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Looking for help troubleshooting a PET 4032 with no video
- Thread starter rglenn
- Start date
daver2
10k Member
The Open2372 adapter will certainly not work (from reviewing the schematic).
I can't find the schematic for the other adapter unfortunately.
Dave
I can't find the schematic for the other adapter unfortunately.
Dave
daver2
10k Member
Ignore what I said about the Open2372 adapter. I had a thought whilst eating my tea!
Give me a few minutes...
Dave
Give me a few minutes...
Dave
daver2
10k Member
I think I have got it...
Your 27C256 device is a 32K EPROM whereas a 2716 device is a 2K EPROM.
You need to create a 32K image by merging multiple copies of my PETTESTER together in BINARY and programming your 27C256 device.
This now means that the higher address lines should have no effect on the contents of the EPROM, as multiple copies are contained within the one device.
After programing your 27C256 device you need to mess up two pins out of the 28!
Bend pin 2 of the 27C256 EPROM out and attach it to pin 27 of the 27C256 EPROM (still allowing the EPROM to be inserted into the adapter socket). This cross-connects A12 with A14.
Likewise, bend pin 20 out and attach it to pin 23. This cross-connects /CE with A11.
Insert the 27C256 EPROM into the Open2327 adapter and cross your fingers...
Dave
Your 27C256 device is a 32K EPROM whereas a 2716 device is a 2K EPROM.
You need to create a 32K image by merging multiple copies of my PETTESTER together in BINARY and programming your 27C256 device.
This now means that the higher address lines should have no effect on the contents of the EPROM, as multiple copies are contained within the one device.
After programing your 27C256 device you need to mess up two pins out of the 28!
Bend pin 2 of the 27C256 EPROM out and attach it to pin 27 of the 27C256 EPROM (still allowing the EPROM to be inserted into the adapter socket). This cross-connects A12 with A14.
Likewise, bend pin 20 out and attach it to pin 23. This cross-connects /CE with A11.
Insert the 27C256 EPROM into the Open2327 adapter and cross your fingers...
Dave
daver2
10k Member
This (I believe) is the modification I suggested:
I don't think R4 and jumper JP1 exists on your board. Yours looks to be an earlier revision than the schematics currently available.
From your photograph, I believe R1, R2 and R3 are 0 Ohm resistors - so a short-circuit to +5V.
Bend pins 2 and 20 of the 27C256 at 90 degrees to the IC body so they do not make contact with the IC socket, and link them as shown in red.
The schematic showed a 27128 EPROM. Your 27C256 EPROM has an extra address line (A14) on pin 27.
The code within the EPROM is accessed as 1110XXXXXXXXXXX - however, as we have duplicated my 2K PETTESTER code throughout the entire EPROM contents, it doesn't really matter about the higher address lines. The key thing is that when A11 is LOW (EPROM pin 23) that this also drives the /CE (pin 20) of the EPROM to enable the EPROM. And when A11 is HIGH, the EPROMs /CE pin is also HIGH - thus disabling the EPROM.
Dave
I don't think R4 and jumper JP1 exists on your board. Yours looks to be an earlier revision than the schematics currently available.
From your photograph, I believe R1, R2 and R3 are 0 Ohm resistors - so a short-circuit to +5V.
Bend pins 2 and 20 of the 27C256 at 90 degrees to the IC body so they do not make contact with the IC socket, and link them as shown in red.
The schematic showed a 27128 EPROM. Your 27C256 EPROM has an extra address line (A14) on pin 27.
Code:
AAAAA
11111AAAAAAAAAA
432109876543210
HHHLXXXXXXXXXXXThe code within the EPROM is accessed as 1110XXXXXXXXXXX - however, as we have duplicated my 2K PETTESTER code throughout the entire EPROM contents, it doesn't really matter about the higher address lines. The key thing is that when A11 is LOW (EPROM pin 23) that this also drives the /CE (pin 20) of the EPROM to enable the EPROM. And when A11 is HIGH, the EPROMs /CE pin is also HIGH - thus disabling the EPROM.
Dave
daver2
10k Member
Well done...
The ROM checksum display indicates that UD7 (901465-21) appears to be faulty (the d=9f59 should be d=a425).
The DRAM passed the initial memory testing. Let the 'countdown' reach 0 and it will enter a full MARCH-C memory test. See if this test runs continuously up to about a count of 20 (or so) and that the display indicates that 32K is being tested. If it does, the RAM is OK.
