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PET 8032 - No Chirp, No Cursor

KevinO

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Working on a PET 8032. It was a completely black screen until I used the Tynemouth RAM/ROM Replacement board. With the RAM and ROM switched to the test board, I get what looks like a normal boot screen, but without a cursor. I also don't get a chirp.

Poking around the CPU, I see that IRQ and NMI are high. I'm no expert, but I think there should be pulses here.
Also, I get pulses on all the address lines Except A7 is stuck high, and A8-A12 are stuck low.

At this point I could use a poke in the right direction, if anyone can help. Thanks!

20220922_214314.jpg
 

Hugo Holden

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I'm not as familiar with this model, but I have a feeling that it will boot like this, with no cursor, if the Keyboard PIA IC is missing or defective. You could try a swap of the PIA's if you don't have any spares on hand.

If address lines are stuck it could pay to check it out with the NOP generator. However, if any were always stuck, I doubt it would have been able to boot at all.
 

KevinO

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Thanks for the response. Sorry I forgot to mention I did swap the PIAs, but there was no change. Oddly, they weren't socketed on this board! While logic analysis is not my strong suit, I'm pretty good with a soldering station, so no problems with that swap.

I'm going to need a refresher on how to use the NOP generator. I know I wired one up some years ago with the help of someone here, I just have to find it. My memory is horrible, but my first PET repair was about 10 years ago. Maybe that thread still exists...
 

Hugo Holden

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Thanks for the response. Sorry I forgot to mention I did swap the PIAs, but there was no change. Oddly, they weren't socketed on this board! While logic analysis is not my strong suit, I'm pretty good with a soldering station, so no problems with that swap.

I'm going to need a refresher on how to use the NOP generator. I know I wired one up some years ago with the help of someone here, I just have to find it. My memory is horrible, but my first PET repair was about 10 years ago. Maybe that thread still exists...
You probably fitted sockets for the PIA's ? It is possible to have more than one defective PIA, so rather than just the swap you should get more PIA's as spares.

All the NOP generator does is disconnects the Data bus DA0-DA7 from the CPU's data pins, and applies the byte EA to those pins, which is a NOP byte code. So then , if the CPU is working, you end up with a pattern of square waves on the address lines which look very similar to those you would get from the output of a clocked binary counter or binary divider chain. This way the address circuitry can be checked.

But, it appears that your CPU was able to read enough of the ROM code to boot to BASIC, so it appears that the address lines are likely ok. I would concentrate on the Keyboard PIA area for now, is there anything going on that could stop a good keyboard PIA IC from working ?

Or, maybe something has stopped the CPU after the boot message (probably less likely than a PIA issue) and there is possibly ROM corruption. It is easy to check the ROM firmware as it is on Zimmers and you can read the code from the ROMs with the inexpensive GQ-4x programmer to check it, or make new ROMs.
 
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daver2

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The clue is no /IRQ signal. /NMI should be high.

The keyboard is scanned only when an /IRQ is generated. So, no /IRQ no keyboard operation and no cursor flashing either.

Can you check for a signal on UB12 (6520 PIA) pin 18. This should be the vertical drive and is responsible for generating the /IRQ via the keyboard PIA.

Having to use a Tynemouth RAM/ROM replacement board indicates that the RAM and/or ROM is faulty (...).

I think the 'stuck' address lines may be a red herring. The BASIC ROM code will not be doing much at all until it gets an interrupt (which it appears not to). The 'chirp' is partially driven by the keyboard PIA and partially vie the VIA (UB15). Either (or both) of these being faulty could account for your lack of a chirp...

The fact that the CRTC is displaying an image is a good indication that some of the I/O device decoding is working.

There is also a PETTESTER diagnostic that I wrote that can be programmed into an EPROM that replaces the EDIT ROM. This is a fairly low-level set of test routines. This would allow you to interrogate the keyboard PIA and key matrix to see what is working. It doesn't use interrupts.

Dave
 

KevinO

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The clue is no /IRQ signal. /NMI should be high.

The keyboard is scanned only when an /IRQ is generated. So, no /IRQ no keyboard operation and no cursor flashing either.

