The 27C256 is a standard 32 KB (256-kilobit) Erasable Programmable Read-Only Memory (EPROM) chip. It is the core memory chip used to store custom fuel and ignition calibrations in chipped OBD0 and OBD1 Honda ECUs.

1. Crucial Speed Ratings#

Honda ECU internal processors read data at high speeds. When sourcing 27C256 EPROMs, pay close attention to the access time (speed rating) printed on the chip.

• Requirement: The ECU requires EPROMs rated at 200 nanoseconds (ns) or faster.
• Compatible Speeds: 90ns, 100ns, 120ns, and 150ns chips will work cleanly.
• Label Decodes: Chip suffixes indicate speed:
  • -90 or -9 = 90ns
  • -12 or -120 = 120ns
  • -15 or -150 = 150ns
  • -20 or -200 = 200ns

Warning: Avoid chips rated slower than 200ns (such as 250ns or 300ns), as they will cause timing sync errors, throwing a solid check engine light (CEL) and preventing the engine from starting.

2. Chip Equivalents & Packaging#

• DIP-28 Package: The 27C256 is housed in a standard 28-pin Dual In-line Package (DIP-28) with 0.6-inch pin spacing.
• Factory OKI Chips: Stock Honda ECUs contain a pre-programmed mask ROM chip labeled OKI 38256. This is simply a factory-printed, write-once equivalent of a standard 27C256.

3. DIP-28 Pinout Layout#

Below is the standard pinout mapping for a 27C256 EPROM:

       +------()------+
   VPP |1           28| VCC (+5V Power)
   A12 |2           27| A14
    A7 |3           26| A13
    A6 |4           25| A8
    A5 |5           24| A9
    A4 |6           23| A11
    A3 |7   27C256  22| /OE (Output Enable)
    A2 |8           21| A10
    A1 |9           20| /CE (Chip Enable)
    A0 |10          19| D7 (Data Bit 7)
    D0 |11          18| D6
    D1 |12          17| D5
    D2 |13          16| D4
   GND |14          15| D3 (Data Bit 3)
       +--------------+

Pin Explanations:#

• A0–A14: Address Lines (input lines used by the CPU to select specific memory bytes).
• D0–D7: Data Outputs (8-bit data bus returning byte values to the CPU).
• VPP: Programming Voltage pin (typically requires 12.5V during write operations on standard burners).
• VCC: Power Supply pin (+5V logic power during normal operation).
• /CE (Chip Enable): Actively selects the memory chip when pulled low (0V).
• /OE (Output Enable): Actively gates data onto the data bus when pulled low.