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Booting Issue w/ PET2001-8 - Issue appears to be PIA 1 - replaced w/ MC68521 - no joy :-(

BrainStruck

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Jun 5, 2020
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Location
Jupiter, FL, USA
Good evening, day or morning all,

I have a Commodore PET 2001-8 that's giving me some trouble. When I first got it, I couldn't get it to boot at all. It would give me a semi-random screen. After removing all of the chips, cleaning the system board and spraying all of the chip sockets with Deoxit, I reinstalled all but PIA 1, PIA 2 and all memory except for the I1, I2, J2 and J2. At this point the system would boot to the Ready prompt. I cycled through all of the RAM chips and found one defective so it's now running on all but the last 4 chips. I reinstalled PIA 2 (MOS 6522) and it boots to a Ready prompt, but no keyboard. I install PIA 1 (MOS 6520) and it hangs at the random character screen. I read that the MC68521 was a modern replacement for the 6520 so I ordered 1 (well minimum of 10 per order - anyone need any?) and it still hangs at the random character screen.

I'm not sure what the troubleshooting procedure is from here. I do have a multimeter and oscilloscope. Can someone please point me in a direction to go next?

Thanks in advance!
Brian
 
Hello and welcome to the forum. A few things mentioned in your post are confusing me:

1. A semi-random character screen is usually indicative of a video RAM fault (early 2001-8 only). Have you investigated the video RAM and if so, how?

2. Presumably you mean that with only four RAM chips installed in I1, I2, J1 & J2 the system booted to the READY prompt which implies that the RAM chips are 6550s rather than 2114s. Please confirm as requested in post #2.

3. Please explain what “cycled through all of the RAM chips” actually involved.

4. PIA2 should be a 6520 not a 6522 which is a VIA so presumably that’s a typing error since the PET booted. Please clarify.

5. As far as I can tell there’s no such thing as a 68521 PIA. The 6821 and 6521 are both equivalents to the 6520. The 6521 and W65C21 definitely work in my 2001-8 PET. Please clarify.

It’s difficult to comment on possible next steps without some clarification of the above points. The oscilloscope in particular is a useful tool to have as long as it has a reasonable bandwidth (20MHz or more).

Alan
 
Welcome to VCFED.

+1 for Alan's post.

Are you able to burn a 2716 EPROM?

If so, you can download my PETTESTER code (completely free), program a 2716 EPROM and swap it for the EDIT ROM. This will test out a fair amount of the PET functionality.

There is one limitation of PETTEST V4 (that will be fixed in V5) - that is 4K of RAM is required as a minimum.

Be aware that you will be moderated for your first 10 posts.

If that is not an option - we can walk you through a procedure using your oscilloscope - but it will be a bit easier once you are free of the moderators (so let's take it easy until then).

Dave
 
6550 ram? Or 2114??
6550

Hello and welcome to the forum. A few things mentioned in your post are confusing me:

1. A semi-random character screen is usually indicative of a video RAM fault (early 2001-8 only). Have you investigated the video RAM and if so, how?

2. Presumably you mean that with only four RAM chips installed in I1, I2, J1 & J2 the system booted to the READY prompt which implies that the RAM chips are 6550s rather than 2114s. Please confirm as requested in post #2.

3. Please explain what “cycled through all of the RAM chips” actually involved.

4. PIA2 should be a 6520 not a 6522 which is a VIA so presumably that’s a typing error since the PET booted. Please clarify.

5. As far as I can tell there’s no such thing as a 68521 PIA. The 6821 and 6521 are both equivalents to the 6520. The 6521 and W65C21 definitely work in my 2001-8 PET. Please clarify.

It’s difficult to comment on possible next steps without some clarification of the above points. The oscilloscope in particular is a useful tool to have as long as it has a reasonable bandwidth (20MHz or more).

Alan
Hi Alan, thank you for your reply.

1. The screen isn't semi-random, it's the fully random screen:
Random Screen

2. Yes they are 6550 RAM chips

3. I put RAM chips in until I had a failed boot pulled the one that it failed to boot on out and kept going until all but one RAM chip was installed

4. I did the troubleshooting last week and in my post work, late evening stupor I mistyped my original post (sorry about that). I just rechecked and yes the 6522 is in the VIA slot as it should be. If PIA #1 slot is populated by either of the 6520 chips I get the random screen I posted above. I can swap the MC6821P in PIA 2 out and the system will boot to the ready screen:

Ready Screen (sorry for the glare)

5. Sorry, typo on the MC6821P, there's no 5 in the part number.

The oscilloscope is a Rigol DS1054 with 50 mhz bandwidth.

Welcome to VCFED.

+1 for Alan's post.