Can you check for a signal on UB12 (6520 PIA) pin 18. This should be the vertical drive and is responsible for generating the /IRQ via the keyboard PIA.

I will check that when I get home tonight. I know we're 'out of phase' time-wise (GMT-6), so whenever you can reply would be great.
Having to use a Tynemouth RAM/ROM replacement board indicates that the RAM and/or ROM is faulty (...).
Agreed. In fact, if I try to switch EITHER the RAM or ROM on the 8032 back in, I get a blank screen. So my logic, and feel free to tell me if it's faulty, was to bypass the RAM and ROM using the Tynemouth board, and try to fix the OTHER issues first. RAM and ROM are relatively easy, just a lot of soldering. I was also short on 4116 RAM chips, which I have ordered a bunch.
I think the 'stuck' address lines may be a red herring. The BASIC ROM code will not be doing much at all until it gets an interrupt (which it appears not to). The 'chirp' is partially driven by the keyboard PIA and partially vie the VIA (UB15). Either (or both) of these being faulty could account for your lack of a chirp...
Thank you. In wondered if I might not be barking up the wrong tree there.
The fact that the CRTC is displaying an image is a good indication that some of the I/O device decoding is working.

There is also a PETTESTER diagnostic that I wrote that can be programmed into an EPROM that replaces the EDIT ROM. This is a fairly low-level set of test routines. This would allow you to interrogate the keyboard PIA and key matrix to see what is working. It doesn't use interrupts.

Dave
I may try that. Anticipating this repair, I picked up a couple of Jim Brain's 23xx substitution boards at VCF Midwest two weekends ago, so I can easily use 27xx chips in place of the originals. Does it require removing of the other ROMs first?

Also, who gave me that quick-and-dirty ROM checksum program years ago, was that you? I still have that somewhere, so I can check ROMs when I finally get around to desoldering them.

Kevin
 

daver2

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A 2716 or a 2516 EPROM (I will have to double check which one) will fit directly in place of the EDIT ROM without any adapters. You will need a partially working kernel ROM to get my PETTESTER V4 to work.

My PETTESTER also gives you a checksum of the ROMs without unsoldering them... This is why it is good not to remove the existing ROMs! The EDIT ROM should always be in a socket - as this was customised to the keyboard locale etc.

Dave
 

KevinO

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You probably fitted sockets for the PIA's ? It is possible to have more than one defective PIA, so rather than just the swap you should get more PIA's as spares.
Yes I did. Good thinking. I agree, it's possible that they could both be bad. I can pull one from my working 8032 to test.
All the NOP generator does is disconnects the Data bus DA0-DA7 from the CPU's data pins, and applies the byte EA to those pins, which is a NOP byte code. So then , if the CPU is working, you end up with a pattern of square waves on the address lines which look very similar to those you would get from the output of a clocked binary counter or binary divider chain. This way the address circuitry can be checked.
Yeah, I kinda remember that now. It's been a few years, and I think I only used it the one time. I'm not sure how it would work with the Tynemouth board in place though, since it's actually bypassing the 8032's RAM and ROM, so you might not see what you expect.
But, it appears that your CPU was able to read enough of the ROM code to boot to BASIC, so it appears that the address lines are likely ok. I would concentrate on the Keyboard PIA area for now, is there anything going on that could stop a good keyboard PIA IC from working ?

Or, maybe something has stopped the CPU after the boot message (probably less likely than a PIA issue) and there is possibly ROM corruption. It is easy to check the ROM firmware as it is on Zimmers and you can read the code from the ROMs with the inexpensive GQ-4x programmer to check it, or make new ROMs.
 

KevinO

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A 2716 or a 2516 EPROM (I will have to double check which one) will fit directly in place of the EDIT ROM without any adapters. You will need a partially working kernel ROM to get my PETTESTER V4 to work.
I know I don't have any 2516s. I'm sure I have 2716s, or at least 2732s.
My PETTESTER also gives you a checksum of the ROMs without unsoldering them... This is why it is good not to remove the existing ROMs! The EDIT ROM should always be in a socket - as this was customised to the keyboard locale etc.
Thanks for that explanation. I don't have the ROMs memorized, but you're correct...ONE of the stock ROMs is in a socket. I assume that's the EDIT ROM. There's also an oddball ROM in one of the spare sockets that I wasn't able to identify, but I have that removed for the moment. I'll get back to it when the machine is fixed.


