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Commodore Pet 2001-16N Garbled Screen Issue.

jbste

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Jun 7, 2021
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6
Problems with garbled screen on Commodore Pet 2001-16N.

I bought the Romulator (Ram/Rom) replacement and diagnostic board by bitfixer so I could test my Commodore pet 2001-16N.

I have a garbled screen issue and wanted to rule out a rom or ram issues.

Well I've now ruled out a ram or rom issue and the screen output is still bad.

I removed f10 character ROM (901447-10) the screen still filled but now was full of square blocks and not characters.
I believe with the chip removed it should read all pixels as ON and help verify the problem is the character ROM chip and not the related circuits that read the chip..

I have a few questions I could do with help with please.

What would be a good and reasonably priced Eprom programmer that will programme a 2716 Eprom? I think a replacement character ROM should be my next step?

Any suggestions on how to repair the faulty video output anyone?

Thanks in advance.
 

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First of all, welcome to VCFED (as this is your second post).

With the character generator ROM removed it is possible to ‘jumper’ the data outputs from the ROM to trst the video shift register. These have been known to fail. Do you know how to do this or would you like me to explain?

Where are you based? Someone may be able to ship you a character generator EPROM providing you cover their costs.

The ROMulator should contain a copy of my PETTESTER. The first test checks the video RAM, so I would be interested in what it displays...

Dave
 
Hi Dave
Thanks for the reply. I don't know how to ‘jumper’ the data outputs from the ROM to trst the video shift register. Could you please explain how to do that?

I'm based in Leeds in the UK, I'd happily cover costs of a character generator EPROM.

"PETTESTER. The first test checks the video RAM so I would be interested in what it displays" If the screen is garbled how would I be able to see the results?

Thanks
Regards Steve
 
Ah ha. M62 land. I am in Worcester.

Our location used to appear along with our Avatar. This all went with the forum ‘downgrade’...

I will tell you how to do this tomorrow. It’s bedtime now for me!

That’s what I want to check, to see what is displayed on the screen, garbled or not. Because I know what I am writing, it would be interesting to see what we can see.

I must admit, I suspect your character generator is a duffer, but I also see a pattern to the screen display which is interesting.

What test equipment do you have?

Dave
 
I should note that you have not ruled out the video RAM, only possibly the system RAM. I would not immediately suspect the character ROM. It also looks to be typical uninitialized video RAM. There are many reasons why it might not be initialized.
Dave will help more when he gets up.
Dwight
 
I have an old 20Mhz oscilloscope, a logic probe and my digital multimeter :)

I have a pic start pro programmer but I'm pretty sure it won't programme EPROMS

Oh and my Romulator, which also has the following software tools installed:
NOP generator
PETTEST
Low RAM test
VRAM/RAM/ROM

Steve
 
Hi Steve,

So everything you need then :)!

Just need to get yourself an EPROM programmer.

The schematics and parts layout for your machine are over on Bo Zimmer’s website. I will post a link shortly if you haven’t found them yet. EDIT: http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/index.html.

How many of your devices are socketed? Does this include the VIDEO RAM by any chance?

Dave
 
Last edited:
Hi Steve,
If you have a Raspberry Pi handy, you can hook it up to the ROMulator and download the contents of the full memory map after startup. This will show you what the CPU tried to write into video ram. If you see what looks like a normal basic startup screen, that would at least show you that the cpu was running. If you do this with VRAM/RAM/ROM test, that can show you snapshots of the tests running, and includes a VRAM test so you can somewhat rule out bad VRAM.
The readme on the ROMulator GitHub should have decent instructions on doing this, but if anything is not clear please let me know and I can help walk through it and update those instructions.
 
Steve,

If I am correct, this is the schematic for your machine (well, the video part of it I am interested in) http://www.zimmers.net/anonftp/pub/c...N/320349-8.gif.

UF10 is the character generator ROM. With the power OFF, remove it from the socket - observing which pin is pin 1 - and put it somewhere safe. Ideally, observe anti-static precautions.

Pins 9, 10, 11, 13, 14, 15, 16 and 17 are the eight data lines from the character generator forming the horizontal lines of each character.

