The Votem

The Votem, to play on James J. Carr’s book Digital Interfacing to an Analog World (Tab Books, Summit, PA. 1978), is an analog interface to Sinclair’s digital world.

Analog/Digital Conversion

Analog (related to analogous) refers to relationships by ratios. The analog device relates numbers to a turn on a wheel such as our rapidly disappearing analog wrist- watch; iv me movement of a rod or shaft such as the slide rule; or to continuously changing voltages or currents such as in an electronic analog computer.

The electronic digital computer, of course, also works on voltages but in discrete packages or digits as in the digital wrist-watch with the liquid crystalline dis- play. Digital computers, unlike the watch- es, do not count in decimal (0-9) in their digits, but rather use a bistable digit which is voltage or no voltage, in other words, on or off, 1 or 0, true or false.

What Carr is driving at in the title of his book is that most of the devices we use to measure and control things are analog in nature. Transducers convert physical energy such as strain, thermal energy, or light energy into electrical energy in a continuously changing voltage or current related to the amount of energy, yes, its analog. In order to control things, voltages or currents have to be output.

The Votem is a device which can measure external events relating Analog/Digital (A/D), but it does not go the other way for control, Digital/Analog (D/A).

A/D by the Votem

How does the Votem manage the marriage between the analog and digital worlds? It takes advantage of the tact that the Z80 processor uses a crystal to generate a pulse at a given frequency. This produces a clock to take care of the timing of events in the operating cycle of the machine.

The heart of the Votem is a voltage to frequency converter (V/F, AD537JH). The V/F relates the frequency of the Sinclair clock to voltage applied to the external connection of the Votem. This is done via a machine language program (provided in the manual) which converts the clock pulses to a count/volt. Basic programs can then convert the voltage to something meaningful such as the temperature at the end of the probe and display it periodically on the TV screen.

Temperature Measurement

The Votem comes with a temperature probe of the resistance type with the change in resistance proportional to temperature. By calibrating this probe to C (ice) and 100 C (boiling water) one can display on the screen the room temperature using a Basic program. The calibration is all important in the final accuracy of the result because the software uses these calibration factors in the calculations. Since clock frequencies can vary from computer to computer, accuracy is to a certain extent in the hands of the user.
The Votem can interface with positional devices (e.g., joysticks), photocells (e.g., spectrophotometers), and virtually anything with an output voltage in the 0-1V range (e.g., ionization detectors).

Tape Load Circuit (Earphone)

The Votem connects via the tape earphone port and thereby leaves the expansion port of the computer free. A RAM pack or Byte-Back/RBI control module or both can still be used. The BB1 device can be programmed to turn on relays which can switch on 110VAC items such as lights or coffee makers. The ZX/TS owner has an opportunity to do some practical things not previously possible.

Getting the most mileage possible out of the device, the frugal designer also employed unused portions of the circuitry to condition the tape signal to improve LOADing, providing audio output of the signal and an LED which glows during the LOAD. We found this feature to be quite useful. Those with LOADing problems might find the the Votem a useful addition for this reason only.

With just three integrated circuits, only one of which is in a DIP, it is a relatively easy kit to assemble, for those who like to become intimately involved.

For those who want to make the most out of a Votem/Sinclair combination Carr s book is recommended reading.

The Votem is probably not for those who cannot quite remember who Ohm was, or which parameter goes on top and bottom in his equation. But for those who do and would like to expand the cognitive horizons of their computers the Votem is a must.