Maplin Modem
Maplin Electronic Supplies Ltd
1984

MAPLIN ONLINE

John Dawson found the thought of putting together and using a Modem quite daunting until he actually went ahead and did it. Now he finds it reliable and useful. Here he look at operating procedures for the unit with particular reference to the Maplin Modem.

I have been using the Maplin Modem and it has worked well, giving me access to a number of amateur and commercial databases around the country. The Modem has proved reliable and useful. It has worked satisfactorily on both A BBC Micro and an Apple II fitted with a Computech Diplomat serial card.

The Maplin Modem connects directly to British Telecom's telephone lines and this ensures the best signal possible in the UK and, consequently, low error rates. The alternative method for putting a signal on to the telephone line is to use a small loudspeaker near the mouthpiece and a microphone close to the earpiece to send and receive the tones that carry a message.

There is no electrical connection to the telephone lines in this type of Modem, instead it is acoustically coupled - the computer produces a digitaI signal which is turned into two tones by the Modem and these are fed to the loudspeaker which whistles and chirps and sends the information down the line.

Unlike many acoustic couplers which are restricted to imitating calls, the Maplin Modem can both originate and answer calls. That means that the receive and transmit filters in the Modem can be switched between incoming and outgoing messages. This is important because two originate-only acoustically coupled modems will be unable to communicate with each other - one Modem must answer the other's incoming call.

The Maplin Modem relies on the operator to switch the Modem into the telephone circuit, it is not an auto-answer device. Using the Modem is quite straightforward, you dial the number you wish to communicate with and listen on your telephone until the call connects. A steady tone indicates that the computer at the far end is waiting for the out going tone from your Modem.

You switch the Maplin line switch on and replace your telephone handset on its rest. The remote computer will send a welcome message and then you can start to communicate. At the end of the exchange of information you switch the Maplin line switch off.

Whenever I have used the Modem with either a BBC micro or an Apple II computer, the number of corrupted characters has been very low. In radio terms the best way to describe the results is solid copy. All the computers I have accessed - most of which are listed in Figure 1 - use full duplex which means that a character typed onto the BBC micro does not appear directly on the VDU but is sent, instead, to the Modem and the remote computer.

The remote machine then echoes the character back down the line, through the modem and into the BBC micro, where it is displayed on the VDU. Full duplex allow the simultaneous transmission and reception of data.

Half duplex, also available on the Maplin Modem, means that the terminal - the BBC micro plus Modem - transmits and receives data consecutively, and transmitted data is displayed locally before it is sent to the remote machine.

Many of the telephone numbers printed in one computer magazine or another turned out to be incorrect. Some of the computers are only connected to the Bulletin Board service between certain hours. You also, quite reasonably, have to be prepared for human voices on the other end of the line - "Sorry love, he's on holiday for two weeks". The numbers listed in figure 1 were correct at the tune of writing.

The hardware is only part of the story because the Modem and computer on their own are just pieces of equipment with no function. In common with all peripherals you need some software to make the computer recognise and use the equipment you have connected. The BBC machine is particularly clever in this respect. The machine operating system, or MOS, allows you to work at a high level if you simply wish to control the flow of data out of the machine and the built in assembler makes the development of a terminal package quite easy.

Figure 2 lists a Basic program which will set up the BBC micro either as a terminal or to transfer a file of information held in the machine to another computer. The terminal section of the program - lines 140 to 780 - is taken largely from Mike James' excellent book, The BBC Micro - An Expert Guide. The book is worth buying and the operation of the program is explained in some detail The additional part of the terminal program stores each incoming character at the address pointed to by the variable text.

When you have completed the exchange of data with the remote computer you can return to the main menu by pressing the Escape key. The program is very crude, uses far too many Gate instructions and could be turned into a number of neat, faster running procedures. It was developed in a hurry, it does work, but it needs refining - it is not an example of good programming.

Lines 790 to 900 send the data stored between "&2000" and "textend" to the serial printer port - the same RS-423 output as was used by the terminal program. The data is sent at 300 Baud - line 820 - and the file is concluded by sending a Control Z character - line 880. The purpose of this instruction is to terminate the transfer of information to a CP/M file using the Peripheral Interchange Program or PIP.

Someone said the other day that the Maplin Modem is over-engineered. That may be but it certainly works reliably and, despite the recent availability of single chip Modems, it represents good value for money. The isolating transformer is essential for any Modem that is to be connected to BT lines - careful construction is necessary to ensure that the completed equipment is electrically safe.

I expect to use the Modem and the BBC micro together to access an American database Colleague by going through the packet switched network for work purposes. The different frequency tones that are used in the US are not a problem because all you are doing is to make a local phone call to the packet switched entry point.

