In my project, one section I eliminated from the updated circuit is the IC shown below.
(it was dropped as it related to equipment that was already rare when it was state of the art, and no examples likely survive)

I was just curious as to what this circuit actually does.

A0 and A1 are driven by an addressable latch, and X and Y go out of the unit completely to said rare piece of equipment - the documentation I've found so far suggests that this was a calibration signal source?

 

Impossible to say without more information.

 

Can you elaborate what information I am missing?

 

When two or more signals share the same signal path they are multiplexing.
A television remote does more than one thing maybe those controls use the same signal path.
There are too many applications that use MUX. that it would be difficult to say what.

A better question might be: What are some common applications for a multiplex system ?

 

a1/a0	X	Y
00	12k	gnd
01	1.6k	2k
10	2k	1.6k
11	gnd	12k

The resistors are to +5V. I didn't check the pin numbers with the chip doc but the above is enough to get the idea...

 

It might be used to make a sinewave made with steps. I have a very low distortion sinewave generator made with steps that are smoothed to have extremely low distortion with a switched-capacitor Butterworth lowpass filter IC.

 

Interesting AG, I found one design after the Butterworth sounds logical must have been before this
Article shows multiplex sine highlights the switched capacitor filter Maxim UK uses Analog switch DG508
https://gr33nonline.files.wordpress.com/2019/07/wireless-world-1991-08.pdf page 62

Way back EDN magazine August 20, 1973 page 86 used only an op amp filter not distortion rated like <2% THD
Also published in Cmos Circuits J.R.Tracey, arbitrary waveform generator used 2 CD4151 I have schematic
It's the ones after this

Another promising design was a triangle generator also used dual polarity 5V supply
A multiplexed triangle CD4053 TL084 D1 VR1,2 You have to click on the picture to open it.
https://electronicsproject.org/triangular-wave-generator/

 

Could it be a stepped waveform generator feeding a filter to produce quadrature sine waves.

 

Impossible to say without more information.

Since I am quite familiar with the the CD4052 binary controlled 4 position switch, the answer is that it supplied the 5 volts or common to the outputs, X and Y, depending on the 2-bit binary code presented at A0 and A1. It would not be very close to a sine wave with only 3 levels and zero, but it could serve as a calibration source for some sort of system.

 

Another search based on number of levels for the history of multiplexed sine generation. levels shown in figure 2 and 3
I found 5 levels Siemens 1983. made from discrete components switching was limited in frequency by the CD4018 Johnson ring counter, today counters run much faster.
( I saw a 15 level signal had a much better wave shape ) soon after the trail ends and engineers are using digital for sine wave generation.

 

Certainly an approximation is possible BUT the 4 level arrangement was not very close. And certainly here are other applications for such a circuit, such as speed control and driving a light dimmer.

 

on my way out the door, an example of DEMUX simulation with leds

 

Certainly an approximation is possible BUT the 4 level arrangement was not very close. And certainly here are other applications for such a circuit, such as speed control and driving a light dimmer.

Given that the TS mentions that this is a part of a larger circuit, we can possibly learn a bit more and explain exactly what the circuit does.

 

I had to dig out the documentation I was able to find on this equipment, and I now see:

"...an analog multiplexer is connected to the data bus and address decode device (the latch that controls the A0 / A1 lines on the 4052). The multiplexer, under command of the microprocessor (within the main unit), selects one of a plurality of analog signal sources (I assume the resistors attached to X0-Y3), each of which provides a different known level of analog signal and supplies the selected source as an analog test output signal."

On the other end, the unit this circuit eventually connects to in testing has capacity of up to four transducers (current, voltage, or watt), connected to a multiplexer and an ADC. It says the multiplexer on that side was the 4529B and the ADC was a 3711, both available (at the time) from National Semiconductor. I haven't been able to find datasheets for either...

 

OK, so now we know that this circuit is multiplexer that probably also provided reference levels for checking that noting had changed.
The 4529 is a dual 4-channel analog data selector with the same 2 lines controlling both selection switches, but with separate enable lines provided. So this was not a 2-bit resistor type D/A converter.