On 2023-10-08 10:34, Rob Jarratt via cctalk wrote:
Perhaps it isn't the 2114 or its associated circuit at all. Maybe some other
device is being incorrectly selected by that address and driving (half) the bus
low? Just a thought.
Many thanks for the suggestion. This hadn't crossed my mind, so I checked. All
the things that I could identify on the schematic that connect to the bus
(UART, interrupt vector, flag buffer and modem signals) seem not to be enabled.
I have looked at what is sinking the data bus, there is a buffer which seems to
be OK and the 8251 PIC. The PIC is harder to check but I can see it is not
selected and the input pins don’t appear to be shorted.
Put a scope on the data bus. You should see it, if two chips are driving
the bus at the same time ...
