PETunia's Repair Log

[Nivag Swerdna]

I guess it is likely to be another bad connection. I have been putting wires in various holes whilst using my logic analyser to verify the address decoding for the buffers so maybe that has pushed things over the edge.

In particular I am very conscious of the fragility of the decoding in the $Exxx part of the address space. In particular nCS2 is active low and the primary mechanism for keeping the PIAs quiet when not selected... so a dodgy or broken connection on nCS2 means they will become active and start blasting data over the address bus.

I guess I have to replace the PIA socket at G8... that's a 40 pin job... not easy.

 

[Nivag Swerdna]

Making progress again... swapping the G8 PIA socket wasn't actually too hard. It is quite easy to decimate using side cutters unlike the solid plastic sockets used by the ROMs.

I also discovered how to make the LED flash which is rather fun and a good PIA test (maybe that could be added to PETTESTER?)

Write 0F to E810 ; PA0..PA3 outputs
Write 04 to E811 ; Set DORA/ORA bit in CRA

repeat
Write 0B to E810 ; PA0, PA1, PA3 HI, PA2 LO... makes LED turn ON
delay 0.1s
Write 0F to E810 ; PA0, PA1, PA3 HI, PA2 HI... makes LED turn OFF
delay 0.1s

 

[Nivag Swerdna]

I think I am done now for a bit... I ended up replacing three 40 pin DIP sockets... both PIAs had connectivity issues for nCS2 and the VIA had a dodgy RnW.
The old sockets were black and you could not see the contacts down the holes whereas the new ones are open at the top so the contact condition is much more obvious.
I found a couple of the old sockets had contacts that had been very compressed and lacked springiness.. this would have been obvious with the open topped variety.
[TBH it was probably partially my fault for sticking wires into the holes whilst playing with my logic analyser to verify the addressing]

 

[Nivag Swerdna]

So I put everything back together again and it boots but to no cursor! I tried PETTESTER and the PIA registers keyboard presses so all good...

I did a bit of chip swapping and discovered that it will boot to a flashing cursor if I remove the VIA.

Any ideas?

 

[daver2]

Check the /IRQ line for being stuck LOW.

That's where my money is bet on...

Dave

 

[Nivag Swerdna]

Check the /IRQ line for being stuck LOW.

Thanks for the pointer... will give that a go at the weekend.

 

[dave_m]

Check the /IRQ line for being stuck LOW.

That's where my money is bet on...

Dave

Agreed and if the low goes back high when the VIA is removed and the cursor starts blinking (which indicate the interrupt is working) then it may be that the 6522 VIA is bad. Do you have a spare to test?

 

[Nivag Swerdna]

Do you have a spare to test?

I don't think I have a spare 6522. I also am getting a bit worried about my monitor now due to the constant turning off and on... might have to work on a video solution... maybe using a composite adapter since I do have an old monitor that could probably manage to go down to 15kHz.

 

[AndyG]

PM me as may have a 6522 in my box of stuff.. for you to try. Ironically I have found these to be reliable (well apart from legs falling off due to corrosion) and the 6520 PIA’s often fail.

Andy

I don't think I have a spare 6522. I also am getting a bit worried about my monitor now due to the constant turning off and on... might have to work on a video solution... maybe using a composite adapter since I do have an old monitor that could probably manage to go down to 15kHz.