I finally got around to completely reverse-engineering a spare Micro-SCI floppy disk controller, and have managed to duplicate it. It has been running in my ACE1000 all morning, and appears to be working correctly.
I've placed the KiCAD project at https://codeberg.org/cryu/micro-sci-a2-controller if anyone is interested in building one.
- there is a difference between the original Micro-SCI board and the Franklin re-badged board; this works around the non-compliant-with-the-][+ timing issues in the ACE1000. I couldn't get a vanilla Micro-SCI board to work with Steve's FloppyEmu, although a real floppy drive will probably work. To convert the board to a Franklin-and-FloppyEmu-compatible unit, redirect U2/11 from U1/14 to U1/12, and redirect U5/2 from U1/11 to U5/1 (yes, the two gate inputs are common).
- the "main" branch contains an exact replica of the Micro-SCI board. That means that it needs a 2708 EPROM (or an EPROM on an adapter that makes it look like a 2708).
- the "2716" branch contains a modified board that incorporates two jumpers to select between Micro-SCI and Franklin timing, and replaces the 2708 with a 2716/2732. This branch has not yet been tested (the PCBs are inbound), but the changes are straightforward and thus it should work out of the box.
- the "rework" branch is a "let's use ground pours and modern footprints" work-in-progress, and should therefore not be used yet.
- while troubleshooting the board, I stumbled across Franklin-sourced schematics for the board. They, along with the R1/R2 calibration procedure, are in docs/service_and_schematics.pdf
- the contents of the 2708 EPROM were disassembled, lightly commented, and placed in firmware/. The diagnostic comments are from S.Elliot (thanks!), the 16- and 13-sector comments from https://6502disassembly.com/a2-rom/C600ROM.html, and the "the jumpers go like this" splash screen by me.