CODE 240 REM GIRL-F (FILE TRANSFE R) Al 82 BOO GOSUB 1760:REM RUN FILE TRANSMIT 510 RETURN 1:?.EM BACK TO M 296 OD 290 REM CTRL-Z (TERMINAL MOO AIN LOOP BC 250 POKE R1•37 , LR: POKE RI.38 E) 54 620 ,HR Fa 260 REM CTRL-C (EXIT PROGRAM 42 530 23 270 POKE RT-1-49,LT:POKE ,HT 35 270 70 640 REM RESPOND TO FILE TRAN SEER REQUEST CS CS 17 EF 280 Nii..INT(NA/266):NL.NA-N5° 266 290 POKE RT. 1,NH 300 PRINT CHRS(14) . 310 SYS RI 12.NL:POK E 23 69 13 F9 280 290 PRINT "csAlwAITING CSE,S N,S0] DR KFYPRESS" 300 GOSU8 1210: REM GET FuNc 'ION 310 ON C EDSUB 400,530,800,3 GO 13 BC 97 3C 813 690 PRINT "CSFDILE TRANSFER REQUEST ...- 660 FRS- -670 GOSUB 1380:REM GET NAME " FROM RS232 680 cosus 900 :REM SEND ACK 690 GOSuB 1980:REM FILE RECE R T 4 . 1 4 YOUR COMMODORE january 1987 89
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