Following on from my done and dusted Commodore PET-2001 clone, this is my second project to recreate a classic 8-bit machine from the beginning of the personal computer age.
Erratum.
16 Jan 2020 — Uploaded a corrected version of the project document. Part of the technical description on page 15 pertaining to the MCM6670 is rewritten as I had erroneously stated that this part had 6-bit, not 5-bit words. Also found and corrected a couple of typos. Viewed in my browser, I had to hit "refresh" to clear the old document out of cache and load the new.
Obsolete parts updates.
1) The pair of PCB-mount 296XD-series potentiometers from manufacturer CTS, which I used in the video generator for the raster horizontal and vertical position controls, are now discontinued and are no longer readily available. I am not aware of any substitute with a compatible footprint. A pair of panel-mount potentiometers wired to the PCB with lengths of hook-up wire, preferably no longer than a couple of inches, would be the most practical substitution in most cases.
2) Intel, who bought out Altera, have since discontinued a whole bunch of their low-end CPLDs, which includes the EPM7064AETC44 device that I used in the design of the optional PS/2 keyboard interface board. This part is currently (12 June 2022) out of stock at Digikey with a specified status of “Obsolete”. Mouser has a limited quantity and still lists the device status as “End of life”. The design could be revised using the ATF1504AS by Microchip Technology instead, but lead times for this part at the moment are rather long and this is not currently on my to-do list.
Alternative CPU
There are five speed grades of the Z84C00 microprocessor; 4, 6, 8, 10 and 20 MHz. I specified the 6 MHz version (available at Mouser) because the 4 MHz version is obsolete, though either one is suitable. I do not recommend using any of the faster grades. These parts have less forgiving worst case timing specifications, that would make them potentially unreliable in this functionally faithful recreation of the old TRS-80 Model 1 logic.
Incidentally, although I don’t have one to try, the computer should work fine with an original NMOS Z80, Z80A or Z80B.