Uh oh - A2000 thinks fire button is being held down

[JimS]

Try swapping the 8520's the fire signals go through U300.

[JimS]

I like this idea the most, but I swapped the chips and didn't get any change. Could they both be borked? What symptoms should I be looking for if U301 is fried, too?

Regardless, I've ordered some replacements. Always good to have spare 8520s around

Well, as I read the schematics, the same failing bit in U301 would show up as a constant RTS or DTR from your modem.  You could check the emi filters at emi238 or emi241. Sometimes they short to ground. I even saw one where loosening the mobo mounting screw at the mouse port fixed a problem.

[JimS]

Thanks for the advice, everyone.

What exactly are these EMI things? (i.e., what's the part called that I should use as a replacement?)

EMI: ElectroMagnetic Interference. Basically there to keep your machine from spewing out radio frequency interference thru it's connectors. It's a passthru connection with a capacitor to ground to shunt off RF noise.  Some have ferrite beads on them but these are 6800pF capacitors to ground, according to the schematics. If I found one bad, I'd just remove it, and jumper across. ;-) But that's just me, the FCC scofflaw. ;-)