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Commodore PET 2001 with no cursor

R

richterh

Member
Joined
Apr 30, 2022
Messages
45
Location
Germany, Crailsheim
Hello everyone,

I am new to the forum and am asking for help. I have got a PET 2001, it worked fine except for the tape drive because an old belt broke. After an hour I kept getting a garbage screen after booting. Bought a ROMulator tool to solve the problem because it looked like a RAM/ROM issue to me. The ROMulator fixed the garbage screen, but now I get a screen with no cursor. Just a weird Asterix sign in the middle of the screen.
I have already exchanged the 6520 PIAs without any effect.
Does anyone have any ideas what i could try?

Greetings Harald
 

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Hello and welcome Harald from another recent recruit who has an early PET 2001 like yours.

If you have tried replacing the PIAs with new ones the fault is most likely to be something to do with their supporting logic chips. Do you have fault finding equipment such as a logic probe or an oscilloscope?

There is still likely to be a RAM/ROM fault as well of course if you if you are interested in tracking it down. The ROMulator may help with that but it would probably be best to solve the cursor problem first.

Alan
 
Welcome,

Too late now to give you a more full explanation but welcome to VCFED.

I would check the interrupt pin of the CPU (/IRQ) to see if it is being held permanently LOW.

Failing that it is likely to be one of the PIA's or the VIA or something associated with the vertical blanking signal.

I will hunt out the information for you tomorrow (unless someone can do it whilst we are sleeping!).

There are also some threads already on VCFED with the self-same problem.

Of course, it could be a ROM/RAM fault. Who's ROMulator did you buy and what have you mapped from the PET to the ROMulator?

Dave
 
ajgriff said:
Hello and welcome Harald from another recent recruit who has an early PET 2001 like yours.

If you have tried replacing the PIAs with new ones the fault is most likely to be something to do with their supporting logic chips. Do you have fault finding equipment such as a logic probe or an oscilloscope?

There is still likely to be a RAM/ROM fault as well of course if you if you are interested in tracking it down. The ROMulator may help with that but it would probably be best to solve the cursor problem first.

Alan
Click to expand...
Hello Alan,
thanks for your fast responce. I already ordered a logic probe, hope to get it in the next days.
Harald
 
daver2 said:
Welcome,

Too late now to give you a more full explanation but welcome to VCFED.

I would check the interrupt pin of the CPU (/IRQ) to see if it is being held permanently LOW.

Failing that it is likely to be one of the PIA's or the VIA or something associated with the vertical blanking signal.

I will hunt out the information for you tomorrow (unless someone can do it whilst we are sleeping!).

There are also some threads already on VCFED with the self-same problem.

Of course, it could be a ROM/RAM fault. Who's ROMulator did you buy and what have you mapped from the PET to the ROMulator?

Dave
Click to expand...
Hello Dave,
Thanks to you too. I bought the ROMulator from Mike (bitfixer). I use setting 9, BASIC 1, PET 2001-8 to get the COMMODORE BASIC display.
I am just waiting for a logic probe to improve debugging.
Harald
 
Would it boot to basic if the IRQ is being held permanently low?

From memory it's PIA#1 that interfaces with the keyboard so I'd start the hunt for the cursor problem in that vicinity.

It's a bitfixer ROMulator from the US (see photo) and I'm not familiar with its capabilities but hopefully Harald will have access to the necessary documentation.

As I mentioned above there is likely to be an underlying ROM/RAM fault since the PET boots to basic with the emulator but not without it.

Alan
 
My post #6 refers to Dave's post #3. Moderation delay may make the thread a little confusing for a while. I should have quoted Dave's post to make things clearer, sorry.
 
ajgriff said:
Would it boot to basic if the IRQ is being held permanently low?

From memory it's PIA#1 that interfaces with the keyboard so I'd start the hunt for the cursor problem in that vicinity.

It's a bitfixer ROMulator from the US (see photo) and I'm not familiar with its capabilities but hopefully Harald will have access to the necessary documentation.

As I mentioned above there is likely to be an underlying ROM/RAM fault since the PET boots to basic with the emulator but not without it.

Alan
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The IRQ ist permanently high. It does not boot when I pull it down to low. By the way, I got an oscilloscope now.
 
/IRQ being permanently HIGH is good (well, better than permanently LOW at any rate).

I didn't say to pull it LOW incidentally!

So, the first thing to check with your new oscilloscope is pin 7 (SYNC) on the CPU (IC F3). This pin should be pulsing HIGH every time the CPU fetches and executes an instruction. If the CPU has stopped executing instructions - the machine will freeze...

It will also be worthwhile seeing what is on UG8 (6520) pin 18 and UA5 (6522) pin 15. They should be identical and have the monitor SYNC signal on them.

If UB8 (6520) is in a socket, it is not required during this part of debugging (it is used for the IEEE488 port). Removing it may help to rule it out from causing a problem.

Dave
 
Hi Dave,

yes there is the SYNC on CPU pin 7 and the identical signals on the 6520 and 6522.
I have removed the UB8 6520.

Harald
 

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Excellent, that's good news.

Are you able to download my PETTESTER code and burn it into a 2716 to exchange the EDIT ROM with?

I can provide you with a link tomorrow.

This code will test out quite a lot of PET hardware functionality for us.

Dave
 
daver2 said:
Excellent, that's good news.

Are you able to download my PETTESTER code and burn it into a 2716 to exchange the EDIT ROM with?

I can provide you with a link tomorrow.

This code will test out quite a lot of PET hardware functionality for us.

Dave
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Hi Dave,
unfortunately I don't have an eprom burner. But I can look to get one if it helps to find the error. Any recommendation what typ I need to write the fitting eproms?
Harald
 
Hi
You have a ROM substitution device... You should be using it! Get the PETTESTER image and load it into the device, contact bitfixer for help if you don't know how to do that.

As far as no cursor is concerned... That's probably a PIA problem. Are the chips in the correct orientation? Post a photo

If the PIA is OK but it is not being addressed then you should suspect address decoding to the PIA... In that case use your ROM substitution gadget to serve up EAs (NOP), again ask bitfixer if you are unsure, and scope the CS lines on one trace and A15 on the other.

Regards
 
Agree with Nivag. For example, is there any change in cursor behaviour if the bitfixer is mapped to BASIC 4? By the way the processor chips (all socketed) are aligned correctly in the original photo (post #1). PIA#1 and its associated logic remain the prime suspects. Dave's PETTester will really help if you can get it up and running.

Alan
 
Yep, you can download my PETTESTER code into Bitfixer’s ROMulator.

Contact Bitfixer for details (if you haven’t already got it).

Dave
 
Hello Harald,
Fortunately, Dave's PETTESTER is pre-loaded for one of the default settings on the ROMulator.
It's setting 13, Switch 1-4 on, on, off, on.
 
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