Pet 8296D no signs of life

[daver2]

Ok,

So /RESET (pin 40) is OK.

The clocks are all OK (pins 3, 37 and 39).

The /NMI and /IRQ pins are both HIGH - so OK.

But the CPU is either not executing instructions; or starts executing instructions and then stops.

So, next step?

Dave

 

[Desperado]

Ok,

So /RESET (pin 40) is OK.

The clocks are all OK (pins 3, 37 and 39).

The /NMI and /IRQ pins are both HIGH - so OK.

But the CPU is either not executing instructions; or starts executing instructions and then stops.

So, next step?

Dave

IC 6545 ???

 

[daver2]

Nope.

How about the NOP generator?

That will see if you now get pulses on pin 7 (i.e. the CPU is running fine).

Dave

 

[Desperado]

Ok! Nop Generator, it's true!

 

[daver2]

Pin 7 TRUE or pin 7 PULSING?

Dave

 

[Desperado]

Pin 7 TRUE or pin 7 PULSING?

Dave

WITH Nop i have pulses on Cpu pin 7!

 

[daver2]

Excellent, so we know the CPU works!

What's the next step with the NOP generator in place?

Dave

 

[Desperado]

Excellent, so we know the CPU works!

What's the next step with the NOP generator in place?

Dave

A0 to A15 pins?

 

[daver2]

Yep!

So, this schematic will help you: http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8296/324644-1of9.gif.

Check on the output pins (on the right) of the 244 buffers at UD8 and UC10 first.

Then check the /CS pins (pin 20) of each ROM for activity (UE10, UE9, UE8 and UE7).

Finally, check for activity on the /CS pin of the CRTC (pin 25 of UC9).

Dave

 

[Desperado]

Yep!

So, this schematic will help you: http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8296/324644-1of9.gif.

Check on the output pins (on the right) of the 244 buffers at UD8 and UC10 first.

Then check the /CS pins (pin 20) of each ROM for activity (UE10, UE9, UE8 and UE7).

Finally, check for activity on the /CS pin of the CRTC (pin 25 of UC9).

Dave

Thanks so much Dave

 

[daver2]

Off to do some chores...

Dave

 

[Desperado]

Yep!

So, this schematic will help you: http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8296/324644-1of9.gif.

Check on the output pins (on the right) of the 244 buffers at UD8 and UC10 first.

Then check the /CS pins (pin 20) of each ROM for activity (UE10, UE9, UE8 and UE7).

Finally, check for activity on the /CS pin of the CRTC (pin 25 of UC9).

Dave

Dave, with logic probe or scope please?

 

[daver2]

Always with the oscilloscope.

Don't forget you are looking for A0 to be 250 kHz; A1 to be 125 kHz; etc. Each higher address line being half the frequency of the previous. You are also looking for 'good' TTL signal levels (both HIGH and LOW).

You can't count the pulses on your logic probe...

Dave

 

[Desperado]

With logic probe, i have pulses on every output pins of UD8 and UC10

 

[Desperado]

Always with the oscilloscope.

Don't forget you are looking for A0 to be 250 kHz; A1 to be 125 kHz; etc. Each higher address line being half the frequency of the previous. You are also looking for 'good' TTL signal levels (both HIGH and LOW).

You can't count the pulses on your logic probe...

Dave

Ok i see correct waveforms also!

 

[daver2]

That is good, but I would redo everything with the oscilloscope after your have done them with the logic probe...

Dave

 

[daver2]

OK,

What about the ROM and CRTC chip select pins?

Dave

 

[Desperado]

OK,

What about the ROM and CRTC chip select pins?

Dave

I am desperate! I have only Ue7 and Ue8 on board!!View attachment IMG_20221023_181821.jpg

 

[daver2]

That is correct - you can still test the chip select pin of the IC socket can't you?!

Alternatively, just ignore the IC sockets that do not have anything plugged into them.

No need to be desperate (just yet)...

Dave

 

[Desperado]

Ok, i see waveform on every pin 20 of Roms (also where rom is missing)...also on 6545 pin 25!