The 28C256 is a 32 KB (256-kilobit) parallel Electrically Erasable Programmable Read-Only Memory (EEPROM) chip. While it has the same memory capacity and uses the same physical 28-pin Dual In-line Package (DIP-28) as the standard 27C256 EPROM, their pinouts differ.

Because of these pinout differences, you cannot plug a 28C256 directly into a standard chipped Honda ECU socket without hardware modifications.

1. Pinout Comparison: 28C256 vs. 27C256#

Most of the pins—including data lines (D0–D7), ground (GND), logic power (VCC), and chip select (/CE)—align perfectly. However, the highest address line and write control pins are swapped:

2. DIP-28 Pinout Layout (28C256)#

       +------()------+
   A14 |1           28| VCC (+5V Power)
   A12 |2           27| /WE (Write Enable)
    A7 |3           26| A13
    A6 |4           25| A8
    A5 |5           24| A9
    A4 |6           23| A11
    A3 |7   28C256  22| /OE (Output Enable)
    A2 |8           21| A10
    A1 |9           20| /CE (Chip Enable)
    A0 |10          19| I/O7 (Data Bit 7)
  I/O0 |11          18| I/O6
  I/O1 |12          17| I/O5
  I/O2 |13          16| I/O4
   GND |14          15| I/O3
       +--------------+

3. Modifying the Chip for ECU Compatibility#

To adapt a 28C256 EEPROM to function in a standard DIP-28 ECU socket designed for the 27C256:

1. Bend Pin 1 and Pin 27 Outward: Before inserting the chip, bend Pin 1 (A14) and Pin 27 (/WE) outward at a 90-degree angle so they do not enter the socket contacts.
2. Reroute Address A14: Solder a jumper wire from the bent Pin 1 (A14 on the chip) to Pin 27 of the ECU socket (which is where the ECU expects the A14 address signal).
3. Disable Write Enable: Connect the bent Pin 27 (/WE on the chip) to Pin 28 (VCC on the chip/socket). This pulls the Write Enable pin high (+5V), putting the chip into read-only mode and preventing the ECU from accidentally executing write cycles to the memory space.