double density floppy cable adapter for Intel Series II or III MDS

Eric Smith spacewar at
Sun Dec 6 03:26:03 CST 2015

Last month I made a trivial little cable adapter PCB to use with the
Intel SBC 202 double-density M2FM floppy controller in an Intel Series
II or III MDS (normally part of an MDS 720 subsystem). The usual SBC
202 cabling has two DC37S connectors on the MDS back panel, one for
drives 0 and 1, and the other for drives 2 and 3. (There's a variant
that has a ribbon cable to the internal drive as 0, and only one DC37S
for drives 2 and 3.)


Schematic and board layout in Eagle and PDF:

The adapter has no active electronics; it just wires a DC37P to a
50-pin header for a cable to one or two normal 8-inch floppies.

I just got around to testing it yesterday, and was pleasantly
surprised that my SBC 202 and the adapter worked on the first attempt.
The SBC 202 was of unknown provenance so I didn't actually have much
expectation of it working. I haven't yet tried more than one drive on

Intel used radial ready signals from the drives, so the drive(s) have
to be configured slightly differently than the factory defaults to be
fully compatible with the MDS 720.

I have one spare bare adapter PCB which can be made available for
$5.95 plus shipping from Colorado if anyone else needs such a thing. I
am NOT willing to source the connectors or assemble the boards.

You can also order them in increments of three pieces directly from OSH Park:

If there's demand for a few units, I can have a small run made in
China very inexpensively, probably under $4 each plus shipping from

Richard Main has made fancier adapter PCBs that also support 34-pin
cabling for the use of high-density 5.25 or 3.5 inch disks.

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