The first Acorn add-on for the Electron

The eagerly awaited Electron Plus 1 turns out to be a very interesting product indeed. Take out your Electron, unplug everything, turn it over and make sure the two supporting screws of the Plus 1 are flush with the plastic edge. Now push together. Your Electron now has an Analogue to Digital port for making use of joystick controllers or for measuring analogue voltages, a Centronics printer interface for connection to most popular printers and a couple of ROM cartridge slots for instant access to games and applications software.

It turns your Electron, appearance-wise at least, into a two-thirds size BBC. As usual, an excellently produced Acorn manual accompanies the Plus 1. It gives details on how to wire up the connectors for the two ports (if you fancy do-it-yourself), offers advice on the sort of printers and joysticks available and goes into the basics of programming in BASIC and using (official) assembler in conjunction with the new facilities.

Acorn also feel it necessary to advise on how to deal with problems that might occur, in both hardware and software. For instance, the need to specify the tape filing system after using ROM cartridges. Those who have recently moved over to discs with their BBC will recognise the problem.

When the Plus 1 is slotted onto the edge connector and the screws tightened home, there is very little give and, one would imagine, no room for any misalignment. However, on first connection I did have difficulties and could not get the printer interface to function. Don't panic! I followed the advice given in the manual, checking for dirt on the edge connector, giving it a quick wipe clean and reconnecting. Hey presto, it worked!

The Centronics interface you get with your Plus 1 is not based upon the 6522 VIA. The BBC uses the 6522 (its two bi-directional input/output ports) for the Centronics and User ports but in the Plus 1, unlike some other Electron expansions, only the Centronics has been implemented. From designing a most advanced chip, the uncommitted logic array, of the Electron itself, Acorn has now implemented a simple TTL Centronics port.

This need be of no concern whatsoever to the Electron owner who wants to utilise his/her computer alongside a printer, but it is difficult to see how with the Plus 1, anyone could also add a user port. Acorn plainly do not see this as a widely required option. Broadway Electronics employ the 6522 in their interface, utilising it for a user port. There are even firms which use a 6522 and ignore one side completely, merely implementing the Centronics interface - an uneconomic proposition.

The printed circuit board of the Plus 1 carries an 8K EPROM which contains the software to drive the printer interface, analogue to digital conversion, and a RS423 interface. The software is needed to queue whatever is being sent for printing and to talk to the printer to check if it is ready for further information.

All the software features characteristic of the BBC are present with the Plus 1. *FX 5,0 selects the printer sink - output for printing is ignored - while *FX 5,1 selects the default situation, the printer driver in operation. CTRL-B and CTRL-C have their turn on and off effects from the keyboard. VDU 2 and 3 replace them within programs. *FX 3, with a choice of second parameters, effects the output to screen and printer in various combinations. *FX 6 selects the printer ignore character specified by an ASCII code and *FX 21,3 flushes the output buffer, into which all output to the printer is first sent.

The Plus 1 then supplies the facility to print out program listings, wordprocessed text (if you can find a good wordpro on tape) and graphics dumps. The Centronics port supports most of the printers which an Electron owner would wish to use and if, for some reason, there is a serial printer available, then it may be possible to connect up via the cartridge sockets.

The next most important of the rather cheap-looking plugs on the back of the Plus 1 is the Analogue to Digital port. The ADC allows the use of the multitude of BBC-compatible joysticks now available to the games player. Acorn's own have been somewhat surpassed by more sophisticated games controllers with numerous buttons which can be programmed to take on the function of keyboard controls. The range is remarkable and now immediately available to the Plus 1 owner.

The converter reads four channels like the BBC but the resolution is 8 bits (1 in 256) rather than the 12 bit resolution of the Beeb. The software still generates numbers in the range of 0 to 65280 (the BBC manual gives 65520) for BBC compatibility. The interface still enables other devices to be read but its usefulness is reduced by the less accurate resolution of the chip. If greater accuracy is required, there are suitable BBC-related products which should do the trick. *FX 16 calls are implemented for masking channels in and out.

The most interesting feature of the Plus 1, and a complete departure from the BBC, is the implementation of two cartridge sockets. These two sockets enable the use of games cartridges, programs in paged ROM and a serial link for a printer, modem or any other of the many devices which employ a RS232 interface.

The ROM filing system is accessed with *ROM and programs can be auto-run with SHIFT-BREAK, the familiar !BOOTing of disc systems. Other commands available are *CAT, LOAD and CHAIN. *TAPE is required to return your cassette system. The front cartridge port takes preference over the back. Paged ROMs (Lisp alone is available at the time of writing!) are called by entering an asterisk and the language name. Not only languages are available in this form for the BBC but wordprocessors, spreadsheets, programming aids and so on. There is no reason why a ROM box or board should not be connected to one cartridge port and a number of ROMs paged in and out. The parallel program takes the place of BASIC between &8000 and &C000.

Games currently available for the cartridge system include old favourites like Snapper and the new Countdown To Doom. A new favourite at some time in the future may be the addition of a modem to the Electron system, since this is one of the fastest growing areas in personal computing. The software in the afore mentioned 8K EPROM, ELK 1.00 tells us that it is for the Electron Expansion 1.00 (C) 1984 Acorn, ADC/Printer/RS423. In conjunction with the RS423 there is the necessary software to run an Acorn Prestel adaptor. The commands familiar from the funtion key strip are all implemented. This means that the Electron is not only adding facilities which the BBC has had from the start, but is it actually catching it up.

Indeed, with the Plus 1 cartridge facility, the Electron moves into an area as yet unrealised by the BBC Micro. Much of the more 'useful' BBC software available on disc and EPROM may well find a home in an Electron cartridge. The ROM filing system is sufficiently the same to make the transfer fairly simple. Like BBC owners in the past, Electron owners will become impatient with tape loading - although it will still have to be used for filing purposes - and the ROM filing system brings both speed and convenience.

The Plus 1 is not an imposing piece of hardware and it takes a long look to realise that Acorn has very successfully and fairly cheaply (£59.95) supplied four very necessary features for a home computer: analogue to digital conversion, printer output, cartridge filing system and RS423 communication. As well as being an important step for those who already own an Electron, it should also prove an important draw for those considering a new computer this Christmas. The Plus 1 of course requires the services of the 6502 and can slow up all but the best commercial games. From BASIC programs it is simple to turn off an unnecessary ADC channels or indeed the whole servicing of the Plus 1 with *FX 163,128,1. This can be done during a program whenever Plus 1 operations are not needed and switched back on as soon as they are.

The software does not appear to allow for a user specialist printer routine to be called and other than the setting up of the function keys and a couple of error messages, the RS423/Prestel adaptor link is undocumented. Micro Power and Microdeal games for the Electron worked a treat with their joystick option but A&F's Cylon Attack inexplicably did not.

The Plus 1 is purpose-built and seems to have been the planned extension during the development of the Electron itself. It is a reasonably priced enhancement allowing access to the sort of facilities which have become so popular with BBC and other micro owners. The documentation is a straight addition to the Electron manual, covering all the new relevant commands. Acorn continuity obviously counts for a great deal and the Plus 1 is a worthy product.

Dave Reeder