 

daver2

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2716 are fine (as Colin says). A 2732 should work, providing you program the second half 2K of the EPROM with my code.

Dave
 

KevinO

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The clue is no /IRQ signal. /NMI should be high.

The keyboard is scanned only when an /IRQ is generated. So, no /IRQ no keyboard operation and no cursor flashing either.

Can you check for a signal on UB12 (6520 PIA) pin 18. This should be the vertical drive and is responsible for generating the /IRQ via the keyboard PIA.
So, a signal IS present on UB12 Pin 18, and UB15 Pin 16. I didn't look at it with a scope, just hit it with a logic probe and got pulses.
 
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KevinO

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Ok, so HERE'S an interesting development....Hugo said you could have more than one bad PIA, so just for giggles I put BOTH of the 8520s into my working 8032, and got a chirp, but NO CURSOR! So looks like you were right. I should have checked this...both of these are bad! Then when I put my good 8520 into the bad PET, it boots to the monitor and gives me this.

20220923_171052.jpg
 

Hugo Holden

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I have used Daver2's Pettester ROM too and it works great, also it has a good manual to read explaining how to use it.

For making the replica ROM's I used the TMS2532JL. These work well. Except for the character ROM and the UD8 Edit ROM (in my Dynamic PET) where I used 2716's.

The GQ-4x programmer can program the TMS2532JL very well. Be cautious of the TMS2532A, these have a different programming voltage to the JL type and are not fully supported by the GQ-4x. They can be programmed as a 2732A in an adapter in the GQ-4x, to acquire the correct programming voltage, but its awkward, so its better to keep way from that variant I think.
 

KevinO

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I have used Daver2's Pettester ROM too and it works great, also it has a good manual to read explaining how to use it.

For making the replica ROM's I used the TMS2532JL. These work well. Except for the character ROM and the UD8 Edit ROM (in my Dynamic PET) where I used 2716's.

The GQ-4x programmer can program the TMS2532JL very well. Be cautious of the TMS2532A, these have a different programming voltage to the JL type and are not fully supported by the GQ-4x. They can be programmed as a 2732A in an adapter in the GQ-4x, to acquire the correct programming voltage, but its awkward, so its better to keep way from that variant I think.
My programmer is an original TopMax, and should be able to handle those old chips just fine. Thanks for the tip on the TMS2532JL. I might have to get some of those. Can use those without needing an adapter!
 

daver2

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Post #14 indicates the firmware has detected the DIAG input to the keyboard PIA being pulled LOW during a startup.

This is a thing called TIM (the machine code monitor). There is a manual for it. You can dump various areas of memory for example.

Can you type anything into TIM using the keyboard?

Dave
 

Hugo Holden

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........your post above, your CPU has gone into the machine language monitor mode. It went to execute some code somewhere at some address, it looks like it (your CPU's program counter) would have gone to address 20C1h, where it found the byte 00 there, so it opened the M/L monitor.

For example if you type SYS "some address in decimal" such as SYS 1024 and there is an 00 there at address 0400h (as there often is) it will open the M/L monitor (or TIM as it is sometimes called).

The correct way to open the M/L monitor is to type SYS 64785

I'm not sure why it went into lower case, though I have seen that happen when various faults are about.

While you are in the TIM, assuming your keyboard is working, you could type M 20C0 20C7 and the TIM will (should) display the byte values there at memory addresses 20C0 20C1 20C2 .......20C7. In theory 20C1 would be an 00
 
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KevinO

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Post #14 indicates the firmware has detected the DIAG input to the keyboard PIA being pulled LOW during a startup.

This is a thing called TIM (the machine code monitor). There is a manual for it. You can dump various areas of memory for example.

Can you type anything into TIM using the keyboard?

Dave
Nope. Just tried, and it doesn't accept any keyboard input.
 

daver2

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As I mentioned in my post above, if you pull the DIAG line low when you reset (or power up) the PET you will also enter TIM.

This can also happen if (when the CPU tries to read the port from the PIA containing the DIAG bit) it fails to read correctly.

Dave
 
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