One of my favorite diagnostic hacks is Morse code messages blinked out
via the power LED on an Amiga. If the processor isn't totally wedged
and the hardware isn't so broken as to allow I/O port access, you can
get your message.
Not Morse code, but the Whitechapel MG1 blinks out an error code using
long ans short flashes (0 and 1, I can't remember which way round they
are) on a frontpanel LED. This would be great if the error codes were
correcly described in the technical manual (hint : a 'multi-bit DRAM
failure' can simply mean there's not enough RAM installed for the boot
ROM you're using. That one let me a merry dance...)
HP9000/200 machines have 8 LEDs connected to an output port on the
processor bus. Reset turns them all on, they are then turned off one
after another if the CPU is running by the self-test code. The
diagnostics may the put up an error code if something has failed. And the
error code is 'beeped out' (high and low beeps for 0 and 1) through the
internal speaker. Apparently if you 'phoned HP service they would
sometimes ask you to run the self-test and listen to the beeps over the
'phone so they knew what board had failed. Again, be careful, the error
code (and on-screen messages) don't always accurately describe the fault
(as I found when restoring a 9836CU...)
-tony