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Old 24th Feb 2021, 4:57 pm   #181
Mark1960
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Default Re: Commodore PET 2001

Did WDC change the interrupt output from open collector to totem pole, meaning its not possible to connect them wire-or as they might be in the PET?

Edit: I took a quick look at the W65C21N/S spec, the interrupt outputs are open drain, so not a problem in the PET.

Last edited by Mark1960; 24th Feb 2021 at 5:05 pm.
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Old 24th Feb 2021, 5:27 pm   #182
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Default Re: Commodore PET 2001

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Originally Posted by Mark1960 View Post
Edit: I took a quick look at the W65C21N/S spec, the interrupt outputs are open drain, so not a problem in the PET.
That's a relief!

Alan
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Old 24th Feb 2021, 5:56 pm   #183
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Default Re: Commodore PET 2001

I’ve been experimenting with the emulator and used its diagnostic tools to prove that the PET’s original ROMs are working just fine. In fact, with the emulator’s ROMs and RAM disabled the machine runs as originally intended using the 8k RAM board instead of the unreliable 6550s. It could reasonably be argued that I didn’t need to buy the emulator but on balance I don’t regret the purchase for the following reasons:

1. The emulator makes fault finding much easier with a choice of two diagnostic tools (there’s an NOP option available as well) and let’s face it these are not the most reliable of machines.

2. The PET can now be set to run with 32k of RAM like some of its later brethren.

3. ROM functions can be taken over by the emulator in the event of 6540 failure.

4. The emulator offers the opportunity to run with one of four versions of BASIC.

5. There are other options which either I haven’t investigated or don’t fully understand yet.

Overall the PET is now future proofed as far as it’s possible to be without having lost much, if any, of its originality. There doesn’t seem to be a lot of choice when it comes to off the shelf emulators for the 2001-8 and, bearing in mind the suspension of Tynemouth’s operations, I could only find two currently available alternatives:

- The bitfixer (US based) ROMulator

OR

- NivagSwerdna’s (UK based) ROMulan RAMulator

I opted for the latter partly because I find it easier to deal with UK suppliers but I also found the associated documentation more understandable and simpler to follow. Even after taking import charges into account the UK product does cost a bit more. However, I was impressed with the same supplier’s 8k RAM board and don’t regret my choice although clearly I’m not in a position to comment on the ROMulator as a product.

So far I can definitely recommend NivagSwerdna’s ROMulan RAMulator for use with the PET 2001-8 and this is a link to the offering for anyone who might be interested now or in the future:

https://www.tindie.com/products/niva...zy-nopulation/

Alan
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Old 24th Feb 2021, 7:10 pm   #184
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Default Re: Commodore PET 2001

Forgot to mention that the emulator board is relatively small and unobtrusive once fitted. The attached illustrates.

Alan
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Old 25th Feb 2021, 8:51 pm   #185
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Default Re: Commodore PET 2001

As the PET's waiting patiently for its new PIA to arrive I thought I'd do a bit more cosmetic work on the datasette. Masking up (first photo) so that the tape door lining could be sprayed took a great deal longer than applying a few squirts of silver paint. I used a combination of modellers' masking tape and the standard decorators' variety. The result of the paint job is shown in the second photo. Overall the case is starting to look ok with a bit more detailed cleaning needed. The last shot is a reminder of what the datasette looked like when it arrived at my door.

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Old 25th Feb 2021, 9:23 pm   #186
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Default Re: Commodore PET 2001

Absolutely amazing job, the erstwhile owner would be astonished to see it looking like that. I just wouldn't even think of trying to spray something with such narrow lines, just one leak or bad tape seal and there would be silver mess all over the cassette door. I'd probably have tried to use a very fine hand held brush instead.

One thing, when your replacement 652x arrives, get into the strict habit of never connecting or disconnecting the datasette or anything else to / from the PET while it is powered on. I don't know if the machine was on when you first connected the datasette but if it was, that may have been what killed the 6520.
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Old 25th Feb 2021, 10:34 pm   #187
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Default Re: Commodore PET 2001

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Originally Posted by SiriusHardware View Post
I just wouldn't even think of trying to spray something with such narrow lines, just one leak or bad tape seal and there would be silver mess all over the cassette door. I'd probably have tried to use a very fine hand held brush instead.
The key is the modellers' tape as it doesn't bleed, adheres well and won't pull at the paint when removed. I used this because I had some in stock but there are plenty of other suppliers:

https://www.ebay.co.uk/itm/Modelcraf...kAAOSw2s1Uuh1L

I don't think anyone other than a time served sign writer would have had much luck with brushing by hand.

Quote:
Originally Posted by SiriusHardware View Post
One thing, when your replacement 652x arrives, get into the strict habit of never connecting or disconnecting the datasette or anything else to / from the PET while it is powered on. I don't know if the machine was on when you first connected the datasette but if it was, that may have been what killed the 6520.
Yes that had crossed my mind as a possibility even though I'm aware of the risks associated with hot-plugging I could have been careless. Interestingly the datasette interfaces directly with the VIA (6522) so its hard to see how the PIA (6520) might have been damaged if I did sin. I have to admit however that it's an odd coincidence that the PIA failed after I'd been testing the datasette. Yet another salutary lesson perhaps.

