squelch41 wrote on 2022-02-08, 17:37:

Hi,
Doesn anyone know which (e)eproms that 3com 3c905B-TXNM network cards support?
Want to use one for XT-IDE but it seems that the AT28C256 doesnt work but it may be I have it configured wrongly.

Made the same mistake once because I mis-remembered the pinout. So first AT28C is an EEPROM, not EPROM. A big difference, it can be electricaly erased and written to in-circuit. Which means it needs /WE (a write enable) input signal, that was added on pin #27. Problem is, a 27C256, the EPROM you want to be in that socket, has addres line A14 on pin 27. Where is A14 on 28C256? On pin #1, which is Vpp (so tied to +5V during normal operation) on 27C256.

Not only that screws up the addressing, any attempt to read bottom half of a chip like 27C256 will drive A14 low, which will be interpreted as write strobe by 28C256 and bytes get over-writen by whatever floats on the bus.

Solutions: Get a chip called "EPROM emulator" which has exactly the pinout of 27C256. It's Flash or EEPROM inside but interprets 12V on Vpp pin as write enable, so it's a modern direct replacement and doesn't need UV to erease. Or, obvisouly, use an actual UV-erasable EPROM part. Or OTP (one-time-programable) EPROM if the image you want in there is final and wont ever change.
Alternatively the card could be modded and re-wired to accept a 28C256 instead of 27C256. Not sure if anybody would want to go that route but it's very much doable.

Here's some more info about usable EEPROMs:
Re: XT-IDE BIOS with 3com 3c509B

1541 wrote on 2022-02-08, 20:35:

Here's some more info about usable EEPROMs:
Re: XT-IDE BIOS with 3com 3c509B

that's a different card