The cornerstone is the ADC0838 8 multiplexed channel 8 bit ADC with serial I/O.

The parallel port is used for communication with the PC. Everything is electrically isolated using 3 dual optocouplers, the CNY74-2, aka ILD 74.

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