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. ( Just a note about the above three speed ratings. W27E257-15 = 150 ns access time (slowest 'speed') (lowest 'speed') W27E257-10 = 100 ns access time (fastest 'speed') (highest 'speed') 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. I use these on my IBM 5155/5160/5170 motherboards. The Winbond W27E257 EEPROM is a 27C256 equivalent, but electrically erasable instead.
