> -----Original Message-----
> From: [email protected] <[email protected]>
> Sent: 08 October 2023 04:15
> To: [email protected]; Rob Jarratt via cctalk <[email protected]>
> Subject: Re: [cctalk] VT100: Failing 2114 Chip Replaced With One With The
> Same Fault
>
>
>
> > On 10/07/2023 5:35 PM CDT Rob Jarratt via cctalk <[email protected]>
> wrote:
> >
> >
> > I find this really hard to explain. It can't be the chip selection
> > logic because then the addresses 0x2400-0x2407 would also fail and I
> > checked the CS signal with the logic analyser just to be sure. I also
> > checked the address lines directly on the RAM chip for any stuck bits
> > and they seemed fine too.
> >
> >
> >
> > What are the chances of two 2114 chips failing at exactly the same address?
> > Is there some failure mode I might not be considering?
> >
> > Rob
>
> 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.
Not really sure what else to consider.
>
> Will
>
> If you want to build a ship, don't drum up people to collect wood and don't
> assign them tasks and work, but rather teach them to long for the endless
> immensity of the sea.
>
> Antoine de Saint-Exupery
