Some time passed and Gerrit returned to the subject shortly before 2013 Christmas break. He’s got an ingenious, “out-of-the-box” type of idea for approaching the four latest C-64 boards. By looking at various truth tables of commonly available logic ICs he realised how to transform a “Dual 4-Input Multiplexer with common select inputs” into a non-normally existent Dual, Independent 2-Input Multiplexer in a single (74153) chip. This one thing made it possible to design the circuit in a clean and elegant way. Only two logic ICs, the same number as with my previous approach (one slightly larger) and no additional, discrete semiconductors. Moreover all switches operate on static signals’ lines, meaning you can have practically arbitrary length of the cables connecting them to the PCB and you don’t need to take any particular care about where to route the wires. It was brilliant! I admired the idea so much that I quickly drew the schematic:

The schematic (2-ROM narrow boards):


and laid the PCB out:

The PCB (2-ROM narrow boards):


And while being at it I immediately thought of adapting the same, inspired idea to the three-ROM-chip versions of the main PCB, adding the ability use both 27C256 and 27C512 (with two independent ROM sets), and a jumper provision to select what EPROM lines are driven (either both CS (CE) and OE or OE alone - if that ever happens to be necessary for some very old’n slow EPROM chips) on my way:

The schematic (3-ROM wide/SX-64 boards):


And in order not to create three or more layouts for all the main boards variants with three ROMs, I laid out one PCB, which fits all of the 3-ROM versions of the C-64 boards as well as the CPU board of the SX-64.

The PCB (3-ROM wide/SX-64 boards):



  1. Default CHAROM/KERNAL images are active when no jumpers are attached to the CHAROM/KERNAL pinheads or attached switches are in their “open” position.
  2. Assembled PCB is to be press-fit placed in the KERNAL ROM socket with two single-pins installed in 1 2 3 location, tapping on pins 20 of BASIC and CHAROM IC’s sockets respectively. Please use correct pair of pins with the same number.
  3. If your original ROMs were not socketed, you may decide to solder the above PCB in. I have to say, however, that I find little to no excuse for not soldering good sockets in place of the original chips, once those are removed.
  4. ROM content is to be laid out as shown here (supplied GERBER files have this map in the silk-screen too).
  5. If you plan to use the version for 3-ROM boards you can use either ‘256 or ‘512 type of EPROM. If you program the ‘512 type you can program two independent sets of ROMs with for example additional KERNALs or additional CHARSETs, etc. The ROMSET pinhead will allow switching between both sets.
  6. As always - if you want a ready made one, there is a chance I may still have some. Check the appropriate section of my For Sale pages or contact me and ask.
For Sale: