Electronics > Projects, Designs, and Technical Stuff
Cloning a Tandy TRS-80 Model 1
kizmit99:
Parallel loading of the state registers should prevent you from getting spurious key-presses at the Z80 side, even if you happen to change just as the Z80 is scanning, since from it's perspective it would just be the same as the person typing (which is obviously async to the scanning).
To me, it seems an almost perfect interface from the PIC would be 6 'address' lines, a single state line, then a strobe to load the state of a single register at a time. It would only be 8 lines, and in most cases the PIC would only need to modify a single key-state bit at a time anyway (sometimes 2 or 3, but mostly just one key going up or down). I'm not familiar with your CPLD, but since you couldn't fit an extra 3:8 decoder into it, I suspect it would support the ability to individually address each state-bit either (effectively a 128bit LUT in front of each register).
GK:
--- Quote from: kizmit99 on July 06, 2019, 11:20:01 pm --- I'm not familiar with your CPLD, but since you couldn't fit an extra 3:8 decoder into it, I suspect it would support the ability to individually address each state-bit either (effectively a 128bit LUT in front of each register).
--- End quote ---
Not even close unfortunately. The 3-7 line decode alone bumped the compile from 64 macro cells to 69. These CPLDs are just really inefficient at LUTs. Anyway, the design is now complete and I think I'm happy with it. Like I did with my PET clone, I've designed the keyboard interface as a stand-alone PCB which could interface a PS/2 keyboard to any TRS-80 model 1, not just my clone and the motherboard itself will be compatible with an original keyboard (not that I have an original keyboard).
The fitter didn't give me the luxury of assigning my own pinout to a useful degree due to the fact that the whole thing is such a tight fit, so the CPLD "wiring" is a bit of a jumbled mess with a more or less random pin assignment. Who said you cant have power planes with just 2 layers? :P
German_EE:
I guess that this will be a US layout PS2 to Tandy interface?
GK:
--- Quote from: German_EE on July 07, 2019, 08:41:11 am ---I guess that this will be a US layout PS2 to Tandy interface?
--- End quote ---
I don't think that it should make any difference as far as the PS/2 serial protocol is concerned :-//
German_EE:
Well, just as an example, shift-9 produces a ) and shift-3 gets me a ยง on my German keyboard but on the International US layout shift-9 produces ( and shift-3 is #.
This could get confusing.
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