Erm, no the
serial/parallel conversion isn't common to both ports
on the chip.
Hmm... curious. I'd downloaded the datasheet for the chip and it wasn't
clear either way so figured I'd try the inverter trick.
This implies that some buffer chip that feeds to
8530 is
faulty.
If they are totally seperate channels (no common internal buffering
first, say) then yep, it looks that way. There's no buffers immediately
by the serial logic, so I'll look further afield.
Even though there is (obviously) separate parallel/serial shift registers
inside the chip for each channel, I am assuming that the parallel data bus
enters the chip through only one set of pins - it is entirely possible that
the input gate, or some internal buffer common to both channels has failed.
An easy test would be to trigger on chip select, scope the affected data
bit on the bus inputs, and write patterns to the chip with the affected
bit in both hign and low positions - it should be very easy to determine if
the data bit is making it to the chip or not. If yes, replace the device,
if no, move to the next upstream component and repeat the test (all the while
keeping triggering on the device chip select). It should be a very easy matter
to identify the defective device in this fashion - no need to remove anything
other than whatever device is defective.
Even if you don't have an extender board, it is far easier to tack a lead
on to points you want to test than to be removing and replacing entire devices
in the hope of being lucky. -- Just MHO.
Regards,
Dave
--
dave04a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools:
www.dunfield.com
com Collector of vintage computing equipment:
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