PAL-2 - [ST] not working

[Jeremy Starcher]

Just realized my [ST] button isn't working ... Button tests out OK.

Reset works though.  Hmmm.

[GN Liu]

To use the ST, you need to first set $17FA to 00 and $17FB to 1C to specify the NMI vector.

[Jeremy Starcher]

Oh I know, and I had vectors said but the pressing the button does NOTHING

On the PAL-1, pressing ST without setting vectors crashes the CPU and the display freezes on one digit.

On the PAL-2, pressing ST does *nothing* -- Even with all vectors set, pressing ST does nothing.

Single Step mode works too, tested that.

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[GN Liu]

That’s strange, the PAL-2 is supposed to behave like the PAL-1 when using the ST key. Also, the SST mode relies on NMI, so we might need a more experienced user to help with this… 

Does the system shows any other abnormal behavior?

[Jeremy Starcher]

I've not noticed any other odd behavior.

I haven't had a chance to dive into it again but my current guess is that the  debouncer circuit isn't working for some reason.

I'm a software developer -- I should never be trusted around screwdrivers, debugging equipment and a soldering iron, but I'll figure it out.

[GN Liu]

I'm also learning through the project. It's very interesting and different from modern software development.

[Hans Otten]

Do you have a way of measuring to see what happens on the NMI pin of the 6502 when you press ST?
It should show a pulse as output of the 556 debounce circuit.

A LED with a resistor  (1K e.g.) can be enough to see this.  

[Jeremy Starcher]

I've got a logic analyzer and access to a good scope. 

I'll probe things out next time I'm at the community workshop.

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[Jeremy Starcher]

Chalk it up to another great case of Jeremy being blind.

A cold solder joint is the cause of many woes.