Hello,

> - did you try a 2006 pic in a new board? (if you have a leftover pic
> from the past)

Not yet. I have to take one from the old assembled boards.

 
> - was there a layout change between old and new boards? (error in re-
> layout or
> new layout prone to noise)

No. No change.

 
> - are the new pullups the value you think they are? how about other
> parts in
> that area? (new parts not in spec)

Triple checked. Everything ok.

 
> - is the new board located differently in the final assy? (noise
> introduced)

I used the board in the lab and in the application.
No change in behavior.

 
> - is power good all the time? what about brown out issues?

Yes, 5V are perfect.

 
> - any other components change, like the brand of motherboard power
> supply?
> (maybe the signal isn't stable)

No. Anyway, the problem is not related tot he motherboard and the
motherboard supply.
I checked also with other brands, and with nothing connected, supply from 5V
lab-PSU. I also connected the Pin 5 direct to GND of the PIC, so noise
should be no issue.


> - are the motherboard and pic grounds tied together and good?

See above.

 
> and there may be others. Make a small circuit for test that also
> samples that
> line and is edge triggered, but could drive a buzzer, etc (another pic,
> a 555,
> etc). Put it on the line with your circuit in test and physically see
> (hear)
> when it falsely triggers. It may be only when someone touches
> something, on a
> regular interval, etc, which will give you more ideas on what the
> source is,
> and if the problem goes away when you add to the line, that's important
> too. It
> could even be a pic on a protoboard that also samples the line (or
> drives an
> interrupt), and could measure the time it stays high. Use a different
> model pic
> for that. If it never sees that, and the 877 does, it will get
> interesting. ;)
> I would also verify (compare hex files) that the code hasn't changed
> since 2006
> from a master archived copy. Also write something very simple for the
> 877 that
> doesn't do anything but sample that line and report it. Maybe use a
> slightly
> different, but simple, approach, like a loop. These tests might narrow
> down a
> software issue.

More testing is scheduled during the week.
And I want to write a small test software ...
 
> All that said, this reminds me of a similar mystery 40 yrs ago or so
> (TTL at
> that point). Turns out someone had built a _small_ Van de Graf gen on
> the other
> side of [a very big] building in their space for some testing, and when
> it ran,
> a bit would mysteriously be read wrong over on my side. Of course, it
> was only
> be accident that we discovered it and solved it formally...

I know that. A long time ago, we made strange measurements in an EMC/EMI
lab. Than we found out, that on the parkingplace on the supermarket 100
meter ago, a gas-discharge lamp was defect, generating giant noise ...

But here, we tested the unit in lab and factory, and they are 500 km away
from eachother ...

Best Jens 

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