rasz_pl wrote on 2023-07-01, 06:30:

one picture of scope screen would solve it immediately 😀

depending on the scope... screen & settings 😉

I've experienced the same issue with a cold joint in the past - lightly tapping it with a probe made the cold joint temporarily disappear.

As motherboards get older, I'm finding myself reflowing parts more and more.

pshipkov wrote on 2023-07-01, 06:50:

It shows on the oscilloscope screen.

picture of TX signal would show mangled logic levels, turning on serial decoder would reveal its a bad signal
With both 1488/89 level translators missing negative supply there was a considerable chance of receiver interpreting bad input correctly resulting in working loopback test, now that one would be fun to debug with most people arguing physical transmission is clearly working so no need to look at the signal 😀
glad its fixed

Glad you found the issue and was an easy fix !

pshipkov wrote on 2023-07-01, 02:20:

Success ! It was the -12V line. A rail somewhere flaking on and off frequently from even gentle touches of the components. So, a […]
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Success !
It was the -12V line.
A rail somewhere flaking on and off frequently from even gentle touches of the components.
So, apparently when i measure the -12V is mostly ok. So even for a moment it shows 0V i assume it is me not holding the probes right and pushing/tweaking them which brings the voltage back.
Pulled a wire on the back to pin B7 and that was it.

Thank you very much guys and especially to you @Deunan for prompting me to inspect this more carefully.
Command ports and how they function was never of big interest to me, but the information in this thread was a bonus to the fix itself.

Yay!! Good work!

Cheers,