A 28C series EEPROM can be used as replacement for an equally-sized 27C series EEPROM without any issues. A 28C256, or Winbonds 27E257 (later called 27C257) is a suitable replacement for a classic 27C256. The 28C128 has one address pin less than the 27C256. That's pin 27. On the 27C256, that pin is A14. On the 28C128, this pin is /PGM. This means if the card tries to access the low half of the 27C256 (A14 = low), it will enable "programming mode". The Microchip datasheet for the 27C128 I found doesn't specify what happens if you apply a programming pulse without applying 13V programming voltage to pin 1. Likely the chip will not be modified because there is no sufficient voltage for programming, but this is undefined. I couldn't find a 28C128 datasheet, but the same should apply.

So to use a 16KB 28C128-like EEPROM on a device that expects a (up to) 32K EPROM, do not insert pin 27 into the socket, but just tie it to pin 28 (possibly using a pull-up resistor instead of a direct connections, so the device stays programmable).

In addition to the above, you want a chip with equal or faster access times than the original one to avoid potential timing problems. If a slower chip works or not of course depends on how fast the driving circuitry works, but an equal or faster chip is the safe bet.

The way those parallel (E)EPROMS work from the user perspective is actually very simple by the way. Might be worth the time to read up on the basics too, i.e. general pin configuration etc. I'm sure there are plenty of tutorials online 😀

Hi,
I was searchinǵ for a similar question, but when I check the datasheet of eg at28c256 against 27C256 the pins are different, Adressbusline 14 is on the left side pin 1 for at28C256 and for 27C246 on the right side pin 27. Is there in pin compatible version or did I miss something?
The Winbond 27E257 seems to fit.

BR Stefan

I run into an annoyance when trying to read Eproms from my TL866II Plus when I dont want to scrape off an old classic label. I can usually figure out what settings will read the old DIP28 rom through trial and error, but it is time consuming and less fun the 10th time. Seems like the AMD settings are compatible with many, but the ST roms in particular are different