Great news 640K!enough ... you're really earning your name on the frontside of the PCB here 😀
Pretty excited for what you come up with.
The first thing I looked at was the patch parameters in the official FFF files. I didn't really take a detailed look at the 4 MiB set beyond that, but it was interesting to look at the effects settings. I will gradually take a few hints from those settings for future ROM files. For the time being, I have resorted to hex-editing the parameters before generating the ROMs, which can get tedious as the number of instruments grows. I think the sound quality is gradually improving, with a few mysteries that remain to be solved. I think further work on the parser will help us get there.
Prototype 1 has now evoled to a real prototype 😀
Regarding adding the socket, I had to slightly modify the lower carrier because otherwise a decoupling capacitor would be in the way and the jumpers for the SIMM/SOJ RAM selection had to be removed (the plastic standoffs collided with the upper part of the socket). I'll try to incorporate support for sockets in the final version (for the ROM, as well as the SOJ DRAM), that has actually been on my to-do list for a while.
I have to admit that it looks good with the socket in place. Now it really looks like a prototype meant for experimentation.
For the final board, wouldn't the sockets be an unnecessary expense? If they can solder the socket, isn't the chip about the same, in terms of difficulty? Once it's out of the prototype stage, are we really expecting to be swapping chips so often that the cost of sockets is justified? If it's still under consideration, and we can get in-circuit flashing working, that could easily cut at least $15 worth of sockets from the BOM. I have a theory that might get it working with minor hardware changes and some software trickery. Sadly, any attempt at writing that sort of software will have to wait until I can find a board that I wouldn't mind modifying at a sane price; the $400+ they want on eBay is out of the question!
Regarding different pinouts of the IW78C21M1 and 29F800 ... it basically comes down to the highest pin of the ROM that has to be wired up differently depending on the chip used (!RESET on the 29F800 which is constantly held low on GUSPnP boards, which works for the IW87C21M1 but holds 29F800 chips in a permanent reset state). I actually messed that up in the prototype which is what the wire on the backside in above picture is for (which replaces the botch wire in previous photos). That wire will of course not be present in the final run and will be replaced by a jumper for configuring how the board is populated.
Do all of the other pins fall in exactly the same place? WE#, BYTE#, etc. are all sure to be correct? Are there any other free pins that we could borrow for other purposes?