29th Jan 2021, 7:53 pm | #341 | |
Octode
Join Date: May 2012
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Re: Non-working Commodore PET 3016
Quote:
Pin 1 testing PET off, Red pin in Pin 1, meter set to 2k: 9 - 0.628 10 - 0.629 11 - 0.629 12 - 0.630 13 - 0.630 14 - 0.629 15 - 0.630 16 - 0.626 17 - 0.631 18 - 0.630 19 - 0.630 20 - 0.631 22 - 0.630 23 - 0.630 24 - 0.630 25 - 0.556 So Pin 25 looks like an outlier to me? PET off, Black pin in Pin 1, meter set to 2k: All readings were 1. I tried all meter settings and the readings stayed at 1. no matter what |
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29th Jan 2021, 7:54 pm | #342 |
Octode
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Re: Non-working Commodore PET 3016
I watch a lot of those by Jerry Walker and he seems to find a lot of machines with those chips faulty.
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29th Jan 2021, 7:55 pm | #343 | |
Octode
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Re: Non-working Commodore PET 3016
Quote:
Colin. |
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29th Jan 2021, 7:58 pm | #344 |
Dekatron
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Re: Non-working Commodore PET 3016
That slight difference on 25 (A15) isn't enough to alarm me unduly. Although the resistance looks rather low on 2K range it rises to over 10K on your next range, so I understand, so I think this is OK.
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29th Jan 2021, 8:03 pm | #345 | |
Dekatron
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Re: Non-working Commodore PET 3016
Quote:
What I'd like now is for you to reinstate the address pin contacts ie, pins 9-20 and pins 22-25 in your modified socket so that you can insert the 6502 with all the address pins connected, but the data pins (26-33) still disconnected. With the modified socket fitted in the CPU socket and the CPU fitted in the modified socket, power on and use your scope to look for activity on the address lines starting with pin 9, 10, 11.... |
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29th Jan 2021, 8:09 pm | #346 |
Dekatron
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Re: Non-working Commodore PET 3016
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29th Jan 2021, 8:32 pm | #347 | |
Octode
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Re: Non-working Commodore PET 3016
Quote:
Some variants of the 6502 (like the Sally used in some Atari 8 bits) have a /HALT input, and the 65C02 has a BE input to disable the busses but the original 6502 used in the PET and VIC20 does not. |
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29th Jan 2021, 8:50 pm | #348 |
Octode
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Re: Non-working Commodore PET 3016
With power off the resistance between 5v and 0v might be quite low, so red on 5v and black on address lines may just be measuring the parasitic diode from 0v to the inputs of the buffer, which would not be a fault condition.
I would suggest putting a 1k resistor from A0 to 0v, without the 6502 connected to the address lines, then measure the voltage across that resistor with the board powered up. The voltage measured should be less than 0.8v. |
29th Jan 2021, 8:54 pm | #349 | ||
Octode
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Re: Non-working Commodore PET 3016
Quote:
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29th Jan 2021, 9:13 pm | #350 |
Nonode
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Re: Non-working Commodore PET 3016
Would Mark1960's pull-down idea be better as a pull-up ?
NMOS is resistive pull-up and active pull-down. dc |
29th Jan 2021, 9:16 pm | #351 |
Octode
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Re: Non-working Commodore PET 3016
Check the outputs of C3 for activity with the scope, pins 18, 3 and 16, should be the same as 9, 10 and 11 from the 6502.
If that looks OK then verify all the outputs of both 74ls244. If that checks out then it seems to be pointing to ram or rom problems. I think next step would be to connect the data lines again, to see if activity on 9, 10 and 11 stops, just in case you fixed something without realising. Is the ram in your pet in sockets or only the rom? |
29th Jan 2021, 9:19 pm | #352 |
Octode
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Re: Non-working Commodore PET 3016
That was an idea when I thought faulty buffers might be driving the address lines of the 6502 high due to a fault in the buffer inputs, latest from Colin seems to eliminate that.
