11th Mar 2021, 6:18 pm | #921 |
Dekatron
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Re: Non-working Commodore PET 3016
Don't touch UF7 and UF8 just yet, I would say there is also the option of removing UE7 and UE8 to see if doing that allows the SD... bus to become more active.
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11th Mar 2021, 6:34 pm | #922 | |
Octode
Join Date: Mar 2020
Location: Kitchener, Ontario, Canada
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Re: Non-working Commodore PET 3016
Quote:
I think the 9F might be a batch or date code. Is there any other suffix, example -1 etc? This would indicate the speed of the parts in your PET. It should be possible to use the easier to find 2114, but we should make sure we know what speed you need first. Don’t do anything to them yet. |
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11th Mar 2021, 6:36 pm | #923 |
Dekatron
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Re: Non-working Commodore PET 3016
Mark, what do you say to removal of UE7 / UE8 to see if that lets the video RAM outputs show some activity?
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11th Mar 2021, 6:40 pm | #924 |
Nonode
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Re: Non-working Commodore PET 3016
You're quite right of course. I didn't express myself well but I still wonder if the problem lies before the video RAM/ROM circuit. I note that Colin is now seeing a screen something like the attached image which in the 2001-8 is most often caused by a CPU issue although I know that's been thoroughly checked out in this case.
One thing that strikes me about the various 2001N circuits is that 6114 and 2114 RAMs seem to be used interchangeably. 2114s (or equivalents) seem to be relatively cheap so it might be worth considering trying a couple if the video RAMs are still suspects, although personally I'm not sure they are. Alan |
11th Mar 2021, 6:48 pm | #925 |
Dekatron
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Re: Non-working Commodore PET 3016
I think we seem to have two separate problems, one is that the output from the video RAMS is not appearing on the SD... data bus and the other is that the CPU does not seem to try to clear the video RAM, so there is still a question over whether it is executing code from the ROMs.
Given the number of only tenuously related ICs which we have found dead so far I can only think there are probably still a few to find. I do wonder what can have caused all this mayhem originally. |
11th Mar 2021, 6:59 pm | #926 |
Dekatron
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Re: Non-working Commodore PET 3016
If the RAMs are pin compatible with the 2114 then there is a ready baked Arduino 2114 tester sketch lying about somewhere - however, using that would require removal of the RAMs, intact, from the PCB.
I'm minded to try removal of the UE7 / UE8 buffers first, to see if the video RAM data outputs then come to life. |
11th Mar 2021, 7:07 pm | #927 | |
Nonode
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Re: Non-working Commodore PET 3016
Quote:
Alan |
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11th Mar 2021, 7:09 pm | #928 | |
Octode
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Re: Non-working Commodore PET 3016
Quote:
Check activity on 6502 R/W UC4-34, there should be some activity after power on reset, though it might not be easy to capture. Reset could be applied by grounding UA2-2 instead of turning power off and back on each time. If activity found on UC4-34, then follow that through the chain of buffers. UA10-4, UA3-12, UA3-8 Also check activity on UG1-11, UG7-6, UG7-12, UH4-9. These should be fairly regular square waves. And then check activity on UE3-7, UE3-9, UE4-7, UE4-9, UE5-7, UE5-9, UE6-7. |
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11th Mar 2021, 7:13 pm | #929 |
Nonode
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Re: Non-working Commodore PET 3016
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11th Mar 2021, 7:18 pm | #930 |
Dekatron
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Re: Non-working Commodore PET 3016
It is usual for RAM, on power-up, to contain random data, I have never known a case where RAM, without any intervention from anything else, starts up as all zeros or all ones. (I'd be happy to be corrected on that, though, because if someone can assure me that these RAMS initialise with all bits = 1, that would take the heat off the video RAMs for the time being).
If I'm right though, if everything from UF7-UF8 through UF9 and UF10 onwards was working, we would expect to see random characters rather than the same character repeated. We'll get a better picture (literally) when UF9 is replaced with a known working device, because the original UF9 did not appear to be passing what was on its inputs through to its outputs (all inputs high, but some outputs low). We need that out of the way first. Edit: No problem with Mark's suggestion for looking to see why the video RAM does not get initialised, but it might need to be broken down into more specific steps. |
11th Mar 2021, 7:25 pm | #931 | ||
Octode
Join Date: May 2012
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Re: Non-working Commodore PET 3016
For reference, attached is a photo of the chips. I'll leave them be for now.
Colin. Quote:
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11th Mar 2021, 7:26 pm | #932 | |
Octode
Join Date: May 2012
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Re: Non-working Commodore PET 3016
fyi - I have a spare 6502; tried that and I still get the reverse @ signs.
Colin. Quote:
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11th Mar 2021, 7:27 pm | #933 |
Octode
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Re: Non-working Commodore PET 3016
I’m expecting the power on sequence of the PET is first clear the video ram, then check to see how much ram is connected and display a message on the screen.
However if the ram is not working the PET would be unable to maintain a subroutine stack, which might then prevent it reaching the clear video routine. Due to the number of faulty chips found so far, I’m concerned about the condition of the 4116 dynamic ram as these don’t have a good reputation for reliability and are also susceptible to incorrect power supply sequence. |
11th Mar 2021, 7:29 pm | #934 | |
Octode
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Re: Non-working Commodore PET 3016
Hopefully my regular envelope from Cricklewood Electronics should pop through the letterbox and I'll let you know.
re below: "it might need to be broken down into more specific steps" - yes please..... Colin. Quote:
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11th Mar 2021, 7:37 pm | #935 |
Nonode
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Re: Non-working Commodore PET 3016
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11th Mar 2021, 7:45 pm | #936 | ||
Octode
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Re: Non-working Commodore PET 3016
Mark - I get the attached on UC4/34 after power on. It does not go away even minutes later.
Colin. Quote:
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11th Mar 2021, 7:48 pm | #937 |
Octode
Join Date: Mar 2020
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Re: Non-working Commodore PET 3016
I think we have two separate faults,
1. there is no sign of initialisation of the video ram. 2. There should be random data in the video ram, but we see all the same characters. 2. Could be caused by UE7, UE8, UF7, UF8. 1. Is going to be due to some other part of the PET. If we can fix 1. Then we might be able to see if 2. is caused by the buffers before trying to change them. We have a lot of people contributing, which might make it difficult for Colin to decide what to try next, so I would suggest that maybe Sirius could take the project manager role to try and advise the operations to perform next. |
11th Mar 2021, 7:52 pm | #938 |
Octode
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Re: Non-working Commodore PET 3016
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11th Mar 2021, 8:00 pm | #939 |
Octode
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Re: Non-working Commodore PET 3016
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11th Mar 2021, 8:07 pm | #940 |
Heptode
Join Date: Jan 2009
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Re: Non-working Commodore PET 3016
I am following this massive thread with interest but will not be interfering - just to remark that the "@" display reminded me of this page:-
http://www.dasarodesigns.com/project...dore-pet-2001/ (I realise that it is a different Pet and the CPU has been replaced - but might the clock phase circuitry be similar?) They say:- "This screen is usually the result of a failed CPU. Because the CPU contains the clock divider circuitry that produces the clock-phases used by the video circuitry, if it is not present the characters stored in the video RAM (random noise) will not be displayed and instead the character represented by the code 0x00 is displayed." Switching back to lurking mode...
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