Can LPC812 pins (NXP ARM uC) be shorted briefly?

But dicey, especially considering the other experience you and others had. If I fry those things it would not be cool at all because this job is about finding a smoking gun. Not to blame anyone but to figure out how we can prevent such system failures in the future.

Can't do it. Especially since I have to find out why these systems failed so I need to test "as is".

However, then the uC and other stuff on the rail will be compromised in their function.

Or two diodes in series. That is a good idea. I only need to cross the logic threshold on teh other device and even with Schmitt that shouldn't require anything much closer than 1/3rd towards either rail.

No, that is clearly not what it means. When they say VOH =0V they mean exactly that. How would you limit the current while holding the pin at a given voltage???

You need to stop being paranoid and learn to read the data sheet. That said, they don't indicate the max current for the VOL = VDD for the high drive pins on page 35. So for those few pins you don't have data.

Rick C.