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Commodore PET 2001-8 stuck on garbled screen

Nivag Swerdna said:
OK That's interesting.

With all Buffers REMOVED

EXPERIMENT 1:

Can you leave the ROM in but remove the CPU and tell me what you see.

EXPERIMENT 2:

Can you leave the ROM in but put in the NOPPER and CPU and tell me what you see
Click to expand...
Without cpu no interference apart from a couple of flickering lines the size of characters.

With nop and cpu the interference is in a band that passes up the screen like a refresh rate of a camera shutter filming a crt
 
RetroGadgetMan said:
Hi after debugging the video logic with help from some people here which I am really grateful for we now have a garbled screen.
Character set has been replaced and I now get the following image pic 1.
Sometimes some of the characters will change if I press down on the board in various places or even flicker indicating a bad socket or maybe a bad trace.
I have cleaned all the sockets which made no difference.
Pic 2 shows the tynemouth pet rom ram substitute.
This can be configured to replace either the rom or ram or both and allso contains basic 1 2 and 3 selected by dip switches.
Pin 7 of the CPU with this rom ram board is at hi lo and pulsing.
With the original CPU installed with out the ram rom board pin 7 stays at lo.
Moving forward I think it's best to keep using the rom ram board until we get the correct welcome screen and a flashing curser. Then we can start looking at the rom and ram section.
Click to expand...
It has been a long while, since I had, what I believe is the same problem. So, my memory may be faded. However, I believe that there are only 2 Video Ram chips on board, and one is not really used. At least not in the boot process.
If I am remembering correctly, all I did was swap the two video ram chips and that resolved my problem.
 
RetroGadgetMan said:
All buffers back in , same band of interference
Click to expand...
OK. So I guess that with no CPU it is not doing read and writes and that causes the video to be less interfered with.

With the NOP it is doing all READS... we would expect the content to stay the same but with some snow... do we see that or is it changing?

Now with a CPU and the diagnostic ROM it is writing but clearly writing at the wrong time/place and causing mayhem.

We need to establish the Read and Write decoding to the Video Section and the Addressing of the video section.

(We could also have corruption to the data and address buses so the program being run is corrupted.... but let's assume that is not the case).

So clutching at straws...

Put the NOPPer in with a CPU... check it happily flashes.

Then let's check the address bus makes it all the way to the video section...

Scope pins 10, 13 of D2, pins 3,6, 10, 13 of D3 and pins 3,6, 10, 13 of D4
 
Nivag Swerdna said:
OK. So I guess that with no CPU it is not doing read and writes and that causes the video to be less interfered with.

With the NOP it is doing all READS... we would expect the content to stay the same but with some snow... do we see that or is it changing?

Now with a CPU and the diagnostic ROM it is writing but clearly writing at the wrong time/place and causing mayhem.

We need to establish the Read and Write decoding to the Video Section and the Addressing of the video section.

(We could also have corruption to the data and address buses so the program being run is corrupted.... but let's assume that is not the case).

So clutching at straws...

Put the NOPPer in with a CPU... check it happily flashes.

Then let's check the address bus makes it all the way to the video section...

Scope pins 10, 13 of D2, pins 3,6, 10, 13 of D3 and pins 3,6, 10, 13 of D4
Click to expand...
Ok so D2
Pin 10 pic1
Pin 13 pic 2
D3
Pin 3 pic 3
Pin 6 pic 4
Pin 10 pic 5
Pin 13 pic 6

All 50us time div
 

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Nivag Swerdna said:
OK. So I guess that with no CPU it is not doing read and writes and that causes the video to be less interfered with.

With the NOP it is doing all READS... we would expect the content to stay the same but with some snow... do we see that or is it changing?

Now with a CPU and the diagnostic ROM it is writing but clearly writing at the wrong time/place and causing mayhem.

We need to establish the Read and Write decoding to the Video Section and the Addressing of the video section.

(We could also have corruption to the data and address buses so the program being run is corrupted.... but let's assume that is not the case).

So clutching at straws...