The RS-232 interface for the Sinclair ZX-81 computer is made by Cobra Technology Limited and it comes with software on a tape to turn the ZX-81 into a versatile terminal. The terminal software uses the Xon/Xoff protocol to control the flow of data into the computer and can dump the ZX-81 screen to the ZX printer.

One option at the start of the program will set up the system to look like a Telecom Gold terminal. Unfortunately I was unable to borrow a ZX-81 to test the Cobra interface with the Maplin Modem but in theory there should be no problems.

The Microdrives are now on sale for the Sinclair Spectrum. Probably the most interesting part of the new hardware is the interface between the Spectrum and the Microdrives. The interface has an RS-232 outlet and also sockets to set up a local area network or LAN for up to 64 Spectrum computers. The manual does not say how long the leads can be between computers on the net but if the same electronics are use for both the RS-232 and the LAN then you might expect to be able to run up to 50 metres of cable from one computer to the next.

You can direct the output from your computer to another specified computer on the network or you can broadcast a message or data for everyone to receive. The manual says that the output from your computer is marked for the recipient and if she or he is not waiting for the message your computer will wait forever, attempting to get the message through.

Although you must sort out high level collisions, it appears from the manual that more than one pair of stations may use the net at the same time. Low level collisions are apparently avoided. Sharing programs need never be a problem again although one report suggests that the network operates at 110 Kbaud.

Learning about computing happens in a series of steps. Moving information from one computer to another was quite a big step until I did it and then it was easy. The BBC machine is ideal for experimenting in this field. Connecting the Maplin Modem was remarkably simple and extending Mike James' original program was not difficult given the BBC user guide as a reference book.

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REVIEW BY: John Dawson

Blurb: FIGURE 1 - COMPUTER DATABASES Distel 01 679 1888 Display Electronics Maplin 0702 552941 Maplin Electronics Rewtel 0277 Radio & Electronic World Forum 80 Hull 0462 858169 Electronic mail library TBBS London 01 348 9400 Mailbox 80 091 428 8924 Liverpool Electronic Mail

Blurb: FIGURE 2 10 HIMEM = &1FF0 20 text = 12000 30 textend = 62000 40 start = 12000 50 CLS 60 PRINT TAB(10,10), "BBC terminal utility" 70 PRINT TAB(13,13), "PRESS 'T' for terminal program" 80 PRINT TAB(3,15), "PRESS 'S' to send text to serial port" 90 INPUT TAB(10,17) K$ 100 IF K$ = "T" THEN 140 110 IF K$ = "S" THEN 790 120 GOTO 10 130 140 DIM INIT% 20 150 DIM CHAROUT% 20 160 DIM CHARREADY% 20 170 DIM CHARGET% 20 180 SERCON% = &FE10 190 ACIACON% = &FE08 200 ACIABTAT% = &FE08 210 ACIATRAN% = &FE09 220 ACIAREC% = &FE09 230 GISUB 260 240 CALL INIT% 250 GOSUB 630 260 For B = 0 TO 3 STEP 3 270 P% = INIT% 280 COPY S 290 LDA #&13 300 STA ACIACON% 310 LDA #156 320 STA ACIACON% 330 LDA #16D 340 BTA SERCON% 350 RTS 360 J 370 P% = CHAROUT% 380 COPY B 390 .OUT1 LDA ACIABTAT% 400 AND #10% 410 BEO OUT1 420 STX ACIATRAN% 430 RTS 440 I 450 P% = CHARREADY% 460 I DPT S 470 LDA ACIACON% 480 AND AB111 490 STA 1140% 500 RTS 510 J 520 P% = CHARGET% 530 I OPT B 540 LFA ACIAREC% 550 AND &17F 560 BTA LU404 570 RTS 580 J 590 NEXT 9 600 RETURN 610 620 REM VDU LOOP 630 CLS 640 SFX229,1 650 X% = INKEY(0) 660 IF X% = 27 THEN 760 670 IF X% = -1 THEN GOTO 690 680 CALL CHAROUT% 690 CALL CHARREADY% 700 IF A% = 0 THEN GOTO 650 710 CALL CHARGET% 720 text = A% 730 text = text + 1 740 PRINT CHR$(A%), 750 GOTO 650 760 textend = text 770 GOTO 50 780 790 REM SEND TO SERIAL PORT 800 text = &20 810 SFX5,2 820 SFX8,3 830 VDU2 840 SFX6,0 850 REPEAT PRINT CHR$(text) 860 text = text + 1 870 UNTIL text = textend 880 VDU1,26 890 VDU1 900 GOTO 50