Are you able to burn a 2716 EPROM?

If so, you can download my PETTESTER code (completely free), program a 2716 EPROM and swap it for the EDIT ROM. This will test out a fair amount of the PET functionality.

There is one limitation of PETTEST V4 (that will be fixed in V5) - that is 4K of RAM is required as a minimum.

Be aware that you will be moderated for your first 10 posts.

If that is not an option - we can walk you through a procedure using your oscilloscope - but it will be a bit easier once you are free of the moderators (so let's take it easy until then).

Dave

Hi Dave, thank you for your reply.

I do have a UV eraser and PROM burner. I don't have any 2716's, I'll place an order for one. Where is the EDIT ROM?

I have 5K of working RAM.

Thanks again! Also, sorry for the confusion in my original post. Apparently coffee is mandatory after 5pm.

Brian
 
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Welcome

It sounds like you have made excellent progress so far...

You should be aware that the 2001-8 has two PIAs and one VIA.

For the PIAs... They are 6520s and are replaceable with 6821s (I assume that's what you have just bought!).

One thing to note is that the PIAs have differing orientation .. I have to check a diagram every time.. so make sure you install them the correct way around!

So...

The easiest thing to do is to run the PETTESTER diagnostic but this will require you to make up a 6540 image which you might find tricky.

I believe you can ignore the PIA at B8 and if I remember correctly the VIA at A5... Remove them....

So concentrate on one PIA at G8... Check orientation... What exactly do you see?

The keyboard is scanned.... Do you see activity on pin 2,3,4 or 5 of G8?

Are you seeing video sync on pin 18 of G8?

Are you seeing interrupts on pin 38 of G8?

Sounds like you are close... Maybe clean socket G8 and put a 6821 the 'correct' way around and it will all come good?
 
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Ah yes - forgot about the 6540 issue - it would need an adapter. I need more coffee in the morning :)!

Nivag has identified most of the things I was going to suggest you look for in his post above.

Dave
 
If PIA #1 slot is populated by either of the 6520 chips I get the random screen I posted above.

Thank you for responding Brian. You've certainly made good progress and everything is much clearer now. The bottom line seems to be that there's a problem associated with PIA1 although it appears that this is not being caused by the chip itself. Let's see what Nivag and daver2 can come up with in terms of tracing the fault.

Alan
 
Well now I feel like an idiot. PIA1 was in backwards. All of the other chips were notch right, but PIA1 is notch left. So now it boots and I can type... mostly. I need to clean up the keyboard and repaint the pads with conductive paint.

As it turns out I have 3 bad memory chips. I was able to successfully build a chip from: https://github.com/go4retro/Nu6550. I thought I had only one bad chip, but now that I found 2 more I'll build 2 more chips. I'm finding out that SMD soldering is much, much more temperamental than through hole. I already destroyed 2 of the 5 memory chips I ordered. Thankfully I have 2 left to make the remaining defective chips.

I just acquired a PET 4032-12 which should be in next week. So I guess that's the next project once I've wrapped this one up. Does anyone have any PET software they'd be willing to sell for any of these? Not much to be found on eBay.

Thanks again for the help everyone!
Brian

PXL_20220615_013304475.jpg

PXL_20220615_014055930.jpg

PXL_20220615_013312454.jpg
 
My PET does not have your kind of RAM IC's in it (I have the dynamic PET).

One thing I see happen a lot, is that various IC's on adapter boards made to "help out" have pins that are far thicker than an IC pin. So if these adapters are plugged (pushed) in to an IC socket, they stretch apart the IC socket claws, and ruin the socket for later insertion of an original IC.

One of the more elegant solutions I have seen to replacing the 6550 is to use a 2114 & 74HCT138 on a carrier with thin pins, like the offering shown here:


The 2114's are very plentiful and available from multiple old manufacturers, GTE, National, AMD & Intel. I have a large collection of these as spares for my vintage computers. My favorites are the ceramic boded ones made by Intel, they are nearly always perfect, I think they were made to mil spec quality, like these:

 
Told ya! Easy to do.

FWIW my RAM solution... https://www.tindie.com/products/nivagswedna/pet-8k-ram-replacement-for-6550-pet-2001s/

PS
The secret to SMD soldering is.... magnification... I have a widefield microscope but multiple pairs of +2 glasses also works!
That is a good ram solution. My first 2001-8 came with this RAM adapter from 1980. It allows for 2114 ram in it.

Going back to the point of the video ram being faulty. Thats why you have garbage on the screen. You want a modern video ram replacement.


edit: sorry forgot the link! http://mikenaberezny.com/hardware/pet-cbm/ods-ph-001-2114-ram-adapter/
 
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