If the RAM is OK, it could be the faulty ROM in UD7 that is causing the black screen - or the EDIT ROM (that has now been replaced by PETTESTER) is also faulty.
Dave
The ROM checksum display indicates that UD7 (901465-21) appears to be faulty (the d=9f59 should be d=a425).
The DRAM passed the initial memory testing. Let the 'countdown' reach 0 and it will enter a full MARCH-C memory test. See if this test runs continuously up to about a count of 20 (or so) and that the display indicates that 32K is being tested. If it does, the RAM is OK.
If the RAM is OK, it could be the faulty ROM in UD7 that is causing the black screen - or the EDIT ROM (that has now been replaced by PETTESTER) is also faulty.
Dave
daver2
10k Member
I would leave it running a bit longer if it were me - but it is looking good...
Are you able to read the PET EDIT ROM within your programmer/reader? If you are - then you can compare it to the correct image on the Zimmer's website to see if it is 'good'.
Either that, or put the EDIT ROM back into your PET (replacing PETTESTER) and use your Tynemouth ROM/RAM replacement board. Disable the Tynemouth RAM (so you are using the PETs own RAM) and you can 'mess around' with the ROM settings (bearing in mind that the D000 ROM looks faulty). You should be able to test each physical PET ROM by suitable adjustment of the switches. We can talk through this later.
Dave
Are you able to read the PET EDIT ROM within your programmer/reader? If you are - then you can compare it to the correct image on the Zimmer's website to see if it is 'good'.
Either that, or put the EDIT ROM back into your PET (replacing PETTESTER) and use your Tynemouth ROM/RAM replacement board. Disable the Tynemouth RAM (so you are using the PETs own RAM) and you can 'mess around' with the ROM settings (bearing in mind that the D000 ROM looks faulty). You should be able to test each physical PET ROM by suitable adjustment of the switches. We can talk through this later.
Dave
Last edited:
daver2
10k Member
It looks like you have a different board to here: https://oe7twj.at/images/a/a7/PET_ROM_RAM_Board_User_Guide.pdf.
I was just re-reading the posts within the thread. Have we actually replaced anything on the PET logic board?
I note that you stated that when you tested your machine with the ROM/RAM board it did not work.
Dave
I was just re-reading the posts within the thread. Have we actually replaced anything on the PET logic board?
I note that you stated that when you tested your machine with the ROM/RAM board it did not work.
Dave
Last edited:
daver2
10k Member
What state are switches 3, 4, and 5 in when switches 1 and 2 are ON?
But you get a 'random character' screen and not a black screen (as you were before)?
How do you know whether the ROM/RAM board is working or not?
Dave
But you get a 'random character' screen and not a black screen (as you were before)?
How do you know whether the ROM/RAM board is working or not?
Dave
I tried several of the switch 3,4,&5 combinations. I have another PET computer where the ROM/RAM usually lives. It has 8k of ram but shows 32K when the ROM/RAM board is bypassing.
But you get a 'random character' screen and not a black screen (as you were before)? Correct.
But you get a 'random character' screen and not a black screen (as you were before)? Correct.
daver2
10k Member
Ideally you want to select BASIC 4 (ON, ON, ON, OFF).
If it works in another PET, then it is possible that the /IRQ pin of the CPU is being held LOW for some reason.
Use the ROM/RAM board and use your oscilloscope to measure CPU pins 4, 40, and 7. Tell me what you observe.
Dave
If it works in another PET, then it is possible that the /IRQ pin of the CPU is being held LOW for some reason.
Use the ROM/RAM board and use your oscilloscope to measure CPU pins 4, 40, and 7. Tell me what you observe.
Dave
daver2
10k Member
Pin 4 should not be permanently LOW - unless the CPU has stopped executing instructions.
Pin 7 tells me whether the CPU is running. If it is permanently LOW then the CPU has stopped executing instructions. This pin should be pulsing HIGH and LOW.
Let me have a think where we go from here. You definately require a new $Dxxx ROM to move forwards - but this does not address why the ROM/RAM board doesn't work...
Dave
Pin 7 tells me whether the CPU is running. If it is permanently LOW then the CPU has stopped executing instructions. This pin should be pulsing HIGH and LOW.
Let me have a think where we go from here. You definately require a new $Dxxx ROM to move forwards - but this does not address why the ROM/RAM board doesn't work...
Dave