ZX81 ROM Socket pin out and 2764 EPROM pin out
- ZX81 ROM Socket pin out and 2764 EPROM pin out
Note that in the above diagram, "ZX81 PCB" means the socket pin.
"Mask ROM" means an original 24 pin plastic Sinclair ROM chip (the type without a window on top).
If a 2764, 27C64, 27128 , 27C128, 27256 or 27C256 EPROM is being used, some wiring changes are needed.
Pins 1 through to 19 are okay as is.
EPROM chip pin 20 needs to be bent up so it does not enter the socket.
Use a thin insulated solid core wire to link EPROM chip pins 14 (GND) and 20.
Pins 21 and 22 are okay.
EPROM chip pin 23 needs to be bent up so it does not enter the socket.
Use a thin insulated solid core wire to link EPROM chip pin 23 (A11) to socket pin 20 (A11).
Pins 24 through to 28 are okay as is.
If a 27128 or 27C128 EPROM is being used, the ROM image (data) must be programmed to start at chip address 0x2000 (8192). In other words, it must be programmed in the upper half of the chip.
If a 27256 or 27C256 EPROM is being used, the ROM image (data) must be programmed to start at chip address 0x6000 (24576). In other words, it must be programmed in the top quarter of the chip.
Here's a photo showing the modification:
See
this topic for the source of the photo.
Mark