Put the NOPPer in with a CPU... check it happily flashes.

Then let's check the address bus makes it all the way to the video section...

Scope pins 10, 13 of D2, pins 3,6, 10, 13 of D3 and pins 3,6, 10, 13 of D4
Click to expand...
And D4
Pin 3 pic 1
Pin 6 pic 2
Pin 10 pic 3
Pin 13 pic 4
All 50us time
 

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Nivag Swerdna said:
OK. So I guess that with no CPU it is not doing read and writes and that causes the video to be less interfered with.

With the NOP it is doing all READS... we would expect the content to stay the same but with some snow... do we see that or is it changing?

Now with a CPU and the diagnostic ROM it is writing but clearly writing at the wrong time/place and causing mayhem.

We need to establish the Read and Write decoding to the Video Section and the Addressing of the video section.

(We could also have corruption to the data and address buses so the program being run is corrupted.... but let's assume that is not the case).

So clutching at straws...

Put the NOPPer in with a CPU... check it happily flashes.

Then let's check the address bus makes it all the way to the video section...

Scope pins 10, 13 of D2, pins 3,6, 10, 13 of D3 and pins 3,6, 10, 13 of D4
Click to expand...
Here is a pic of the interference that scrolls up the screen with H7 test and nop with CPU.
Activity light blinking away. Reset lamp working for a second then goes off.
 

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Last edited:
Of all those scope traces only D4 p13 looks like a nice square wave.... they should all be nicely square!

D4 p13 should be BA9

I guess we work backwards... why isn't D4 p10 a nice square wave at twice the frequency of D4 p13...

Measure D4 p10 again.... then... E4 pin 6 then E4 pin 5.... are those three the same or different?

Thinking again....

How can that be BA9! BA9 should be 480.8Hz with the NOPPer... it would have a period of 2ms which would be 40 divs at 50us/div! Can you check your timebase for the D4 p13 measurement again please.

1673223757974.png
 
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It's possible the scope traces are confusing us... if you have a DMM with a Hz feature measure the frequency of pins 3, 6, 10, 13 of D2, D3, D4 whilst NOPing
 
Ok I figured out the dmm Hz
And I think I was reading ics thinking they where 14 pins not 16 when I used scope In post 146 and 147. 🙄🤪 Sorry.

So using dmm Hz black probe on pin 1

D2
Pin 3 7.62
Pin 6 7.62
Pin 10 248.5
Pin 13 123.9
D3
Pin 3 61.96
Pin 6 30.98
Pin 10 15.51
Pin 13 7.78
D4
Pin 3 3.90
Pin 6 1.94
Pin 10 968.5
Pin 13 480.3 there's that 480! 🙂
 
I read dmm as damm...

I think your dmm has given up at frequencies higher than approximately 250 kHz.

Dave
 
OK so theory suggests

1673290529516.png

So maybe you could add some units to your measurements! (I guess I should have some on mine! ;) Hz)

PS
My cheapo AN8008 can go up to 10MHz... so if yours doesn't... you were had!
 
Last edited:
Ok I have measured again and included Hz khz where applicable. Sorry for being sloppy with my readings.🙂

D2
Pin 3 7.628Hz
Pin 6 7.628Hz
Pin 10 248.5khz
Pin 13 123.9khz
D3
Pin 3 61.96khz
Pin 6 30.98khz
Pin 10 15.51khz
Pin 13 7.781khz
D4
Pin 3 3.909khz
Pin 6 1.944khz
Pin 10 968.5Hz
Pin 13 480.3Hz
 
Well they all look good to me... pin 6 is R/nW AND nSELE.... R is always high, with a NOPer, so it is nSELE i.e. 1/16th of a complete cycle... that might be 7.6Hz... not sure of my mathematics...

... but...

D2 Pin 3...

1673297609065.png

Pin 3 should be simply pulled to 5V... 0.00000000Hz.

I guess.. measure voltages*.... pin 8 to pin 16 of D2, then pin 8 to pin 2, 3, 5....

*DC volts, include your units! mV, V!
 
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