| ★ HARDWARE ★ INTERFACE SÉRIE AMSTRAD RS232C ★ |
INTERFACE AMSTRAD RS232C: Making the right connections with the RS232 (Computing with the Amstrad) | INTERFACE AMSTRAD RS232C | Serielles Interface RS-232C (CPC Amstrad International) |
THERE comes a time in every computer's life when its limitations begin to be felt. This is as true of mainframes as it is of micros. This point is usually reached when the user finally realises that there is little point in spending hours entering data which is rarely needed and stored elsewhere. There is even less point in writing programs which are already available, fully tested and de-bugged. The key to overcoming these limitations without junking the whole system is either to put it in touch with a more powerful machine, or at least one which has the programs, data and facilities that you need. You might be able to transfer tapes or discs between your Amstrad and another machine, but this is fraught with every kind of problem. The odds are heavily against the formats being similar, never mind needing to stay within the software copyright laws. Having posed the problem, what is the solution? Common denominator At present, the only way of overcoming this lack of consistency between various manufacturers' hardware is to use their sole common denominator - electricity. The technique is to convert your Amstrad's internal codes into standard electrical signals which the device you wish to contact - this need not be another computer - can also understand. Having received these signals along a wire, the device then translates them back into whatever codes it understands. All this coding/decoding is performed by the RS232, thereby bringing either the outside world or specialised hardware within reach of your Amstrad. Of course the Amstrad hasn't got its own RS232 built in - hence the need for the Amstrad RS232C Serial Interface. What you get for your money is a unit the size of a paperback book which is plugged into the Amstrad's expansion socket via its own cable. This lead has a breakout socket so that anything you previously had connected can still be used. The RS232 is powered by its own transformer. Having installed the unit. Old Faithful, my trusty CPC, began to look like a bird's nest. Having no confidence in the ruggedness of electronic connectors, I am extremely reluctant to insert/remove cables from sockets unless absolutely necessary. Consequently I now had a second disc drive, AMX Mouse, cassette, special adapter for a non-Epson standard printer plus the RS232 hanging off the back of the computer which made my desk look like a riot in a string factory. This tangle is my first regret since it spoils an excellent product. Making contact You can use the RS232 in three basic modes:
Whichever mode you need. you should have little trouble in making contact, since the RS232's handbook is comprehensive and clear, despite being decidedly curious in its presentation, each feature being described as a spell. It took me only a few minutes to build a simple menu driven program in Basic to perform all my standard RS232 operations. All I now need to do is plug in the transformer and select from my menu. Consequently I can be "on the air" within a minute. If I need to I can "do the same in CP/M, since this is also possible. There is no limit to the data that can be transmitted. You can access publicly-owned databases, transfer letters/ messages, run programs, retrieve data spooled by a mainframe or merely send codes to operate a device. These transmissions can be made at speeds from 50-1 9200 bits/second (approximately 5-2000 characters/second) in both full or half-duplex. Communication is normally triggered by a series of special commands contained in the unit's own ROM. These steal about 1k of User memory and trigger the RS232 into performing the function required or set up the parameters for the dialogue between the Amstrad and the device it is contacting. There are 30 commands in all, plus dozens of parameters, which provide more flexibility than the average Amstrad is ever likely to need. If these simple commands are not sufficient, which should rarely be the case, the handbook contains several program listings, machine code and Basic, to provide yet more facilities. Even wiring diagrams for connections to other popular micros, and standard pin-outs are provided for those who enjoy soldering. When communication is complete. I leave the RS232 in the CPC but switched off, thereby preserving my sockets — and reclaiming the 1k.
Jo Stork , CWTA |
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