The 27 C256 is the basic EPROM commonly used to chip ECUs. It is a 32k byte (256kbit) chip. 27C256 chips are generally either OTP or UV-Erase. Beware tho! Most honda ECUs require chips rated at 200ns or faster- always check the speed ratings! 90, 100, 120, 150, 175 and 200ns chips will work fine. -9 means 90ns, -12 means 120ns, etc.
27C256 UV-Erase chips can often be found very cheaply on ebay. I recently acquired 100 27C256 UV-Erase chips for ~$35 shipped.
Note: the Oki 38256 chip present in many ECUs is simply a 27C256 type chip.
Datasheet for the Microchips 27C256 (PDF)
name: 27C256
function: 32k x 8 paged parallel EPROM.
package: DIP-32,32
pinout:
+------()------+
VPP |1 28| VCC
A12 |2 27| A14
A7 |3 26| A13
A6 |4 25| A8
A5 |5 24| A9
A4 |6 23| A11
A3 |7 27256 22| OE
A2 |8 21| A10
A1 |9 20| CE
A0 |10 19| D7
D0 |11 18| D6
D1 |12 17| D5
D2 |13 16| D4
GND |14 15| D3
+--------------+
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