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Old 12th Feb 2021, 3:39 pm   #121
ajgriff
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Default Re: Commodore PET 2001

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The PET 2001-8 uses the early #320009 assembly.
Wrong again! The assembly in this datasette is actually #320275 (dated 1979). I was guilty of making assumptions before I'd looked closely at the PCB.

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Old 14th Feb 2021, 2:12 pm   #122
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Default Re: Commodore PET 2001

Turned on the PET this morning and was presented with the random character screen.

Have checked video RAM and system RAM by substitution with no success. I suppose I might be looking at a failed ROM. For now I'm just depressed.

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Old 14th Feb 2021, 3:21 pm   #123
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Default Re: Commodore PET 2001

I would like to hope this is more likely a re-emergence of an old problem than a brand new problem like a ROM failure - why would that happen at first, then un-happen, then happen again? Usually, a dud chip stays dud.

I'd be more inclined to suspect an intermittent contact on one of those famously dodgy sockets, if you have the patience to meter them all out.

If you manage to convince yourself that you really do have a ROM problem I'd be happy to read / verify them for you, as I'm sure others here would as well, if you don't mind letting them go out of sight for a few days.

The PROMs I got from Colin over in the other thread had such heavily oxidised pins that I could only read one of them to begin with, I had to scrape the edges of the pins (where the ZIF socket contacts on my programmer make contact) before I was able to read them all. It appears that machine has spent a long time in a damp place. Yours appears to have been stored in a better environment.
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Old 14th Feb 2021, 4:59 pm   #124
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Default Re: Commodore PET 2001

Thank you for your support and encouragement. I certainly need it at the moment.

Lke you I don't think something's happened, unhappened and happened again. The original problem was undoubtedly down to failed RAM chips and that doesn't seem to be the case now. My feeling is that this is a different failure from the original RAM issue.

One thing that makes me suspicious of the ROMs is the readings I'm getting at the 5V power rails. My recollection is that these were all at more or less the same value when I checked them back at the beginning of January. Now three of the 7805s are giving readings of 5.06V and one is slightly lower at 4.96V. Not much of a difference but the low 7805 happens to be the one that's supplying the ROMs.

As for corroded pins, I've removed and re-seated most of the socketed ICs at least once during this adventure without finding the slightest sign of corrosion. Although the unreliability of the PET's sockets is well documented I really don't think that's the issue here especially as the machine has been kept in ideal storage conditions, albeit for a good few decades.

Alan
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Old 15th Feb 2021, 3:45 pm   #125
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Default Re: Commodore PET 2001

Don't give up hope...

For the Video Screen to clear there needs to be quite a lot working!

Reset, clock, 6502, Address decoding, buffers to video RAM, ROM (Kernel, Editor and possibly BASIC)...

You could burn the VOSSI tester https://github.com/vossi1/2001test into a replacement 6540 ROM to eliminate most of the ROMs; you would need an adapter or use a ROM replacement board.

On the bright side you have a working monitor and characters on the screen, that proves you have at least a partially working machine.
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Old 16th Feb 2021, 12:53 pm   #126
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Default Re: Commodore PET 2001

Having carried out some further checks this morning with 'scope and meter I think it's worth summarising the evidence as I see it. Please bear in mind that the PET was fully operational using NivagSwerdna's 8k system RAM card until Sunday morning when the random character screen re-emerged.

CPU Reset - circuit is working with the expected delay and pin 40 of the CPU then stays high.

Clock Signal - working with the expected waveform at pin 39 of the CPU.

CPU Data Lines - all showing hi-lo signals.

CPU Address Lines - all held high.

System RAM - substituting four known good 6550s for the 8k card makes no difference.

CPU - the random character display would not normally be associated with complete 6502 failure.

Video RAM - the nature of the random character screen suggests no problem and substituting known good RAM confirms.

Video ROM- again the random nature of the character screen suggests no issues here.

PIAs - removing these two chips makes no difference.

PSU Rails - the 5V rail supplying the ROMs appears to be being pulled down by 0.1V.

My overall conclusion to this is that one of the system ROMs has failed (likely) or that one of the unsocketed logic/buffer chips has given up the ghost (less likely). Although I'd not originally envisaged spending much money on this little project I'm inclined to think in terms of buying a ROM/RAM emulator. All thoughts/suggestions welcome.

Alan
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Old 16th Feb 2021, 2:20 pm   #127
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Default Re: Commodore PET 2001

The offer remains to read / verify those for you at no cost, other than whatever it costs to send them back and forth and the minor risk that they might vanish in the post.

You could also venture into the world of Arduino, where an Arduino Mega 'Clone' costs about £11-£15. You could then use that to read the code out of your PROMs (the code to make the Mega 'be' a PROM reader is already essentially written and available) and after doing that you'd still have the Arduino as a very useful multi-purpose experimentation / test tool.

The Raspberry Pi, one with a 40-pin I/O connector, could also be pressed into this role with some additional hardware but if you have not used one before that would send you off on a huge diversionary arc while you learn your way around the Pi before coming back to the main quest.

Alternatively, if you can find someone selling adaptor PCBs which will convert the footprint of a common EPROM like a 2716 / 2732 etc to that of one of your PET proms you could use that to substitute your ROMs one by one - but then you are into the realms of either having to programme the EPROMs or get them programmed.

For the other ICs, You can buy dedicated logic IC testers for not much, I don't have one myself because that functionality is a feature of my device programmer so I'll leave it to others to suggest recommendations.
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Old 16th Feb 2021, 3:42 pm   #128
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Default Re: Commodore PET 2001

Sadly the 6540 ROMs are not terribly reliable, as I mentioned before even back in the 70s they were failing after a few weeks of use, I suspect I will be making a set of 2716->6540 adapters when I finally get my hands on my PET.
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Old 16th Feb 2021, 4:24 pm   #129
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I've just had a look at the pinout for what I think was the 6540 - five CS inputs and a clock input! On a PROM??!!

