Interfaces from Cambridge - Victor O' Neill plugs in a couple

Hot on the heels of the DCP interface for the BBC (Issue 50) comes a pair of IEEE interfaces: the IEEE488 from Acorn, at £325, and CST's Procyon IEEE controller, at £385.

By attaching an interface to your BBC you get the chance to send and receive information to a variety of equipment. The type of equipment you want to use defines the choice of interface. The BBC already comes with parallel (Centronics) and serial (RS232) interfaces, but hooking up electrical instruments or the better (and more expensive) printers and plotters often calls for an IEEE interface. IEEE 488 is a universally accepted data transfer standard in the electronics world, and is also widely used with a number of add-ons for micros: e.g. the new high density Commodore disk drives.

The Acorn arrives well packaged and with a 75-page manual, the IEEEFS EPROM, a single IEEE-488 compatible connecting lead and the interface itself. This comes in Acorn's BBC-matching cream box, about one-third the width of its host, as used for second processors ot Teletext adapters.

By constrast, the CST is much smaller, encased in an industrial looking box about the same size as a 3.5" disk drive. This is small enough to fit onto the computer but looks clumsy compared to Acorn's model.

Three LEDs across its front indicate the bus status and the back panel houses an IEEE socket, a 34-way connector (for the 1MHz bus on the BBC) and a mains inlet. It has no fixed cables, just plug and socket connectors keeping it tidy and making transportation easy. The unit is supplied, like the Acorn, with the ROM-based filing system and manual, and with 34-way and mains cables.

With both devices one of the first things to do is fit the ROM. Both manuals give detailed instructions for this but Acorn stresses that, if in doubt, a dealer should do the job.

Once the ROM is fitted, you can plug in the interface and attach the kit to work with it. This is simple on either device, though since Acorn's ribbon cable has single connectors at either end, it can only be attached to one device. The IEEE standard specifies double-ended connectors (enabling them to be stacked onto one another) and furthermore the transmission characteristics of ribbon cable could not meet the IEEE specification. But this won't matter to most users, particularly since cables and connectors that do meet IEEE standards are very expensive.

The documentation for both interfaces is good. Acorn provides a spiral-bound A5 manual which, though thorough, doesn't match the standard of CST's 110-page ring-bound file, which lets you add notes and contains a first-rate index.

Both systems send and receive control signals and data via the filing system. They control handshaking routines in part of the data transfer cycle. With the Acorn you must address the current device as a listener or a talker, so programs are littered with 'talk', 'untalk', 'listen' and 'unlisten' commands. In the CST this is controlled automatically and the corresponding software is easier to read - as shown in the two example programs.

One of the most successful commands on the Procyon is *HELP IEEE, which returns a screenful of information summarising the commands and options available in its filing system. The same command on the Acorn gives only filing and operating system numbers.

Acorn makes you work harder than CST, especially on programming. On the Acorn, two channels have to be opened from software: a COMMAND channel, via which the interface standard is monitored and IEEEFS commands are transmitted, and a DATA channel for reading and writing data to other devices on the bus.

For most applications, the BBC can process data while reading it because of its fast Basic, but sometimes, particularly with a long experiment, it may be necessary to store data and process it later, so you'll need a disk drive if you want to do a lot of processing.

The extra cost of the Procyon seems worthwhile, the cheaper Acorn version is harder to use and its matching box doesn't compensate for practical disadvantages.

The interfaces are likely to be widely used in educational establishments, as well as in research and development offices.

The two programs shown measure the response of an amplifier. The input is provided by a signal generator and the output is measured by a digital voltmeter. In the program for the Acorn, lines 10-50 set up the files and channels needed, and lines 60-100 specify the device addresses and initialise them. The loop from lines 110-220 measures the response as the signal generated is incremented and stores the data to disk. Line 180 converts the voltage (read as a string) into the amplifier's response (measured in dB). The last few lines close the files and return to the DFS.

The Procyon program is shorter for three reasons: By making default assumptions lines 60-80 of the Acorn program can be left out. Lines 40, 50, 120, 140, 150 and 170 are not necessary because the talking and listening commands aren't used, and simpler file handling allows the OPEN and CLOSE commands in 90, 100, 230 and 240 to be dropped.

  1 REM PROGRAM TO MEASURE FREQUENCY RESPONSE USING ACORN INTERFACE
 10 *DISC
 20 RESULT%=OPENOUT("RESULTS")
 30 *IEEE
 40 CMD%=OPENIN("COMMAND")
 50 DATA%=OPENIN("DATA")
 60 PRINT #CMD%,"BBC DEVICE NUMBER",1
 70 PRINT #CMD%,"CLEAR"
 80 PRINT #CMD%,"REMOTE,ENABLE"
 90 SIGGEN%=OPENIN("7")
100 DVM%=OPENIN("3")
110 FOR FREQUENCY%=1000 TO 10000 STEP 100
120 PRINT #CMD%,"LISTEN",SIGGEN%,"EXECUTE"
130 PRINT #DATA%,"0.1V,"+STR$(FREQUENCY%)+"HZ"
140 PRINT #CMD%,"UNLISTEN"
150 PRINT #CMD%,"TALK",DVM%
160 INPUT #DATA%,READING$
170 PRINT #CMD%,"UNTALK"
180 RESPONSE=20*LOG(VAL(READING$)/(0.1*0.7071))
190 *DISC
200 PRINT#RESULT%,FREQUENCY$,RESPONSE
210 *IEEE
220 NEXT FREQUENCY%
230 CLOSE #DVM%
240 CLOSE #SIGGEN%
250 *DISC
260 CLOSE #RESULT

  1 REM PROGRAM TO MEASURE FREQUENCY RESPONSE USING CST INTERFACE.
 10 *DISC
 20 RESULT%=OPENOUT("RESULTS")
 30 *IEEE
 90 SIGGEN%=7
100 DVM%=3
110 FOR FREQUENCY%=1000 TO 10000 STEP 100
130 PRINT #SIGGEN%,"0.1V,"+STR$(FREQUENCY%)+"HZ"
160 INPUT #DVM%,READING$
170 RESPONSE=20*LOG(VAL(READING$)/(0.1*0.7071))
190 *DISC
200 PRINT#RESULT%,FREQUENCY$,RESPONSE
210 *IEEE
220 NEXT FREQUENCY%
250 *DISC
260 CLOSE #RESULT