The 27C256 is a 32 KB (256-kilobit) Erasable Programmable Read-Only Memory (EPROM) chip. It serves as the primary storage medium for fuel and ignition calibration maps in chipped OBD0 and OBD1 Honda ECUs.

1. Speed Requirements#

Honda ECU processors require specific access times to read data reliably. When sourcing 27C256 EPROMs, ensure the chip meets the minimum speed threshold.

• Requirement: Use EPROMs rated at 200 nanoseconds (ns) or faster.
• Compatible Speeds: 90ns, 100ns, 120ns, and 150ns.

Speed Suffix Reference#

Chip manufacturers typically denote speed via a suffix on the part number:

Warning

Avoid chips rated slower than 200ns (e.g., 250ns or 300ns). Slower chips will cause timing synchronization errors, resulting in a solid Check Engine Light (CEL) and a no-start condition.

2. Packaging and Equivalents#

• Package: The 27C256 utilizes a standard 28-pin Dual In-line Package (DIP-28) with 0.6-inch row spacing.
• Factory Mask ROM: Stock Honda ECUs utilize a factory-programmed mask ROM labeled OKI 38256. This is a write-once equivalent to the standard 27C256.

3. Pinout Configuration#

The following table outlines the pin mapping for the 27C256 EPROM.

Signal Definitions#

• A0–A14 (Address Lines): Input lines used by the ECU processor to select specific memory addresses.
• D0–D7 (Data Bus): 8-bit output lines that transmit stored calibration data to the ECU.
• VPP: Programming voltage pin; requires 12.5V during the EPROM burning process.
• VCC: Logic power supply (+5V) required for standard operation.
• /CE (Chip Enable): Selects the memory chip when pulled to ground (0V).
• /OE (Output Enable): Gates data onto the bus when pulled to ground (0V).