Replacing 2708 EPROM with 28C16 EEPROM

Restoring vintage computers is sure to mean dealing with old EPROMs.  

Many restorers have a modern EPROM programmer (i.e it was cheap, made in China and I got it on eBay) or two but there is a very good chance that neither will write (or even read) older parts even if the "supported parts list" suggests they will.

I ran into this problem with the 2708 EPROM on the Northstar ZPB Z80 CPU board.  To debug the machine I needed it to run some known software.

An easy replacement for a 2708 is a 28C16 EEPROM with a home made adaptor.  As shown in the diagram below they are pin compatible for all pins except 18,19 and 21.

The wiring for the adaptor is shown in the 28C16 image.

  • Pin 21 /W is connected to pin 24 VCC.  This ensures it is not connected to the -5V supplied by the 2708 socket and ensures the chip will only be read.
  • Pin 19 A10 is connected to pin 12 VSS.  This ensures it is not connected to the +12V supplied by the 2708 socket and allows only the first 1K of the 28C16 to be read by forcing A10 to always be 0.
  • Pin 18 /E is connected to pin 12 VSS.   On some 2708 sockets this may be omited but on the Northstar ZPB board, wiring like this ensures the 28C16 is always enabled with pin 20 /G latching data on pins DQ0-DQ7.  The 28C16 is low power compared to the 2708 so having the chip always enabled is OK.

I built the adaptor from a small pice of prototype board with offset pin header and pin socket rows.

Adaptor - Top

Adaptor - Bottom

Solder bridges are used to connect pins where no wiring adjustment is required. 

The red wires are the 28C16 wiring adjustments.

Ignore the two blue wires... they are used in place of a solder bridge for pins 8 and 9 because the pads had come off the prototype board!



Powered by Easytagcloud v2.1

Contact Andrew Quinn