28C256 EEPROM Pinout and ECU Compatibility
Technical specifications, pinout mapping, and socket modification instructions for using the 28C256 EEPROM chip in standard Honda ECU sockets.
The 28C256 is a 32 KB (256-kilobit) parallel Electrically Erasable Programmable Read-Only Memory (EEPROM) chip. While it shares the same memory capacity and physical 28-pin Dual In-line Package (DIP-28) as the standard 27C256 EPROM, the pinouts differ.
Because of these pinout differences, the 28C256 cannot be plugged directly into a standard chipped Honda ECU socket without hardware modifications.
1. Pinout Comparison: 28C256 vs. 27C256
Most pinsâincluding data lines (D0âD7), ground (GND), logic power (VCC), and chip select (/CE)âalign. However, the highest address line and write control pins are swapped.
| Pin Number | Standard 27C256 Pin | 28C256 EEPROM Pin | Impact |
|---|---|---|---|
| Pin 1 | VPP (Programming Voltage) | A14 (Address Line 14) | Mismatch: ECU socket carries +5V/12V on Pin 1. |
| Pin 27 | A14 (Address Line 14) | /WE (Write Enable) | Mismatch: Forces write state or reads incorrect address. |
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 for use in a standard DIP-28 ECU socket designed for the 27C256:
- Bend Pins: Bend Pin 1 (
A14) and Pin 27 (/WE) outward at a 90-degree angle so they do not make contact with the ECU socket. - Reroute Address A14: Solder a jumper wire from the bent Pin 1 (
A14) on the chip to Pin 27 of the ECU socket (where the ECU provides theA14address signal). - Disable Write Enable: Connect the bent Pin 27 (
/WE) on the chip to Pin 28 (VCC).
Important
Connecting Pin 27 to Pin 28 pulls the Write Enable pin high (+5V). This forces the chip into read-only mode, preventing the ECU from accidentally executing write cycles to the memory space.
Caution
Ensure all jumper wires are insulated and do not short against adjacent pins or the ECU PCB. Failure to properly isolate these pins may result in memory corruption or damage to the ECU logic circuits.