Maplin Modem

Personal Computer News

Geof Wheelwight

Ian Scales

Kenn Garroch

PCN MICROPAEDIA VOL 20 PART 2 COMMUNICATIONS, MICROS AND MODEMS Modem summary Phoning the Future Dealing with Duplex HARDWARE TALK Deciding how to connect up your hardware is the next step in the process of using your micro as a communications tool. In choosing the hardware to suit what you think are your needs it tends to be a case of the more you pay the more features and flexibility you get. One of the main variables with modems is transmission speed. Buying a modem with a wide range of these tends to put the price up rather substantially. So remember, while this flexibility may be useful if you plan to use the facility for a wide range of tasks, under normal circumstances you may find that a simple 300 baud send/300 baud receive setting will suffice. 300/300 seems to have become a standard for most of the hobbyist bulletin boards worldwide. However, all this seems to be changing. There is at least one hardwired modem with a wide range of transmission/receive speeds for under £100 (see overleaf). Once you've decided on speed, it's time to start thinking about whether you should go acoustic or hardwired (see the boxes on this page). The relative prices of the two types seem to have swapped about over the past year or so, with simple hardwired modems coming out cheaper. Most of these use the new Telecom standard plug arrangement, so it's a simple process to install a two-way adaptor at the Telecom plug in the wall and run your telephone and modem without any inconvenience. Most hobbyists won't require facilities like auto-answer. If your computer already has an RS232 interface it should be possible to configure a modem for well under £100. Micros without serial interfaces are a little more problematical. There are add-on serial interfaces available for the Spectrum, plus a purpose-built unit complete with serial interface, modem and Prestel firmware built in. All you have to worry about after this is terminal software, the various 'status' parameters which must be set if the interface and modem are to work properly, this will be covered in depth in this issue. RS232 SERIAL INTERFACE Communicating through the telephone system initially requires the configuration of a device to convert the internal parallel binary numbers with which the computer deals into serial form so they can be carried on the line. For most users the device that does this very necessary job is likely to be an RS232 serial interface. The RS232 is usually built into the micro, though in some cases it can be configured as an extra card or add-on. It takes the parallel byte one bit at a time and sends the individual bits down the wire one after the other. An RS232 has at least five wires heading out to the outside world. They handle transmit, receive, request to send, clear to send and signal ground respectively. The bytes are usually sent with a cluster of other bits around them to perform certain functions. The RS232 can usually be configured through software to sandwich the byte in a start bit and end it with a parity bit and stop bit. RS232s work with a series of standard speeds to synchronise the sender and receiver. These speeds are known as baud rates and range from 75 baud (the slowest) to 19,200 baud (the fastest). The most commonly used speeds are 300 receive/300 send or 1200 receive/75 send. Baud rates relate to the number of bits sent per second, but as we observed above, these rates cannot be applied directly to the speed at which the bytes will come through because of those extra start and stop bits. There are more than eight bits per byte. There is also a gap between the end of one byte and the beginning of the next, slowing transmission down even further. With the right software it's possible to transfer information or programs between computers, using the RS232, without the imposition of a modem. This technique is becoming particularly useful with the arrival of the lap-held computer, where the user can transfer standard text files from an applications program in one machine to an applications program in another. All you need is a cable with the correct plugs. The trouble is that finding the right plugs is not as easy a task as it might sound. Although RS232 is supposed to be a communications 'standard', the configuration of the pins on RS232 cables varies from machine to machine. This has led to the development of a market for "smart cables' and 'suss-boxes' which help you to get machines with different cable configurations talking to one another easily. ACOUSTICALLY COUPLED MODEMS There are two basic types of modem available: one plugs directly into the telephone system via a wire and the other simply fits over the telephone handset. Each of these modems has its own particular strengths - and weaknesses. The modem with acoustic coupler has the advantage of being totally portable. This makes it particularly convenient to use in conjunction with luggable or lap-held micros. The coupler simply consists of a 'reverse' telephone handset. The microphone in the mouthpiece of the handset is coupled to small speaker and the speaker in the earpiece is coupled to a microphone in the coupler. Obviously, the coupler can be used with any telephone, but there are two disadvantages. Because it's coupled acoustically there is always the chance that background noise will creep into the transmission causing garbage to appear on the screen at odd moments. This can be reduced somewhat by a rubber wrap-around on each coupling to reduce the noise. But this brings disadvantages of its own as it means that every time you want to communicate you have to fiddle about coupling the two items together - a frustrating process at the best of times. But this method does have the distinct advantage of solving the isolating problem. When you are connecting an electrical appliance like a computer to the telephone system, the people