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First post, by squelch41

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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.

The eprom works fine in one of my other network cards so the rom is ok.

The 3com has a 32 pin socket. The ROM socket is enabled in the card config program.
The manual just talks about "boot roms" but never specifies exactly which chips are compatible!

Thanks!

V4P895P3 VLB Motherboard and AMD 486 133MHz CPU
64mb RAM, CF 4Gb HDD,
Realtek 8019 ethernet + XT-IDE bios ROM, ES1869 soundcard, VLB multi-io card Cirrus Logic GD5428 1mb VGA

440bx MSI 6119, modified slocket , celeron 1.1GHz, 256mb SD-RAM, CF 4GB HDD

Reply 1 of 3, by Deunan

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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.

Reply 3 of 3, by squelch41

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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

V4P895P3 VLB Motherboard and AMD 486 133MHz CPU
64mb RAM, CF 4Gb HDD,
Realtek 8019 ethernet + XT-IDE bios ROM, ES1869 soundcard, VLB multi-io card Cirrus Logic GD5428 1mb VGA

440bx MSI 6119, modified slocket , celeron 1.1GHz, 256mb SD-RAM, CF 4GB HDD