Electronics > Projects, Designs, and Technical Stuff
Found a weird old computer dumpster diving. (z80?)
obiwanjacobi:
--- Quote from: greenpossum on March 29, 2020, 07:26:04 am ---Oh dear.
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Yeah - well you can't know everything ...
Even then - sprinkling LD HL, nn's around requires 3 bytes not 1 and can really mess up the logic if you start loading HL with random stuff.
I still don't understand how this could work.
But I don't want to take the discussion too far off topic so lets drop it.
---
What I find puzzling is that these opto-couplers go to beefy connectors.
I would expect smaller connectors or some sort of switching transistors.
How much current (or voltage?) would this be good for?
T3sl4co1l:
Probably they're just using the connectors for convenience, or consistency. Those are, the uh... Molex something ancient, can't remember the series offhand unfortunately. They'd be rated for some amperes, 8A I think, give or take the usual disclaimers of course. Not that you need to use them at that level or anything.
The optos are good for a few to 10s of mA. They all seem to be pointed inboard, actually, and the current-limiting resistor (on the connector side) seems to be present as well.
They're also rated for some kV of isolation, but the board does not appear to be designed for any significant voltage. It's probably a belt-and-suspenders solution for galvanic isolation (say <100V).
Tim
SiliconWizard:
--- Quote from: james_s on March 28, 2020, 11:37:31 pm ---
--- Quote from: SiliconWizard on March 28, 2020, 10:58:02 pm ---I couldn't see much of anything under the speaker. You should put it away. Knowing how much RAM there really is will give an idea of what could be done with this board. I wouldn't bother with CP/M with less than 32KB.
Next step would be to reverse-engineer the address decoding. I think that's all you'd really need to do. And then write your own programs. I'd probably replace the EPROM with some 5V flash chip so you can reprogram it faster/make your life a bit easier.
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Depending on how the address map is arranged, adding RAM could be fairly trivial. All you need is a single SRAM chip and some address decoding which can either be done the old fashioned way or with a small CPLD.
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Certainly, but at some point, you'd waste less time redesigning a full Z80-based board rather than trying to reverse-engineer and reuse this one, if you have something like this to add - and it may not even be possible to effectively modify the address decoding scheme if the original design had no free zone.
In any case, as I pointed out and you also mentioned, figuring out the address decoding on the board would be the first thing to do before deciding.
james_s:
Sometimes it's just fun to hack something, even if there's no really logical reason to do so. One advantage of using the existing board is that you know it's wired up correctly and works.
rstofer:
--- Quote from: obiwanjacobi on March 29, 2020, 06:23:19 am ---Confused:
--- Code: ---0x21 (LXI H)
--- End code ---
does not sound like a Z80 instruction.
21-hex translates into 'LD HL, nn' ...?
And I have never heard of mnemonic 'LXI'...
Please explain.
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Yes, I used the 8080 mnemonics, oops.
Either way, it is a load immediate of the HL pair with the constant bytes following the command.
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