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List:       classiccmp
Subject:    [cctalk] Re: VT100: Failing 2114 Chip Replaced With One With The Same Fault
From:       emanuel stiebler via cctalk <cctalk () classiccmp ! org>
Date:       2023-10-09 7:05:54
Message-ID: e1019cd5-6041-dd5f-7193-63a64c3c476d () e-bbes ! com
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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 ...


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