Thanks Adrian,
I am not sure if the eprom programmer I have will do what you suggest, I don’t have an Arduino but I do have a Raspberry Pi that I could probably employ in testing the RAM. I may look into that. I will look again at the writes. I have seen that the test pattern (0xAA) is written consistently to many other locations, including some to the seemingly faulty chip, so it is hard to think how the wrong data may be getting written, but I think I can check this fairly easily just to be sure. Regards Rob From: Adrian Godwin <[email protected]> Sent: 08 October 2023 12:38 To: [email protected]; General Discussion: On-Topic and Off-Topic Posts <[email protected]> Subject: Re: [cctalk] Re: VT100: Failing 2114 Chip Replaced With One With The Same Fault Another possible approach is to trigger the logic analyser on a write access to that ram address, preferably with the probes on the ram itself. Look at the resulting captures .. does it seem consistent with the code and other accesses ? On Sun, Oct 8, 2023 at 12:35 PM Adrian Godwin <[email protected] <mailto:[email protected]> > wrote: 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 <[email protected] <mailto:[email protected]> > wrote: > -----Original Message----- > From: [email protected] <mailto:[email protected]> <[email protected] > <mailto:[email protected]> > > Sent: 08 October 2023 04:15 > To: [email protected] <mailto:[email protected]> ; Rob Jarratt via cctalk > <[email protected] <mailto:[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] > > <mailto:[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
