In message , JW
writes
If it's stuck low, then the signal won't be changing, will it?

Yes, it can. It's not stuck/shorted to GND, but fighting with another
latch, driver, or buffer.
Almost the weirdest fault I ever saw was like that, one PC printed fine
to an old Epson dot matrix, another printed garbage.

The printer had a 74LS244 or 245 (can't remember which, was a *long*
time ago) on the parallel input, measured with a meter on ohms the
offending pin was 2-3 ohms to ground. The old Zenith 8088 drove it fine,
the newer 386 machine failed miserably.

One way to trace is to get some thermochromic labels but if it's just
one input then it may not work and can be fairly random depending on bus
load etc.

Another way that's worked well for me in the past is with a component
tester, the sort that injects a signal into the device and displays the
'curve' of the device under test, Huntron Tracker or Hameg 'scope. You
quickly get used to the shape of the trace produced by various different
chip input/output pins and it's extremely easy to spot a dodgy one.

--
Clint Sharp