Do you have one of those eprom programmers which also do device checks ? They might do a check of the supposed faulty ram out of circuit. If you don't have one you could probably write one for any convenient device you have to hand such as an arduino. Exercising the ram with port writes will be painfully slow compared with a normal ram test but with only 2K to test it shouldn't take too long.
On Sun, Oct 8, 2023 at 9:35 AM Rob Jarratt via cctalk <cctalk@classiccmp.org> wrote: > > > > -----Original Message----- > > From: wrco...@wrcooke.net <wrco...@wrcooke.net> > > Sent: 08 October 2023 04:15 > > To: r...@jarratt.me.uk; Rob Jarratt via cctalk <cctalk@classiccmp.org> > > 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 < > cctalk@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. > > 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 > >
