Zendex ZX-200A firmware reverse-engineered

Eric Smith spacewar at gmail.com
Wed Sep 9 03:45:39 CDT 2015

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


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.

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