If you have a
storage scope, you should be able to trigger on the
SYNC signal at the CPU and catch the last few opcode fetches - walking
the data and address bus signals and writing them down will reveal
the last opcode executed and the address from which it was executed.
Alas, I don't. My 'scope's an ancient Tektronix 561A. Works
it's pretty simple. (And maybe a bit underpowered for this particular
You can still capture the first few cycles from power-up. If you scope has
a delay function that can make it even more useful.
Connect external trigger to RESET (and trigger from it)
Connect one channel to select for the device you are interested in.
Use the other channel to probe.
With a nice bright trace, you should be able to see the probe state
when the select goes low after hitting reset - this is where the delay
comes in handy, allowing you to "move away" from reset.
You can also drive RESET with a pulse, causing the trace to repeat which
makes it *much* easier to see - I'd disconnect the monitor while doing
this as RESET may reset the video circuitry and result in unhealty
waveforms to the monitor.
You could also build a "1 bit TTL storage scope" with a very small amount
of logic - latch data to a LED when strobed. I'd consider allowing for
positive/negative trigger, and repeat/single-shot controls. Not as nice
as a scope since you won't see signal problems, but it works for walking
a bus to figure out what is transpiring on a select.
dave06a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Collector of vintage computing equipment: