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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 5th May 2021, 4:57 am   #21
ortek_service's Avatar
Join Date: May 2018
Location: Northampton, Northamptonshire, UK.
Posts: 560
Default Re: Raspberry Pi as a storage device for a PET.

Yes, that's still an issue today - where both are partially-on for a brief period, due to limited rate of change of drive signal (although for High-Power motor-drive etc. FET's, there's deadband drive-controllers). And this mainly first occurred when 'C-MOS' superseded N-MOS, and P-MOS (But the Rds(on) of the original CMOS did limit the current and prevent them blowing-up due to 'shoot-through', if inputs floated).

However, I later realised that when the input is at 3.3V, a significant current will always be flowing into base of the lower T2 transistor - So it will be fully-on, preventing the output from going above 0V.
But if the upper T1 transistor wasn't actually being turned sufficiently-off then there would be quite a significant 'shoot-through' current, this being rather wasteful and potentially damaging these.

So the addition of R4 just limited the current that T1 could pass into T2 (and also the output) to a quite low (< 0.5mA) very-safe level. With T2 then able to ensure the output is always 0V - And that's how it does actually still all work in practice!

T1 can't really provide any extra active speed-up boost, as R4 is still limiting the current it can pass (and being an emitter resistor, also provides negative feedback and so reducing drive-current into to T1 base).
So they might as well just used R4 as load-resistor on T2's collector and never bothered T1 and what seems were original attempts to have better positive output drive / symmetry of source & sink.
But, as well as using resistor dividers I'd suggested, then they could also have inserted a 3.3V etc. zener in series with T1's base, to ensure it wasn't being biased on when input voltage was at 3.3V.
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Old 5th May 2021, 8:56 am   #22
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 6,409
Default Re: Raspberry Pi as a storage device for a PET.

Well, as we all seem agreed that the circuit as originally drawn should work either by accident or design, maybe Colin will get a chance to give it a go some time.

In the long term I think I would be inclined just to get an SD2PET as that seems like a mature and well tested and well made accessory, but as Colin pointed out, he's spent quite enough money for the moment and this project is something which would be cheap and potentially fun to try since he already has a collection of Pis lying about.
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