Alan
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Old 28th Feb 2021, 10:22 pm   #188
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Default Re: Commodore PET 2001

The new PIA arrived yesterday and I've now had a chance to fit it. Everything seems to be fine so the PET is happy once more. It's useful to know that the WDC W65C21N is a satisfactory alternative to the original 6520 especially since I've read that some of the more recent WDC versions of Commodore's chips do not always work as drop in replacements in the early PETs.

All I need to do now is finish refurbishing the datasette and hopefully get it working properly.

Alan
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Old 28th Feb 2021, 10:39 pm   #189
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Well done, Alan. Are you really not going to hang onto it now, after lavishing all that TLC?
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Old 3rd Mar 2021, 9:44 pm   #190
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Default Re: Commodore PET 2001

Still haven't decided on the long term fate of the PET despite all the TLC. My problem is that I like my quite varied repair and refurbishment projects to be capable of doing something useful or at least be entertaining. The PET is really struggling to convince me in either regard.

Refurbishment of the datasette's case is now complete. Of the six anchorage points for the three screws holding the case together four of them had suffered damage although partially successful past repair attempts with superglue were still just about ok for two of them. The four offending pillars have been repaired or reinforced using a combination of epoxy and superglue. Even though the fixings are hidden within the case I sprayed a little black paint around just to tidy things up a bit. The photos give an idea of the approach taken.

I've been testing the datasette and have established that the write (record) and erase circuits are working but I get nothing but hash from from the write (play) circuit. Probing with the scope shows that there’s no sensible signal emanating from the first op-amp. The record/play switch is working after a dose of contact cleaner. I suspect a faulty op-amp but all suggestions are welcome. Copies of the schematic and parts list are attached for reference. I’ve been even more careful to avoid hot-plugging after the earlier PIA failure. ‘Mea culpa’ I suspect.

The PET continues to thwart me at every turn.

Alan
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Old 3rd Mar 2021, 10:09 pm   #191
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Default Re: Commodore PET 2001

It doesn't want to leave you and is afraid when it gets better you will cast it off

Impressive quality repair yet again on the plastic.

Is the coil on the read/write head OK as regards resistance reading? - You mention the write circuit works but, could the R22/R21 voltage divider be masking a problem in the head? Not my area of expertise but, failed heads are quite common on the ZX Microdrives...
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Old 3rd Mar 2021, 10:12 pm   #192
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Default Re: Commodore PET 2001

From something TonyDuell mentioned a while back I don't think these are tape recorder / players in the conventional sense of recording audio onto the tape - instead they record a stream of digital spikes or impulses onto the tape and the playback circuit is geared to 'replaying' those and converting them back into a TTL level data stream.

That being the case, it's possible that if you played a 'blank' tape, the noise / hiss on the tape might be converted into full-height random digital noise by the time it gets to the other end of the playback circuit. A music tape would just be amplified up to flat-out, limited and clipped, and look horrible at the output end. I can't confirm this from personal experience though.

I don't know if there is some way you can record a long stream of output to the tape so that you can see what that looks like when it is being replayed.
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Old 3rd Mar 2021, 10:21 pm   #193
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Default Re: Commodore PET 2001

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I've been testing the datasette and have established that the write (record) and erase circuits are working but I get nothing but hash from from the write (play) circuit.
Should say read (play) of course. Editing option timed out.

Alan
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Old 4th Mar 2021, 12:41 am   #194
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Originally Posted by Timbucus View Post
It doesn't want to leave you and is afraid when it gets better you will cast it off.
I appreciate a bit of humour at this point, thank you!

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Originally Posted by Timbucus View Post
Is the coil on the read/write head OK as regards resistance reading? You mention the write circuit works but, could the R22/R21 voltage divider be masking a problem in the head? Not my area of expertise but failed heads are quite common on the ZX Microdrives...
The head’s coil measures about 320Ω. Furthermore it does manage to write short programmes loaded from the PET and records the sorts of sounds you’d expect to hear when the tape is played back using a conventional cassette player. Not my area of expertise either but I think the head’s ok.

Quote:
Originally Posted by SiriusHardware View Post
From something ‘TonyDuell’ mentioned a while back I don't think these are tape recorder / players in the conventional sense of recording audio onto the tape - instead they record a stream of digital spikes or impulses onto the tape and the playback circuit is geared to 'replaying' those and converting them back into a TTL level data stream.
Yes and this is copied from the datasette service manual:

Write/Record Amplifier - The computer outputs a square wave signal to the datasette. These TTL logic-level shifts are converted by the pre-amplifier and power amplifier to a proportional current output. This current output is then applied to the read/write coil of the head. Through induction, magnetic fields are produced on the tape representing the data.