What do you do with the clock signal when substituting a modern EPROM, just ignore it?
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Old 16th Feb 2021, 6:47 pm   #130
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Default Re: Commodore PET 2001

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You could also venture into the world of Arduino, where an Arduino Mega 'Clone' costs about £11-£15. You could then use that to read the code out of your PROMs (the code to make the Mega 'be' a PROM reader is already essentially written and available) and after doing that you'd still have the Arduino as a very useful multi-purpose experimentation / test tool.

The Raspberry Pi, one with a 40-pin I/O connector, could also be pressed into this role with some additional hardware but if you have not used one before that would send you off on a huge diversionary arc while you learn your way around the Pi before coming back to the main quest.

Alternatively, if you can find someone selling adaptor PCBs which will convert the footprint of a common EPROM like a 2716 / 2732 etc to that of one of your PET proms you could use that to substitute your ROMs one by one - but then you are into the realms of either having to programme the EPROMs or get them programmed.
All informative input as always. I'm normally enthusiastic about learning new things but on this occasion I'm very much focussed on getting the PET up and running again. One of the attractions of an emulator is the future proofing angle as the 6540s and 6550s seem to be so unreliable. Funnily enough though I do have a Raspberry Pi so I may come back to the intricacies of PROM reading/programming in the future.

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Old 16th Feb 2021, 7:18 pm   #131
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Default Re: Commodore PET 2001

If you go down the EPROM emulator route make sure you find one which can specifically emulate the MOS 6540 PROM because they have an unusual (I would go so far as to say - freakish) pinout. It could not be more different to the 2716, 2732, 2764, 27128 types which a typical EPROM emulator will emulate.

You could still use one of those emulators if you also bought an adaptor PCB designed to make one of the aforementioned standard EPROM types 'look' like a MOS 6540 PROM.

Of course it may be that there are emulators specifically aimed at emulating the MOS devices, with PET owners in mind. I didn't look that far.
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Old 16th Feb 2021, 7:19 pm   #132
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Default Re: Commodore PET 2001

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I've just had a look at the pinout for what I think was the 6540 - five CS inputs and a clock input! On a PROM??!!

What do you do with the clock signal when substituting a modern EPROM, just ignore it?
The 6502 times the transfer of data to the high side of the phi-2 clock. The 6540 therefore doesn't output on the bus until Phi-2 is high,

An eprom would have to have the CS deode include the Phi-2 signal.

Attached is a design for a PROM adapter I designed but have never tested
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Old 16th Feb 2021, 7:22 pm   #133
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Looks like this:
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Old 16th Feb 2021, 7:26 pm   #134
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Default Re: Commodore PET 2001

Well, if aj feels like getting the soldering iron out he could always test drive it for you (but it would still require either a programmed 2716 or an EPROM emulator able to emulate the 2716).
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Old 16th Feb 2021, 7:28 pm   #135
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Well, if aj feels like getting the soldering iron out he could always test drive it for you (but it would still require either a programmed 2716 or an EPROM emulator able to emulate the 2716).
Or a 2816 Flash EEProm which can be programmed with an Arduino....
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Old 16th Feb 2021, 7:39 pm   #136
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Default Re: Commodore PET 2001

Happy to get the soldering out! It's the other stuff that bothers me.

Alan
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Old 16th Feb 2021, 8:02 pm   #137
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Or a 2816 Flash EEProm which can be programmed with an Arduino....
Which aj doesn't currently have. In fact I'm surprised to hear he has a Pi, he kept that pretty quiet...

I'm just about to try reading a 2732 EPROM with a Raspberry Pi with only the minimum necessary additional hardware, series resistors between data-out on the EPROM and data-in on the Pi, just to see if it works. The main chore will be the inter-wiring between the Pi and the EPROM. I may be some time...
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Old 16th Feb 2021, 11:08 pm   #138
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Default Re: Commodore PET 2001

After much havering and wavering I've ordered a ROM/RAM emulator. Of course I still have the datasette to sort out which is where this latest escapade began when I switched on the PET in order to align the datasette's head. I'm not sure now whether or not any future owner would even be prepared to cover the costs incurred.

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Old 16th Feb 2021, 11:18 pm   #139
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Default Re: Commodore PET 2001

I gave it a go with the Pi- although driving the address pins with 3v3 works fine the data is occasionally being misread with the odd bit in the wrong state. Obviously a row of resistors between the EPROM data pins and the Pi isn't quite sophisticated enough.

I'm going to try using an 8-Darlington array (ULN2803A, etc) as a data level shifter - they are inverting and open-collector but the Pi's pins can be individually set to have onboard pullups and of course it is a simple matter to de-invert the data after it is read in. Unfortunately I can't find a single ULN2803 in all of my junk here - but I know I have some at work so I will continue tomorrow.

The reason I favour the ULN IC rather than say 2 X 74HC4051 is that they are intrinsically intuitive to wire up - all the inputs are down one side of the IC and all of the corresponding outputs are on the same pins on the opposite side, so it's harder to mess up.

(I note that aj has gone ahead with his idea of using an emulator, but I've started so I may as well finish).
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Old 16th Feb 2021, 11:25 pm   #140
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(I note that aj has gone ahead with his idea of using an emulator, but I've started so I may as well finish).
It's a worthwhile excercise, the results may be useful to some future problem.
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