8 Oct
2023
8 Oct
'23
8:34 a.m.
-----Original Message-----
From: wrcooke(a)wrcooke.net <wrcooke(a)wrcooke.net>
Sent: 08 October 2023 04:15
To: rob(a)jarratt.me.uk; Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
Subject: Re: [cctalk] VT100: Failing 2114 Chip Replaced With One With The
Same Fault
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.
On 10/07/2023 5:35 PM CDT Rob Jarratt via cctalk
<cctalk(a)classiccmp.org>
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.
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