The NASA Deep Space CDU is a coherent demodulator for non-return to zero (NRZ) data, binary phase-shift-keyed (PSK) modulated onto a 16 kHz sinusoidal subcarrier.
In simpler terms, a CDU is a software defined digital radio receiver.
I wrote all the code that ran on the unit, which appears in the listing in section 10. All except the xlatb instruction (on page 10-39 of the engineering report, page 206 of the PDF), for which I have to give credit to my boss, Vic Albrecht, as indicated by the comment on that line.
This CDU was supposed to fly on Mars Observer and all the Mariner Mark II missions. I have no idea which, if any, missions it was used for.
We built a prototype by hand using wire wrap. My desk and our workbench showing the HP logic analyzer and the custom test rig.
All the details are in the Final Engineering Report.
Above is a scan of my copy of the report. You may notice cat footprints.