The original proprietor of Zendex has put the firmware of the ZX-200A in the public domain, so I've put my reverse-engineered source code on Github: https://github.com/brouhaha/zx200a-fw The ZX-200A is a single-board Multibus floppy controller intended for Intel MDS (including Series II and Series III) development systems, replacing either or both the Intel dual-board SBC 201 single-density and SBC 202 double-density floppy controllers. Since it can do both densities, it can replace both Intel controllers simultaneously, allowing the same floppy drives to be used for either density, by using different logical drive numbers. We take it for granted that "modern" floppy controllers often support selectable density, but with those early Intel board-level floppy controllers that wasn't the case. Also, the Intel SBC 202 double-density controller uses an Intel-proprietary M2FM disk format, instead of IBM-compatible MFM. As such, none of the single-chip FDCs are compatible with it, with the possible exception of the Western Digital 1781 (with a huge pile of external support logic), and (less likely) the TI TMS9909. Unfortunately the original ZX-200A manual, which contained the schematics and source code, is not available, hence my project to reverse-engineer it. I'm working on tracing out the schematic, but as the board has 82 chips it is slow going.