Switchable ROM

The Switchable ROM board is very similar to the 8K ROM board with the exception of it being designed for a 64K ROM.  Only A0 to A12 are connected to the Z80’s address line though, so only 8K is visible.  Address lines A13, A14 & A15 from the ROM are bought out to header pins which define which 8K is presented to the Z80.

ROM 27C512

Address lines A13, A14 & A15 on the Z80 need to all be low for the Chip Enable to be triggered, which places it in the 0x000 – 0x1FFF address space.  The RD and MREQ need to be low to trigger the Output Enable.

Click the image below for schematics in PDF format


A 64K 27C512 is divided in to 8 x 8K (0x2000) with 3 jumpers to bring A13, A14 & A15 either high or low as per the illustration below.  Note that all 3 jumpers must be used as leaving any of those address lines could give unpredictable results.


Although this was designed for a 27C512 EPROM, I have tested it with 27E512 (electrically erasable version) and it’s fine.  It should also work with AT27C512R (non-erasable version) or with minimal modification the 27C256 (32K) and 27C128 (16k) chips too.


Buy one here

Bill of Materials

  • 32 pin R/A header
  • 28 pin wide DIL socket
  • 14 pin narrow DIL socket
  • 27C512 EPROM or similar
  • 74LS32
  • 3 x 3-pin header
  • 3 x jumper