Not having a programmer for this chip I decided to use my Arduino. After trying out various projects on the internet I eventually came across TommyPROM which uses 74LS164 serial to parallel shift registers. Wired up to my Arduino Uno, fired up TerraTerm and uploaded the diagnostic ROM to the 28C256.
The 28C256 is a reflashable EEPROM and almost the same pinouts as the 27C256 (OTP)
To replace the 27C256 with the 28C256 you need to go the following:
Comparing them side by side you can see the differences:
- The ’27 has: a. pin #1, Vpp which in circuit is not used (either Gnd or Vcc) b. pin #27, A14 which is a driven address line.
- The ’28 has: a. pin #1, A14 is a driven address line. b. pin #27, /WE Which must be tied high in circuit.
When you place the ’28 in circuit you must:
- Lift pin #1 of the ’28 so it does not contact the Vpp pad
- Lift Pin # 27 of the ’28 so that it does not contact the A14 Pad
- solder or join or bridge the lifted pin 27 to Pin #28 of the ’28 (tying /WE high)
- connect the PAD at Pin location #27 on the board with a blue wire to the lifted pin #1 of the ’28.