The module features two control lines

As well as four input channels for the ADC

An LM324 quad op amp is used to map -10V ... +10V to the 0..5V range of the ADC. The topography is that of a weighted sum inverted adder.

Of the 8 data outputs the parallel port has only 6 (D1 to D6) are actually used.

D2 provides the ADC's clock, D1 its data in, D4 is chip select

This interface does not rely on any bi-directional implementation of the parallel port and instead uses the ERROR status line as a data in for the serial data coming from the ADC. D3 is being held permanently high to provide a current source for the optocoupler

Channel 5 of the ADC is tied back to the +5V supply voltage. I presume for calibration.

Last but not least two 7805 and 7905 voltage regulators in a To-92 package are used to generate the needed +5V and -5V from the two 9V batteries.

Years ago I wrote an extensive library for the MAX 7219 serially Interfaced 8 digit 7 segment Display driver IC. It can display negative / hexadecimal / floating point numbers and individually address each segment and is fully modular. One of these days I need to clean up the code, publish it and finally move this prototype stuff off this breadboard. Has been occupying it since then.

Display modules with a 7219 can be had for very cheap. Although, these are probably counterfeits and I've had them fail on me before.

Test bed system

In case that is ancient and forgotten knowledge of the fore-bearers by now: copy your 9x setup folder from the CD (image) onto the hard disk first, then install from there. Will be MUCH quicker then, and you will never have to dig out the CD again, when Windows asks for it.

Windows 95C OSR2.5 ... It has been stuck at that screen for the past ten minutes or so and that is with a flash medium instead of an old clunky hard drive. Now imagine how slow and awful that must've been back then

Oh boi here it comes