Read/Playback Amplifier - The read amplifier circuit takes the reproduced transition signals from the tape and converts them back to TTL logic-level shifts. This is accomplished by an amplifier limiter which removes the amplitude variations and a switching circuit that toggles the output data between 0 and 5 volts. The signal is then ready for output to the computer.

It’s interesting that the Spectrum (for example) used an ordinary cassette recorder for reading and writing programmes. Presumably there was some form of digitising circuitry built into the machines themselves. I digress.

In the absence of a pre-recorded test tape with a programme on it I started off by recording a series of square waves at different frequencies onto a blank cassette using my audio system and a signal generator. By chance in the search for the original datasette a scrap C64 type C2N (bought many years ago at a boot sale for 10p I think) emerged which now works well enough despite a broken key, hardened belt, cracked case etc. Using the test tape this outputs reasonably clear square waves when in read mode and regrettably the PET style datasette doesn’t.

Alan
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Old 4th Mar 2021, 12:48 am   #195
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Default Re: Commodore PET 2001

I suppose you are at the usual Audio fault finding stage now then of any Caps perhaps that are leaking between stages or as you say simply a failed Op Amp. EDIT: I re-read your earlier post and if there is nothing on the first amp before C1 (i.e Pin 7) then removing one end of C1 will allow it to be tested as a short but, even then some signal should be present - the only other things are R3 and R5 to prevent that stage working.

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Old 4th Mar 2021, 12:55 am   #196
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Unless VREF is not present due to C7?
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Old 4th Mar 2021, 1:16 am   #197
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Ordinarily VREF would be exactly half-supply but in this circuit it is unusually asymmetrical, probably to bias the output into a preferred logic state in no-signal conditions. (In a conventional amplifier the output would sit at half-supply under no-signal conditions). VREF is set by the potential divider R18? / R19? and smoothed / stabilised by C7 as Tim mentioned.

LM358s were very common - in the unlikely event that you have difficulty finding one, I am sure I have some.
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Old 4th Mar 2021, 1:57 am   #198
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Default Re: Commodore PET 2001

I looked at quite a few different methods of interfacing to cassettes for digital storage recently. One of the things I noticed about the commodore cassette interface is that its very similar to saturation recording but still has a separate erase head. I assume from this and the series resistor selected in the write circuit that they are not driving the tape to saturation. I think they must have had a good reason to not drive the tape into saturation at the expense of a separate erase head. Maybe driving to saturation increases the distance between transitions in the data stream.
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Old 4th Mar 2021, 10:44 am   #199
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Default Re: Commodore PET 2001

Quote:
Originally Posted by SiriusHardware View Post
From something TonyDuell mentioned a while back I don't think these are tape recorder / players in the conventional sense of recording audio onto the tape - instead they record a stream of digital spikes or impulses onto the tape and the playback circuit is geared to 'replaying' those and converting them back into a TTL level data stream.

That being the case, it's possible that if you played a 'blank' tape, the noise / hiss on the tape might be converted into full-height random digital noise by the time it gets to the other end of the playback circuit. A music tape would just be amplified up to flat-out, limited and clipped, and look horrible at the output end. I can't confirm this from personal experience though.

I don't know if there is some way you can record a long stream of output to the tape so that you can see what that looks like when it is being replayed.
I don't think pulses would work too well, as would need quite high bandwidth recording to replicate these. However, it seems Commodore did things a bit different (as usual) from other computer tape interfaces and used squarewaves rather than switching between two tones. I presume the squarewaves were low-enough frequency, for enough harmonics to be recorded to playback with a similar waveform.

I'm sure Commodore C64 etc. computer cassette did sound similar to others if played back on audio systems, but you did need a rather custom converter to use a normal tape player to replace the Datasette.
And it seems the same Datasette was compatible with PET VIC-20 & C64, so a similar protocol was used (but maybe slower data rate originally used on PET)

There is quite a bit of detailed info here, on Commodore Datasettes and the waveforms: https://en.wikipedia.org/wiki/Commodore_Datasette
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Old 4th Mar 2021, 10:57 am   #200
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Quote:
Originally Posted by SiriusHardware View Post
Ordinarily VREF would be exactly half-supply but in this circuit it is unusually asymmetrical, probably to bias the output into a preferred logic state in no-signal conditions. (In a conventional amplifier the output would sit at half-supply under no-signal conditions). VREF is set by the potential divider R18? / R19? and smoothed / stabilised by C7 as Tim mentioned.

LM358s were very common - in the unlikely event that you have difficulty finding one, I am sure I have some.
Not to overlook the obvious, I presume the supply voltage on these has been checked? (I couldn't see this mentioned).
And if 2 identical OpAmps are used, then could swap them over (No doubt would need to socket first), if there isn't any compatible spares to hand, to see if different waveform amplitudes are observed / fault changes.

And if a working Datasette has now been found, then could use that to make comparative measurements - assuming it uses the same circuitry.
Plus use it to make some reference PET-recordings as test tapes, if there isn't any pre-recorded ones to hand
- Might also find some VIC-20 / C64 cassettes (if any to hand), might also work OK for making measurements, they use the same recording system.
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