Keytronics KB5151 AT&T keyboard eprom - Page 1
Forum section: electronics / general

author: joelgraff
author_rank: Contributor
timestamp: January 11, 2024, 01:43:51 pm
content: I have a KB5151 for the AT&T (Olivetti) system that’s misbehaving. Likely it’s a bad IC, of which there’s several, including TTL, bespoke chips, an Intel microcontroller and an eprom.At the moment, I’m trying to see if I can dump the eprom. Unfortunately, I don’t think my cheap Chinese TL866 is up to the task. I seem to recall that older EPROMs aren’t readable by newer / cheaper programmers, though. What’s more, I can’t seem to find any documentation on the chip itself.Specifically, the eprom is a National chip, 24-pin DIP, with “UMWA098” and “LB9024” stamped on the chip. The sticker over the window has the number “258701” on it with a 1986 copyright. I suspect that’s in reference to the firmware, as Keytronic part numbers appear to have been much longer.Anyway, I read in the schematics and parts list that this is a “2796” eprom, which, so far as I can tell, doesn’t really exist. I doubt it’s 96K as the Intel microcontroller only required a 4K external ROM, though it may well be.Any thoughts?

author: Andy Chee
author_rank: Super Contributor
timestamp: January 11, 2024, 02:11:59 pm
content: Incompatible pinouts can be rectified with a jerry-rigged DIP IC socket adapter.The high voltage Vpp is obviously not needed for reading. I'm not sure about the negative -5V Vbb (is there a -5V supply on the keyboard?)But yes, you need a datasheet first or infer from the keyboard PCB.

author: joelgraff
author_rank: Contributor
timestamp: January 11, 2024, 02:19:02 pm
content: At a glance, I don’t think there’s a -5 power rail. The AT&T version appears identical to the IBM PC version except that there’s a 7805 in line with the input, because the AT&T socket provides +12vdc.

author: HarryDoPECC
author_rank: Regular Contributor
timestamp: January 12, 2024, 05:01:26 am
content: Since you have schematic, check EPROM data width and address bits - this will tell you the geometry. eg D0-D7 and A0-A11 is 2K x 8 and that leads you to 2716 and cousins. Check those datasheets for best match with the signals on your schematic. That will hopefully get you to data for the chip you have, even if it has a custom part number.Only when you have a pretty good match, then try reading using the specific manufacturer settings that best correspond to your research result. EPROM readers can have read/write sequences that vary between chips of same generic number but different manufacturer - so choose the "specific 2716" before "generic 2716". (Just now I am wrangling a bunch of 27C256's and my programmer has multiple entries for "27C256", including the "M27C256" that I actually have).I'm assuming here that you have a UVEPROM - mask ROMs from the early 70s could be much more quirky and todays ROM readers may understand such chips poorly. In such cases it may be necessary to use a PIC or similar to drive and read the ROM directly, based on the schematic that you have.

author: HarryDoPECC
author_rank: Regular Contributor
timestamp: January 12, 2024, 05:09:06 am
content: Also, re "misbehaving" - are there socketed ICs?I fired up my VT100 a few days ago after a year or more of neglect - blank screen, no beeps, no lights, but at least a nice scan raster collapse on poweroff so the CRT side seemed OK.Totally dead suggested PSU, but voltages all OK. OK, sigh, have to get out the scopes and logic analyzer to chase why the processor seems to not even be getting to self-test.But before that - crack and reseat all the socketed ROMs.-> VT100 restored and Happy Days.So check all those connectors and sockets. Also have a good look for dry/cracked solder.Good luck