responsible for the telephone network (in our case British Telelcom) are understandably concerned to ensure that there is no chance of a short circuit occurring in the computer or the modem, which will send a high-voltage current down into the ,delicate switching systems at the exchanges. An acoustic coupler ensures that the two systems are completely isolated. HARDWIRED MODEMS The second and probably best type of modem is the directly coupled or 'hardwired' one. This hooks directly into the telephone system, but don't be misled by the 'direct' terminology. As always, the part of the modem running off its own power must be completely isolated from the telephone system somehow. On the direct modem this is usually done through an LED system. But to the person using it 'direct' is exactly what it is. These little modems have become very cheap, with several on the market costing under £100. You don't have to worry about connecting them up to your telephone every time you want to use them and you don't have as much trouble with background noise. Many hardwired modems use the new Telecom standard wall plug arrangement - with the incorporation of a double plug adaptor for your wall socket you can have your normal telephone and the modem plugged in and ready to go at the same time - convenience should not be underestimated. Another advantage of this sort of modem is its facility to 'auto-answer' or even 'autodial' a number - though you will always find that facilities like these up the price of the products quite considerably. Telecom regulations are an important factor in this field. Always ensure that the modem you are about to purchase has been Telecom approved. Although it's not an offence for a dealer to advertise a non-approved modem, and it's not an offence for you to buy it, you are liable if you hook it up to the telephone system. This situation is complicated by Telecom's approving individual components, so advertisements you see are likely to make mention of 'Telecom approved components' - not actually of interest to the average buyer. The assembled product is the item which must be approved it it's to be of any use to you. The problem is that the discrepancy in speed of the micro industry and the speed of the British Telecom in approving make things very difficult for distributors - BT has just reduced the time needed for the approval procedure to three months. In fact, BT has also off-loaded direct responsibility for approval of modems to the (BABT) British Approvals Board for Telecommunications - which is hoping to speed up and slim down the approval process (Monitor, issue 52). MAPLINS' MODEM This modem, which you put together from a kit, is the cheapest way to get into telephone communications. At only £39.95, it uses the standard 300 baud CCITT system to communicate over the phone lines and runs at either half or full duplex. Maplin also runs its own mailbox and electronic mail service which lists much of the information in their catalogue and gives you the option to order it through that service. Connection to the computer is via a standard serial RS2320 interface - and if your computer doesn't already have one. Maplin do a number of such interfaces for the Dragon, the Vic-20, Commodore 64 and the Sinclair ZX-81. The modem has indicator lights showing when it's receiving, transmitting and locked. MINOR MIRACLES WS2000 WORLD MODEM This is one of the first of a new breed of international modem that runs on both the North American and British phone systems. Though the Minor Miracles modem does not have British Approvals Board for Telecommunications approval, the components in the modem are approved. It is not illegal to own one, but attaching it to the telephone system is... The specifications for the new modem are impressive; it operates at 300 baud full duplex, 1200 baud send/75 baud receive full duplex and 1200 baud half duplex, it recognises European and American protocols and an auto-dial/auto-answer plug-in can be bought for an extra £39. The Minor Miracles modem is hardwired and plugs straight into a standard British Telecom phone jack. MICRO-MYTE'S 160 IQ/D The Micro-Myte 160 IQ/D is a software-controlled acoustically-coupled modem for the Sinclair Spectrum which uses the cassette port, rather than the more standard RS232C serial interface. It can't be used to communicate with ordinary bulletin board so electronic mailboxes, but is instead designed to allow easy quick transfer of information between Spectrums. The Micro-Myte transfers data at the non-standard rate of 1032 baud, instead of the 300 baud speed used by most low-cost modems. But at the price of only £99, the 'non-standardness' of Micro-Myte's modem may not - in the end - matter. CONCORD'S V.22 DATA MODEM At £547, the Concord V.22 is at the upper end of what most people would be willing to spend on a modem - but it's also at the upper end of capabilities. It's a hardwire affair that runs at 1200, 600 and 300 baud and automatically filters out distortion on the line. The modem also auto-answers calls from other computers and, when it detects the call, analyses it's signal and selects the appropriate baud rate on its own. The system can be run manually if you don't like the idea of your modem answering your phone calls every five minutes. (Some modems - which are equipped with a device that detects whether a human voice or computer tone is calling you - can be run all the time.) The V.22 seems to be a comprehensively equipped, if somewhat expensive, device. SENDATA 700B SERIES ACOUSTIC COUPLER This is a truly portable 300 baud modem. The acoustic cups on either end of the modem mean you can use it with almost any phone and the rechargeable batteries mean you don't have to look for a main plug before you start transmitting. It's just a little more expensive than most acoustic couplers costing between £220 and £280 (depending on whether you buy it from Sendata or Tandy). And, of course, acoustic