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

Scm_SwitchROM

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.

PhotoGrid_1413636650152

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.

SwitchableROM_Tindie

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
3 Comments
  1. […] ROM – an AT27C512 one time programmable (OTP) EPROM (64kb) supported by a 74HCT32 and some jumpers so that different chunks can be addressed to select BASIC, CP/M monitor or Small Computer Monitor. […]

  2. […] have however succeeded in getting EVMBUG running from the switchable ROM module, which allowed me to run HELLO WORLD from the […]

  3. […] wanted to run BASIC and Forth. I’d not been able to get my pageable ROM card working, and the switchable ROM module is too small. But it’s too small because it’s designed to carve a 64K EPROM into 8K […]