all 90-95 (most OBD0 and all OBD1) honda ECUs require a 32k byte (256kbit) ROM. Some common chips used by the DIY-chipping scene are the 27 C256, AT29 C256 and 27 SF256. You can generally find EPROMs at a local electronics store or a major Electronic Part Supplier.

Make sure you pay attention to the speed of the chip. You need a ROM that is 200ns or faster. 250ns or slower MAY work, but your mileage may vary.

• -20 generally means 200ns
• -15 generally means 150ns
• -12 generally means 120ns
• -9 generally means 90ns
• -7 generally means 70ns
• etc.

;:Joseph Davis Says: 200ns is too slow; some ROMS labelled 200ns are in truth faster than that which is where the confusion arises. Honda specifies 170ns or faster, or at least it does for OBD0 . Below is a picture of a factory chipped, window 27C256-17 from an OBD0 PR4 dated 6/89 on the case. Honda proceeded with production of this ECU even though they didn't have their mass production burns in hand yet.

• -17 on "33" PR4 chip:
  

Note: 28 C256 Chips will work in ECUs, but they have a different pinout and cannot be used without modification.