couplers that are cheaper aren't portable. Of course, if portability isn't important for your communications, then you'll probably be just as happy with a cheaper - but more stationary - conventional hardwire or acoustic modem. TANDATA'S TM100 The TM-100 is yet another modem to sell for under £100. The shape of this £99 modem is not dissimilar to a Oric-1 computer with no keyboard, but it is capable of 1200/75 transit and receive on full duplex can store up to eight telephone numbers of up to 16 digits each. Passwords and log-on identifiers for each of those phone numbers can also be stored on the TM100. You can automatically or manually transfer your ID, and you can change the ID and passwords currently stored in the modem. If you're using the modem by yourself and few others are likely to need to use it, you're probably safe leaving all the identifier stored in the modems. The modem is connected easily to any micro with an RS232C serial interface and connects to the phone line through a series modular jack plug. DACOM'S BUZZBOX At £69.95 this is probably the cheapest hardwire modem available (unless you count kit-based modems you have to build yourself). It's small, compact, easy to use and connects to any micro with an RS232C standard interface. The Buzzbox runs at only 300 baud (send and receive), but since that speed is used more often world-wide than any other, this shouldn't be a drawback. It's lightweight and plugs easily into a BT modular jack or the old 420-type and can run either on batteries or the mains though the battery-operated version might cost you a bit more. There are only two switches, one which switches the phone back and forth from answer to originate and one which lets you use the phone for either talking or transmitting data. PACES'S GRAPEVINE MODEM Pace has produced a new modem to rival the Minor Miracles WS 2000. For £125, the Grapevine will allow you to run 1200 baud receive, 75 baud send and 300 baud standard at full duplex. It offers CCITT (European) and Bell (USA) transmission frequencies and a self-test mode. The fact that it's hardwired, like the Buzzbox and Minor Miracles modems, minimises the risk of noise on the line interfering with and corrupting data. The Grapevine sports an integral power supply and uses a 5-pin DIN connector for interfacing with standard RS232 ports on micros. It comes standard with a lead suitable for connection to the BBC Micro, but Pace has said it will supply leads for other machines on request. PHONE TO TERMINAL There is a disturbing amount of jargon and constraints that come between you and getting to grips with using a modem to communicate. Most of the trouble stems from standards - it's not that there aren't any, but that there are so many, it's all a question of setting the right speed, the right number of start/stop bits which surround each byte of data you send or receive. Then there's duplex and parity. Standards demand ensuring the receiving machine expects the same sort of data at the same rate that the sending machine is dishing out. Thankfully, the actual course for the characters which are sent are fairly standard. Just about everything which communicates electronically uses a standard ASCII code. A is sent as 1000001. B comes out as 1000010 and so on up through the alphabet. But to send information you have to know if the receiver is expecting an odd or even parity. That's an easy one, you might say, the standard should be even parity. What could be easier - even parity for nice even data transfer? Not at all. Some systems are configured to accept odd parity. So before you can communicate you have to set your system to the correct parity. Every byte sent has an eighth bit which helps the receiving de vice check the integrity of the data at the other end. All characters are sent as numbers, which are, of course, odd or even. The parity bit is added to the number to make it odd or even - depending on what the receiving machine is expecting. If the status is even parity any character which has an odd number is given a parity bit of 1 to make up the difference, and vice versa if the character is already even it gets a 0 parity bit. Thus, if the receiving machine receives an odd number it can be sure when one of the bits is wrong. The concept of duplex is also complicated. Full duplex means communicating modems can receive and send simultaneously. They do this by sending and receiving on different frequencies, and they do it because it makes a good error check. In full duplex it's obviously a very fast process. Half duplex means the communication is two-way, but not simultaneous, so you're not getting a check back on what's appearing at the other end. Perhaps the most obvious variation is speed, or baud rate. Baud means the number of modulations per second. All this is excusable because transmission speed is a trade-off between speed and safety. The slower you send data over a telephone line, the more sure you are there's not going to be a breakdown in communication. The variables are large. Firstly, there is a wide variance in the quality of the line you happen to be using at any one time. Remember the last time you had to scream down the telephone to make yourself heard. You might be able to cope with that sort of aggravation, but computers can't. Problems of garbage appearing on your screen are certainty not unknown. So, when in doubt, the first thing to do is slow it down. Some of the cheaper modems like the slower speeds. Against this, of course, is the argument of time and money. The 75 baud rate is appallingly slow and a large document costs several minutes in telephone charges. These days a firm favourite is 300 baud transmit/300 baud receive - henceforth termed 300/300. The other common speed is 1200/75, which is the speed used by Prestel. You're also likely to come across 600 or 110 from time to time. When you communicate with another computer you are effectively turning your machine