Note: in the text below - ensure that you do not damage the internal pins of the IC socket when inserting the wires. Just out of interest, is the IC socket that the character generator sits in made of white plastic? If it is , these are notoriously poor and suffer from bad pin connections.

Use some short pieces of insulated wire to connect the above pins of the IC socket UF10 to 0V (pin 12). Power up the machine. The screen should be completely blank (black) indicating that there are no bits of the shift register stuck high. Turn the power OFF.

Move the insulated wires from pin 12 (0V) to pin 24 (however, you need to use a 1k resistor between the eight data lines and pin 24 to be on the safe side). The value of this resistor is unimportant, so anywhere from 100 Ohms to 4.7k should be fine. When you power up the PET you should observe a completely white screen (indicating there are no bits of the shift register stuck low. Turn the power OFF.

You can connect the eight data bits to 0V (pin 12) or high (to pin 24 via the temporary resistor) to make patterns of vertical lines on the screen.

If this is fine, everything from the character generator data lines onwards looks fine.

Power OFF, remove the temporary wires and resistor and install the character generator again (observing antistatic precautions and ensuring it is installed the correct way round).

If your video RAM ( UF7 and UF8 ) are in sockets, you can perform a similar test with them out of circuit. We should then be able to fill the screen with a specific character from the character generator, This will clearly demonstrate whether the character generator is GOOD or BAD.

Dave
 
Hi Steve,

So everything you need then :)!

Just need to get yourself an EPROM programmer.

The schematics and parts layout for your machine are over on Bo Zimmer’s website. I will post a link shortly if you haven’t found them yet. EDIT: http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/index.html.

How many of your devices are socketed? Does this include the VIDEO RAM by any chance?

Dave

Hi Dave, I've got an eprom programmer and a 2716 eprom. I've programmed the 2716 but I've still got the same screen output problem. Any ideas?
 
Steve,

If I am correct, this is the schematic for your machine (well, the video part of it I am interested in) http://www.zimmers.net/anonftp/pub/c...N/320349-8.gif.

UF10 is the character generator ROM. With the power OFF, remove it from the socket - observing which pin is pin 1 - and put it somewhere safe. Ideally, observe anti-static precautions.

Pins 9, 10, 11, 13, 14, 15, 16 and 17 are the eight data lines from the character generator forming the horizontal lines of each character.

Note: in the text below - ensure that you do not damage the internal pins of the IC socket when inserting the wires. Just out of interest, is the IC socket that the character generator sits in made of white plastic? If it is , these are notoriously poor and suffer from bad pin connections.

Use some short pieces of insulated wire to connect the above pins of the IC socket UF10 to 0V (pin 12). Power up the machine. The screen should be completely blank (black) indicating that there are no bits of the shift register stuck high. Turn the power OFF.

Move the insulated wires from pin 12 (0V) to pin 24 (however, you need to use a 1k resistor between the eight data lines and pin 24 to be on the safe side). The value of this resistor is unimportant, so anywhere from 100 Ohms to 4.7k should be fine. When you power up the PET you should observe a completely white screen (indicating there are no bits of the shift register stuck low. Turn the power OFF.

You can connect the eight data bits to 0V (pin 12) or high (to pin 24 via the temporary resistor) to make patterns of vertical lines on the screen.

If this is fine, everything from the character generator data lines onwards looks fine.

Power OFF, remove the temporary wires and resistor and install the character generator again (observing antistatic precautions and ensuring it is installed the correct way round).

If your video RAM ( UF7 and UF8 ) are in sockets, you can perform a similar test with them out of circuit. We should then be able to fill the screen with a specific character from the character generator, This will clearly demonstrate whether the character generator is GOOD or BAD.

Dave

Hi Dave f10 socket is black.
So connect all Pins 9, 10, 11, 13, 14, 15, 16 and 17 together and connect to pin 12(0v)?
Do the same again but connect all pins simultaneously through a 1k resistor to pin 24?

Thanks
Steve
 
Sorry for the delay in replying Steve.

For the first two tests, yes, you are correct.

You can then wire (for example) pin 9 to Vcc (pin 24) via the 1k resistor and the other data pins to Gnd (pin 12).

This should give you a single vertical line of pixels in each character cell (depending upon which data pin you wire to Vcc of course).

Dave
 
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