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29th Jan 2021, 9:30 pm | #353 |
Dekatron
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Re: Non-working Commodore PET 3016
Ah, but... see here and scroll down to 'The KIL Opcodes'
https://www.pagetable.com/?p=39 If the system is broken to the extent that the CPU is not receiving valid opcodes to execute it may very well be ingesting one of these undocumented halt instructions from the databus. This could be the reason why the CPU appears catatonic, but only when it is reading the actual data bus. |
29th Jan 2021, 9:38 pm | #354 | |
Octode
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Re: Non-working Commodore PET 3016
Quote:
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29th Jan 2021, 9:54 pm | #355 |
Dekatron
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Re: Non-working Commodore PET 3016
Again building on what Mark suggested, if you feel like enhancing your Scope-Fu, Colin, with the databus pins still disconnected, try using two probes and both channels of your scope to look at both of these pairs of pins at the same time: (You'll need the second channel turned on to try this, and both probes set to x10 with the dashboard settings also at x10).
The purpose of this is to verify that the buffers UB3 / UC3 are working. If they are you should get the same 'picture' on both channels of your scope, because you are looking at each address line first before, then after it has passed through the buffer. There may be some differences in height / shape but the highs and lows should be in the same places on both traces. As you work higher up through the address pins the pulses will become longer and less frequent, if so keep dialling down the horizontal Time / Div setting on the scope so you can always see at least several pulses. 6502 pin 9, UC3 pin 18 6502 pin 10, UC3 pin 3 6502 pin 11, UC3 pin 16 6502 pin 12, UC3 pin 5 6502 pin 13, UC3 pin 14 6502 pin 14, UC3 pin 7 6502 pin 15, UC3 pin 12 6502 pin 16, UC3 pin 9 6502 pin 17, UB3 pin 18 6502 pin 18, UB3 pin 3 6502 pin 19, UB3 pin 16 6502 pin 20, UB3 pin 5 6502 pin 22, UB3 pin 14 ; Note the jump from CPU pin 20 to 22 6502 pin 23, UB3 pin 7 6502 pin 24, UB3 pin 12 6502 pin 25, UB3 pin 9 You need not necessarily post images of all these checks, just satisfy yourself that each address line looks the same on the output side of the buffer as it does at the CPU. If any don't, then of course we'll want more detail. Last edited by SiriusHardware; 29th Jan 2021 at 10:04 pm. |
29th Jan 2021, 10:46 pm | #356 | |
Octode
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Re: Non-working Commodore PET 3016
Quote:
Colin. |
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29th Jan 2021, 10:49 pm | #357 | |
Octode
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Re: Non-working Commodore PET 3016
Quote:
Colin. |
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29th Jan 2021, 10:55 pm | #358 |
Dekatron
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Re: Non-working Commodore PET 3016
Exactly so.
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29th Jan 2021, 11:00 pm | #359 |
Octode
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Re: Non-working Commodore PET 3016
Thanks - tomorrow.
Incidentally, I found this Televideo service manual which has the first three pages talking about 6502 testing. In particular, page 3 shows the wave forms and which pins should have similar waveforms to the others on the 6502. I know it's not what you are asking me to test but I just thought it was interesting - looks like it should be applicable to the PET doesn't it? |
29th Jan 2021, 11:17 pm | #360 |
Dekatron
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Re: Non-working Commodore PET 3016
I've just had a quick look - it's a cross between what you are already doing and something Mark referred to before, a 'NOP' test. 'NOP' (No Operation) is a microprocessor instruction which doesn't do anything, it just uses up one unit of microprocessor instruction time.
Normally the opcodes / data which the CPU fetches and executes come from either PROM or RAM but there is a trick (which is described in your document) where you disconnect the CPU data pins from the system and hardwire the CPU data pins so that the CPU always reads a 'NOP' instruction - in the case of the 6502 that would be the hex code 'EA' - as mentioned in your document. Why would you do that? Well, because if you keep the CPU fed on a diet of NOP instructions it runs smoothly up through the address range producing nice regular patterns on all of the address lines, and while it's doing that it is also generating chip select signals and doing other generally useful things which can be looked at. So yes, it's a valid technique and we may well ask you to do something like that a little way down the line. You are almost doing it now, except that you aren't imposing a NOP instruction on the data pins at the moment so the CPU is just executing whatever code it sees there over and over again, probably hex FF. |