into a 'terminal' to receive and send information. In the commercial field terminals are used a lot to interact with mainframe or minicomputers. The terminal may be in the same room or miles away from the computer, in an airport for instance. These terminals can usually be split into smart or dumb terminals. A dumb terminal lives up to its name, it has no local facilities. It sends each character straight to its host as it's typed and has no way of retaining information sent to it - the moment it scrolls off the screen it's gone for good. A smart terminal, on the other hand, is assumed to have local capabilities. It can store and buffer ingoing and outgoing information. A micro is really an over-qualified smart terminal but its potential can only be realised with the incorporation of a good 'terminal' program. This is an applications program which should enable you to easily set and store all the variables mentioned above - parity, baud rate and soon. So, if you're in the habit of accessing a variety of remote systems, you can store all the communication variables in the terminal program and call it all up at the appropriate time. Some terminal programs, given the right hardware, even dial the number for you. As well as dealing with all the slog, a good terminal program enables you to easily save text and strip out unwanted control characters ready to insert in another applications program file. You should also be able to prepare text for transmission. Some programs, with the right computer in tow, will allow you to define keys for such things as passwords or frequently used keystroke sequences required by the system or systems you are accessing. FUTURE PLANS The future of computer-based communications is still very much an open question. There are futurists who confidently predict the advent of 'a micro in every home' within the next five years, while others stubbornly hold on to the idea that computer communications are largely going to be a big business concern and won't have much bearing on the home user. You can already buy a cheap modem and computer that opens up many large international mail and database facilities for under £200, but so far the incentive has been small and the 'on-line' cost still relatively high. One thing that might well reverse that trend is the heavy involvement of telephone companies in micro communications. It should come as no surprise that telephone companies are at the forefront of electronic communication, but it might surprise you to learn that some telephone companies are already working towards finding a cheap way to put a micro in every home - as a cost-saving measure. The key to this cost-saving is the annual printing of telephone books. Each year, phone companies all over the world must print and distribute new telephone books to all their subscribers. And in cities such as London - where the telephone directory runs to four volumes - that kind of printing isn't cheap. Some of that cost is off-set by the rates charged for listings in the Yellow Pages, but electronic communication offers a much cheaper way forward. Canada's BC Tel corporation has been looking at the possibility of equipping every home with a cheap communications micro that would allow people to phone up a huge computer database when they wanted to look up a number, rather than using a telephone book. The idea is that the cost of equipping every home with such a terminal would be quickly offset by not having to print new telephone books every year. Numbers would be added constantly to the database so that it was always current. A good deal of money would also be saved on operator information services, which could be largely handled by the computer database, even numbers which had only just come into service. The Yellow Pages would still spin money in the form of full-screen colour advertisements on the telephone terminal which could be charged according to how much storage 'space' is used on the service. In the UK there is an even greater incentive for the development of this kind of service. First of all, it would vastly increase the number of people who could use British Telecom's Prestel and BT Gold communications services and it would provide less costly alternatives to mail delivery. A terminal in every home would give most people an easy facility (with the addition of some form of keyboard) to use electronic mail, and would both speed up and reduce the cost of business communications. BT Gold and the new One-to-One electronic mail services are already looking toward this future by using electronic mail with existing non-electronic technology. On both services you can now send telexes and telemessages via your micro. Telexes feed the text generated by your micro to a larger computer system which puts them into telex format - from which they can then be printed out on standard telexes throughout the world. Telemessages are a slightly different mixture of old and new. You type a message out on your micro with a name and address at the top of it. That message is then printed out by BT or One-to-One on a standard line printer in their offices and the printed message is stuck in an envelope, addressed, stamped and posted. Large-scale use of communications facilities on micro will eventually make the need for such mixtures of old and new unnecessary - but for now they are useful in introducing people to the power of the new communications technology. The future will probably look something like this: you'll send your electronic mail message to a large storage device which the recipient can then ring up and read the messages from. Collecting mail will no longer be a matter of going to the doorstep or letter box - just dial a number. Micropaedia Editor: Geof Wheelwright Design: Nigel Wingrove Contributors: Kenn Garrock and Ian Scales NEXT WEEK: We conclude our look at communications with a detailed comparison of electronic mail and bulletin board services.

