9 Oct
2023
9 Oct
'23
12:05 a.m.
On 2023-10-08 10:34, Rob Jarratt via cctalk wrote:
Put a scope on the data bus. You should see it, if two chips are driving
the bus at the same time ...
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.