The Version 2 of my (and Gerrit's) ROM replacement board is inexpensive, easy to build and works very well. I haven't received a single request for supporting additional CHAROMs or BASICs for example, I received multiple request for supporting more than two KERNALs. The typical comment went like "I like that I can have original and my national (Dutch, Finnish, Swedish, German, Polish, ....) character set but I need to have more KERNALs. The easiest way to achieve this was to allow installation of either a '256 or a '512 ROM IC. This was welcome by people who installed them but I kept feeling uncomfortable about the fact that in the two sets configuration it was not possible to select any of the four KERNALs, four CHAROMs and two BASICs independently. In other words you could not for example select KERNAL from one set and CHAROM from another. This is not a problem when you equip the board with a '256 ROM chip but then you are limited to two KERNALs only.

Another redesign time then. While I was almost sure that Gerrit (obviously) would be able to come up with a logic solution that would require not more than two typical logic ICs, it still seemed to me like a perfect moment to raise the bar a notch. Instead of finding what set of logic ICs I could use/abuse, I elected to put all the required logic into a small SPLD. The smallest with enough capacity and pin count I had at hand happened to be GAL16V8. While most IC vendors no longer produce SPLDs, chips from for example Lattice are still relatively easy to obtain as NOS. Even better option (due to lower power requirement) and a notable exception among the manufacturers is the venerable Atmel... erm... not so venerable Microchip now, which has a range of 5V SPLDs, including ATF16V8 still in their products line as of the time of writing.

The good thing with SPLD approach is that one small chip, with less pins than any potential set of standard logic ICs, gave me all the logic I needed. Even better is that with simple reprogramming of the said SPLD chip you can get different combinations of selectable ROM content on the same adapter. I implemented the following configurations:

Configuration 0:621
Configuration 1:442

but other configurations are also possible.

The slightly less good thing is that you need an appropriate programming device for the GAL type of logic devices and your do-it-all EPROM programmer, the one you programmed all the chips so far, just can happen not to be up to snuff. This is especially true if you choose Atmel to be your PLD vendor. In the links section below you can find link to a page, author of which claims to have successfully programmed Atmel's ATF GAL compatible ICs

  • - GERBER files - C64 wide boards and SX-64
  • romel512_6k2c1b.jed - JEDEC file for programming the SPLD chip
  • conditioning.jed - JEDEC file for "contidioning" new Atmel chips (program twice, then erase)