Your Computer

John Dawson

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.

The British Prestel System has the largest number of users of any videotext system in the world.

Transam's Modem for the Epson HX20.

FIGURE 1 - COMPUTER DATABASES Distel [redacted] Display Electronics Maplin [redacted] Maplin Electronics Rewtel [redacted] Radio & Electronic World Forum 80 Hull [redacted] Electronic mail library TBBS London [redacted] Mailbox 80 [redacted] Liverpool Electronic Mail

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

Sinclair User

MODEM ALLOWS ZX-81S TO SPEAK TO EACH OTHER Maplins has two kits of interest to the ZX-81 user and both can be used to provide an on-line 300-baud modem. The first, which is the modem, was issued as a kit in February, but until now the RS232 and the software was not available to use it. The important thing about 300-baud working against Prestel is that it is used commercially to obtain information from databases for which you have to pay a fee and also you can communicate with other computer users who do not have ZX-81s. Free databases or bulletin boards are also available; a list of them can be obtained from the Association of Computer Clubs. The modem is not the easiest of kits to build as it uses a five-band resistor colour code but if you follow the instructions to the letter the kit can be assembled without too much difficulty. The problem is in setting-up the modem and connecting it to a telephone line. To set up the modem an oscilloscope is essential, except that it is difficult to set up accurately. Most of the controls can be set up using a sensitive voltmeter but to check the mark space ratio the modem is best looped back to itself and, using the RS232, sending Us until Us return on the screen. Having set up the modem, a plug and socket must be ordered from British Telecom to connect it to the line. Once connected to the telephone line, you can telephone Maplins and order components, check your item is in stock and if you are using a credit card, pay for it. The RS232 kit is much easier to build but you will require a motherboard and at least 2K of RAM to run the terminal program - the program to talk to the modem. The software is stored on tape and conversion is made to ASCII characters by using an EPROM. The RS232 can also be used to run a printer or to talk to another computer which has an RS232. The RS232 interface cannot produce negative voltages, so full implementation of the standard is not possible. The modem costs £40.45, RS232 interface kit £25.45, motherboard £8.45, modem case £7.85. Maplins is at [redacted].