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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
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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 And this mainly first occurred when 'C-MOS' superseded N-MOS, and P-MOS (But the Rds(on) of the original CMOS did limit the current and prevent them blowing-up due to 'shoot-through', if inputs floated).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
SiriusHardware
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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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Old 17th May 2021, 9:51 am   #23
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Default Re: Raspberry Pi as a storage device for a PET.

Hi there,

it is great to read about my little project, here!

I'd like to add the following to the discussion:

Quote:
It doesn't appear that they've specified any transistor types, but it should work with an small signal ones like BC547 (NPN) & BC557 (PNP) etc.
Yes! BC547 & BC557 are recommended in the README (https://github.com/RhinoDevel/cbmtap....7.0/README.md).

Quote:
I'd also be inclined to build it a bit more permanently on a bit of matrix board / veroboard, as parts are all quite low-cost, rather than using prototyping breadboard.
That is always a good idea (mentioned in the README, too)!
The breadboard is used to avoid "unnecessary" soldering to not scare away beginners (at least that was the intention).

Quote:
And you could always use a read-made level converter board, for both directions ...
One of the project goals was to use discrete components for the interface, only (to keep it as simple as possible).

Quote:
The only thing I could wish for is, if you were going to connect something as complex and powerful as a Pi, it might as well be emulating the FDD (ie, functionally equivalent to the SD2PET) rather than the datasette as that would seem like a better way to use one.

The Pi would then have to be interfaced to the IEEE 488 port, and that would present its own complications. It might be worth looking around to see if someone has already done it.
CBM Tape Pi is no real tape or disk emulator.
But it uses the tape port, only (you should be able to connect real disk drives at the same time for copying, etc.).
It also has the wedge and its fast mode (~2.5 Kbyte/s).
And it is compatible with VIC 20 and C64, too.

***

About the design of the Pi to CBM READ pin:

The VIC 20 (6522/CA1) and PETs (6520/CA1) do not seem to have a pull-up
resistor (neither internal nor external), but the C64 (6526/Flag) definitely has
one.

R4, R5 and T1 make the connection work with and without pull-up resistor.

By the way, the Pi to CBM SENSE line is always connected to a pull-up,
otherwise a button press would not be detected (C64: 6510/P4, VIC 20: 6522/PA6,
PETs: 6520/PA4). That is way there is no PNP for SENSE.

Please correct me, if I am wrong here or if there is a simpler way to solve this
problem.
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Old 18th May 2021, 7:17 am   #24
SiriusHardware
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Default Re: Raspberry Pi as a storage device for a PET.

Hi RD, thanks for joining and explaining some aspects of your project, which we are obviously very interested in. We have just been engaged in a very long PET repair thread here and we were looking at this specifically with respect to the PET, so we did not have your insight into the fact that it also needs to be compatible with the other Commodore computers mentioned.

Thanks for making the project available to all. As I said several times earlier, it seems clear to me that this is a well tested project, so I can't see any need to change anything without a good reason.
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Old 18th May 2021, 10:02 am   #25
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Default Re: Raspberry Pi as a storage device for a PET.

Thank you!

But as my experience with building hardware is limited (I am "unfortunately" a software developer in real life), I am always interested in suggestions for improvements from people with more knowledge/experience in circuit design, etc.

And now I will read that PET repair thread!
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Old 18th May 2021, 9:02 pm   #26
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Default Re: Raspberry Pi as a storage device for a PET.

Quote:
Originally Posted by ScottishColin View Post
I have a few spare Pis around - I may try this and report back.

https://github.com/RhinoDevel/cbmtap....7.0/README.md

I'd be interested in other people's views.

Colin.
Project looks good , it should also work for other tape based vintage computers like the Nascom etc
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Old 19th May 2021, 2:57 pm   #27
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Default Re: Raspberry Pi as a storage device for a PET.

Good luck reading the whole thread - you'll be there for some time (it was my PET)...

Colin.


Quote:
Originally Posted by RhinoDevel View Post
Thank you!

But as my experience with building hardware is limited (I am "unfortunately" a software developer in real life), I am always interested in suggestions for improvements from people with more knowledge/experience in circuit design, etc.

And now I will read that PET repair thread!
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Old 19th May 2021, 3:29 pm   #28
ScottishColin
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Default Re: Raspberry Pi as a storage device for a PET.

Just a quick update - I have made up the breadboard and got a PET cassette port connector and should be able to try this soon. I'm not 100% clear that I have wired it all together correctly, but that's part of the fun I guess.

Before I plug anything together, is there any possibility I may damage either the PET or the Pi (less of a problem if I'm honest) with this?

Thanks.

Colin.
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Old 19th May 2021, 6:13 pm   #29
SiriusHardware
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Default Re: Raspberry Pi as a storage device for a PET.

No doubt RhinoDevel, who knows this project inside out, will be the best person to answer that question. If he really has gone off to read the PET thread we may not hear from him for four or five days.

I would imagine the main thing is to rigidly follow any of RD's advice about the order in which to turn things on / turn things off, and always have both systems turned off before you connect them together or disconnect them.
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Old 19th May 2021, 10:57 pm   #30
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Default Re: Raspberry Pi as a storage device for a PET.

The interface itself is very low-risk of being able to cause any damage, as it doesn't really source any current into the Tape connector. So there shouldn't be any particular power-sequencing requirements.

Although it might not be a bad idea to add around 220R in series with the 2 outputs, so that there's no direct connection (without a current-limiting resistor) on any connection other than the common Gnd pin. - But this is more to protect this interface's output transistors from being damaged due to a connection fault.

However, much more of a risk is mis-wiring / a short on the tape connector itself - particularly accidentally connecting the higher (9V) Motor-voltage output to anything else.
And I would recommend ensuring your tape connector has a key-way fitted (apparently the cheaper ones sold don't have this), to prevent it being plugged-in the wrong way round as well as aid alignment when plugging-in.
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Old 20th May 2021, 7:20 am   #31
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Default Re: Raspberry Pi as a storage device for a PET.

Quote:
Originally Posted by ScottishColin View Post
Before I plug anything together, is there any possibility I may damage either the PET or the Pi (less of a problem if I'm honest) with this?
As ortek_service said, make sure that you connect the motor signal the right way.

You can also measure the motor signal voltage. Anything between 5.9V and 10.3V should be no problem.

SiriusHardware mentioned the turn on/off order written in the README.
I wanted to be on the save side (e.g. for a very short time after Pi power-on the GPIO pins are in there default states, mostly configured as inputs with/without some pull up/down resistor).

"Unofficially": If you do not follow this order, nothing bad should happen. Resetting the Pi via button when the CBM is on and connected is OK.

If you change the tape port lines' directions by software on the CBM (e.g. via POKEs) you may damage the Pi (but this is also true for a normal datassette drive connected as far as I know).

Don't disconnect something when Pi and/or CBM is powered-on (I think you know that already).

Quote:
Good luck reading the whole thread - you'll be there for some time (it was my PET)...
Impressive thread and a good idea to blog about it! I have a 4032 (and some other stuff) that needs a repair. This will be helpful.
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