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Commodore PET 2001-8 Question

D

dondi

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I have a PET 2001-8 with a 320008 board.

My question is: what is the purpose of the red LED on the board? Mine does not illuminate. Computer does not boot. Just get a random character screen.

Thanks!

Don
 
The red LED was for use with special Commodore diagnostics. It does not light under normal circumstances. It may light if the PET is faulty (a very low probability though!). I had a little test program to illuminate it - but it is pointless (and this LED was not fitted to any future PET designs).

The 'random character' screen is the default.

it is actually good. It indicates that we have some heathy power supplies, the video circuitry (as far as it goes) is working - but the CPU is not working.

The CPU may not be working for a number of reasons though.

I will post some things for you to check presently.

Can you post a photograph of the 'random' screen. What may appear as random to you may not be random to an expert...

What test equipment do you have?

Dave
 
Thanks for the reply Dave.

Attached is a picture of the screen.

I have a multimeter, a logic probe and a Ramulator.

I have tried replacing most of the socketed chips including the CPU. I also replaced the 555 timer.

The computer originally came with Skyles Electric RAM boards which I have replaced with these: https://monotech.fwscart.com/product/mos-6550-sram-replacement-for-pet-2001

If you reply I may not get back to you until tomorrow.
 

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>>> If you reply I may not get back to you until tomorrow.

No problem, we all have lives!

That screen looks fairly random to me. You can generally 'see' where some video RAM data bits are 'stuck'.

I will post some tests for you tomorrow now that I know what test equipment you have...

Dave
 
Even though you have replaced the 555 timer that is used to produce the power-on reset signal - there are other components that can fail that prevents the reset from occuring.

Attach the negative (black) probe of your multimeter onto 0V. All of my measurements will be relative to 0V. Do you know where this point is?

Next, attach the positive (red) probe of your multimeter to pin 40 of the 6502 CPU (this is the /RESET pin). Set the range on the multimeter to something that can accommodate a 5V DC signal.

Power the PET up and ensure that the multimeter reads close to 0V for a full second or so after the power as been applied to the PET, and then the signal should go HIGH (>4.5V DC).

Until your PET is repaired, I would temporarily wire a normally open pushbutton across capacitor C6 (0.1uF). This is wired between 0V and the 555 timer pin 2.

Pushing the button should force a /RESET (the 6502 CPU pin 40 should go LOW). Releasing the button should release the /RESET after approximately a second.

I would then use your logic probe (set to TTL) to monitor the 6502 CPU pin 7 (SYNC) and see if there is any activity immediately after a manual reset has been forced. If your logic probe has a 'pulse detector' on it - use that.

The CPU SYNC pin (pin 7) should pulse each time the CPU fetches and executes an instruction from memory.

See how we get on with those tests first.

Dave
 
You should see a very large power supply smoothing capacitor that is off the main board.

This is a polarised electrolytic capacitor, so it has a positive (+) and negative (-) terminal. The negative (-) terminal of this capacitor is the most convenient 0V point to attach your multimeter's negative (black) probe to.

Dave
 
Dave:

First, thank you for helping me with this. I appreciate it.

So, I installed the switch per your instructions and it works as intended.

The multimeter test shows a voltage of about 0.34V and then goes to about 4.73V. Resetting duplicates this behavior.

The logic probe test on CPU pin 7 shows it pulsing after manual reset.

Don
 
Don,

That is excellent news! That means the CPU is running and executing instructions.

You will find on the board an SN74154 (4:16 decoder). This decodes the 64K address space into 16 lots of 4K.

Can you locate this device and use your logic probe on all of the pins. Post whether the pin is LOW, HIGH or PULSING.

This will tell us what is being accessed in the machine (i.e. main RAM, video RAM and which of the ROMs).

I am not at home at the moment, so I am on my phone and being lazy!

Dave
 
I thought you did...

Can you verify that pin 7 of the CPU is still pulsing and then recheck the pins of G2 (SN74154) again please.

There is something not consistent here...

The CPU seems to be accessing memory (main RAM) at $7xxx - but the '/7' output on the '154 (pin 8) is not being activated.

The CPU should be executing largely out of ROM at this stage not RAM.

Dave
 
CPU Pin 7 Pulsing after Reset

1 Hi Pulsing
2-11 Hi
12 Lo
13 Hi
14 Lo Pulsing
15-17 Hi Pulsing
18-19 Lo
20 Hi Pulsing
21-24 Hi
 
OK, that's all believable. The CPU seems to be executing code out of ROM.

It is very important to check the state of the CPU SYNC pin (pin 7) every time you perform a test. If this pin is not pulsing, it will lead us astray to a false conclusion...

Can you check the SN74154 pin 9 (/8 output = video screen access) to see if there are any pulses that are observable just after a reset please. Obviously (just after what I stated in the last line) there is no time to check the CPU SYNC pin if we need to check this pin first!

The CPU should (fairly quickly after a RESET) clear the screen memory out - so you should observe some initial pulsing on G2 (the SN74154) pin 9.

Dave
 
Another quick question...

The RAMulator you have - is it the "ROMulan PET RAMulator". if so, it has my PETTESTER included, so we should enable that!

Dave
 
So on checking Pin 9 it pulses after Reset but not every time and sometimes one pulse and sometimes multiple pulses.
 
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