Differences on pin 1 between 27C256 & 27C512 E(E)PROM chips \ VOGONS

Differences on pin 1 between 27C256 & 27C512 E(E)PROM chips

Topic actions

Post a reply

First post, by jesolo

Posted on 2018-03-17, 17:19

jesolo Offline

Rank l33t

Rank: l33t
Posts: 2104
Joined: 2014-06-24, 19:04
Location: South Africa

I have an old XT motherboard with two 27c256 EPROM (UV erasable) chips that isn't booting - I suspect that one of the EPROM chips might be faulty and would therefore like to test it with other chips.
I currently only have 27C512 EEPROM (electrically erasable) chips available and my understanding is that one can just fill the extra 64K by "duplicating" the contents of the BIOS (I know how to do this via the DOS copy command).

However, by referring to the datasheets of the two, I noticed that pin 1 of the two are not the same (my understanding of this is limited and I'm therefore looking for some insight into this).
On the 27C256 pin 1 is designated as Vpp, whereas on the 27C512 it is A15.
What does A15 constitute and would this pose a problem in terms of reading contents on the motherboard?

Reply 1 of 4, by Koltoroc

Posted on 2018-03-17, 17:49

Koltoroc Offline

Rank Member

Rank: Member
Posts: 381
Joined: 2016-10-07, 23:21

A15 is an additional address line, Vpp is the pin where the programming voltage is supplied. I don't believe it would cause issues, AFAIR A15 is used to address the upper half of the memory in the 27C512 which your board wouldn't use anyway and I don't think XT boards support writing eproms either, so I believe pin 1 should be unused.

Reply 2 of 4, by lazibayer

Posted on 2018-03-17, 22:08

lazibayer Offline

Rank Oldbie

Rank: Oldbie
Posts: 1065
Joined: 2014-10-21, 21:16

I checked the datasheets of different 27C256 chips and they require Vpp=Vcc during reading operation, so it's likely that your board wired Pin 1 to Vcc. In this case 27C512's Pin 1 will be constantly pulled up so only upper half of your chip's content will be read by your board, which is totally fine.

Reply 3 of 4, by keropi

Posted on 2018-03-17, 22:28

keropi Offline

Rank l33t++

Rank: l33t++
Posts: 9264
Joined: 2003-09-08, 06:45
Location: Greece

fill the 27C512 with 2x C256 data and you'll be fine (copy /b C256.bin + C256.bin C512.BIN -> write C512/bin to your chip, no further steps needed)

🎵 🎧 ⏳ ⚡ PCMIDI MPU , OrpheusII , Action Rewind , Megacard and 🎶GoldLib soundcard website

Reply 4 of 4, by skate323k137

Posted on 2022-01-31, 05:51

skate323k137 Offline

Rank Newbie

Rank: Newbie
Posts: 32
Joined: 2018-07-26, 14:39

lazibayer wrote on 2018-03-17, 22:08:

I checked the datasheets of different 27C256 chips and they require Vpp=Vcc during reading operation, so it's likely that your board wired Pin 1 to Vcc. In this case 27C512's Pin 1 will be constantly pulled up so only upper half of your chip's content will be read by your board, which is totally fine.

I know I'm bumping an old post but this helped save my sanity today. I came to the same conclusions checking the datasheets. Doubled the roms, burned them, no luck. Thank God my board was socketed (and I was prepared to socket it if it wasn't). Finally after verifying and reverifying my roms, I burned another identical pair of 27c512s and it worked fine.

All this after finishing a recap on the unit today. So, thank you for the sanity check.

Go to top of page Go to top of page

Back to General Old Hardware

Main menu

Common searches