27C256 replacements


Winbond W27E257

The Winbond W27E257 EEPROM is a 27C256 equivalent, but electrically erasable instead.

I use these on my IBM 5155/5160/5170 motherboards.

You will still need an EPROM programmer (or other) to write to the W27E257, because the IBM 5155/5160/5162/5170 motherboard cannot write to EEPROM's in its sockets.


W27E257-10  =  100 ns access time    (fastest 'speed')    (highest 'speed')
W27E257-12  =  120 ns access time
W27E257-15  =  150 ns access time    (slowest 'speed')    (lowest 'speed')


( Just a note about the above three speed ratings.  First, all are suitable for IBM 5155/5160/5162/5170 motherboards.  Next, the speed rating (e.g. access time of 120 ns) is akin to the speed rating on car tyres; it is effectively a maximum speed that the chip is to be run at.  The IBM 5155/5160/5162/5170 motherboards always run the ROM's at a fixed speed, irrespective of what ROM you fit.  So, on IBM 5155/5160/5162/5170 motherboards, there is no performance to be gained by using a 'faster' ROM than that specified by IBM for the motherboard.)



Amtel AT28C256

  WARNING:  The Amtel AT28C256 is NOT a 27C256 equivalent.  

It has a different pinout to the 27C256 - see here.

The pinout difference means that if the AT28C256 is placed in a socket designed for a 27256/27C256, the parent equipment (motherboard, card, etc.) is expected to be able to read only the second half of the AT28C56.  That is because pin 1 of a socket designed for a 27256/27C256 is normally tied to +5V.  Pin 1 of an AT28C256 is the pin for A14.  I.e. The AT28C256's A14 address pin will always be high.  From the parent equipment's perspective, the equipment will 'see' that the first half of the AT28C256 is a duplicate of the second half.

According to here, the AT28C256 works on the early (early, not late) version of the IBM 5160 motherboard, but that is because the ROM sockets on that motherboard are wired for MK38000 series ROM's, not 27256/27C256.  More research required.

Out of interest, I tried a couple of AT28C256 on my IBM 5170 motherboard (of type 3).  I observed that after attempting to use them on the motherboard, the contents of both had been corrupted at the half way mark (example), possibly related to pin 27 (/WE) changing state.  Even if no corruption had occured, there would have still been a problem, because on an IBM 5170 motherboard of type 3, pin 1 of the ROM sockets are tied to +5V.