D/A CONVERTERS INCOMPATIBLE WITH MICRODRIVE
EPROM SERVICES has produced two digital-to-analogue converters for the ZX-81 and Spectrum. They allow you to program an output voltage up to +9 volts by outputting a number from the computer. One of them is an independent unit which has three A/D converters and requires no extra port to use it. It is a bare board and an edge connector must be soldered on if you are not using a motherboard. The unit is incompatible with the Interface One, which controls the Microdrive, as both use the same address line A4.
The three outputs appear on screw terminals at the top of the board and there are three sets of variable resistors to adjust the zero volts and the maximum volts setting.
Each output is controlled by an A/D converter chip so that all the programmer has to do is OUT a number to set up the system - the port used is based on a 8255 chip - select the output required and output the number for the voltage required.
The device is one of the few which will allow you to obtain more than 2.55 volts DC. The devices can be very useful for measuring where a standard voltage is input and the output is measured by a D/A converter for the device reaction, transistor testing and resistance measurements.
The other D/A is very similar but will fit only on to the Mk 1 version of the company's I/O port.
Both work well but are incompatible with other devices you may want to use on the system.
The three-output D/A board costs £27 and the single output one for use with a Mk1 port only £10.
Eprom Services is at 3 Wedgewood Drive, Leeds LS8 1EF. Tel 0532-667183.