For ZX81 and Spectrum computers, the Interspec from DCP Microdevelopments Ltd. is an add-on interface with a remarkably wide range of facilities. The Interspec provides an 8 input 8-bit A/D (Analogue to Digital) converter, four relay outputs which enable your computer to switch things on and off, four inputs for external switches, an 8-bit input port, 8-bit output port and a facility for further expansion through a 15-way connector called the DCP Bus.

The Interspec is housed in the familiar black plastic case used by so many manufacturers. DCP have worked wonders with this case! It really looks good. The unit lies flat on the desk with the edge connector that plugs into your computer at one end of the case. No rear edge connector is provided on the Interspec so ZX81 owners who have Sinclair RAM packs are left to fend for themselves.

The remaining three sides of the case have rows of sockets for external connections. 2mm sockets are provided for the 8 analogue inputs, four switch inputs and four relay outputs. These provide good plug retention and no problems should be experienced with wires coming unplugged. 0.1" 'Molex' connectors are provided for the 8-bit input and output ports and the DCP Bus.

The Interspec electronics are mounted on a double-sided printed circuit board and the component layout is neat and professional.

The instruction booklet gives full details on programming the Spectrum and a supplementary leaflet contains two short machine code subroutines which provide the equivalent of IN and OUT commands on the ZX81.

An A/D (Analogue to Digital) interface converts a voltage at an input to a number in the computer. Eight analogue inputs are provided on the Interspec which can be used for signals in the range 0 to 2.7 volts. DCP quote a tolerance of +/- 20% although on the unit I tested, accuracy was well within 10% in the range 0 to 2 volts. The interface has a resolution of 10 millivolts (10mV); if you were to put a signal of 10Mv in, your computer would read this as 1: 500mV would read as 50, while 1 volt would be 100.

Programming the A/D interface is easy. Each of the 8 analogue inputs has a number from 0 to 7. To select one of these inputs, the command OUT 31,n is used where n is a number from 0 to 7: then LET x=IN 31 reads the numerical equivalent of the voltage on the selected input to variable x. The OUT command must be repeated before each IN since this initialises A/D conversion. The Interspec uses the AD0809 A/D chip; DCP quote the conversion time as being typically better than 1 millisecond.

While anyone familiar with electronics theory knows how a graph showing the charge and discharge curves of a capacitor will appear, seeing it happen in real time with the computer drawing a graph on the screen is a fascinating demonstration of the theory.

I must admit to spending some time charging and discharging capacitors and watching the varying voltages across them being plotted by my Spectrum. A simple D/A (Digital to Analogue) circuit using the ZN428 is described in the Interspec literature, details of the connections to be made to the Interspec DCP Bus are included. Personally I would have liked to see this built into the Interspec - perhaps in place of one of the A/D inputs; however, the extra circuitry is very straightforward and should not require much experience to build.

Four sub-miniature relays are built into the Interspec. These have contacts rated 1 amp at 12 volts. Each relay output has a binary value of 1, 2, 4 or 8. Alongside these outputs is a socket marked 'relay common' to which are connected one half of each of the four pairs of relay contacts. These four outputs can be used to directly switch small motors, solenoids, lamps, etc. If you want to switch mains powered equipment on and off, you will have to use the Interspec relays to switch larger 'heavier' relays whose contacts are used to switch the mains. Again, programming is by means of IN and OUT commands. OUT 63,4 would switch the relay four's contacts on; OUT 63,0 would switch all four relays off.

Four inputs for switches are provided which give an easy means of connecting external switches to your computer. A fifth socket supplying +5 volts is mounted next to these. Switches should be wired so that they connect this 5 volt line to a switch input. Programming for the switch input port takes the form LET X=(IN 63)-240. Each of the four inputs has a binary value of 1, 2, 4 or 8. So if switches 1 and 4 were closed, for instance, the variable x would return the value 5 (i.e. 4 + 1 = 5).

The 8 bit input and output ports are programmed with IN and OUT 95. IN 95 reads in a byte from the 8 bit input port, while OUT 95 outputs a byte of data (i.e. a number from 0 to 255) to the 8 bit output port. Data is latched to the output port and remains constant until changed, so this port could be used, for example, to light LEDs.

DCP have also devised a simple 15-way connection system which allows you to expand your computer's capabilities with further electronics. DCP market other equipment which plugs into this DCP Bus including a D/A converter and a fast A/D converter. Both these devices cost £19.95 + £1.95 postage.

The DCP Interspec is undoubtedly a very useful tool for educational purposes and should prove an interesting accessory for computer and electronics hobbyists. At £49.95 + £1.95 postage, it is perhaps a little expensive for the average micro user but you get a lot for your money.

The Interspec can be obtained directly from DCP Microdevelopments Ltd. at 2 Station Close, Lingwood, Norwich.