An historical nit about FDDs

Peter Corlett abuse at
Fri Jul 13 06:37:45 CDT 2018

On Wed, Jul 11, 2018 at 11:12:59AM -0700, Tom Gardner via cctalk wrote:
> Anyone know where the Step/Direction version of the FDD interface
> originated.
> So far as near as I can tell the earliest FDDs (IBM 23FD Minnow and Memorex
> 650/651) used Step In/Step Out. The IBM 33FD Igar used direct control of the
> motor.

Having step-in and step-out gives the opportunity to do both at the same time,
which is unlikely to go well. Step and direction just seems neater to me
because it's essentially equivalent to clock/strobe and data, but perhaps
that's just my modern prejudices.

The choice of which scheme probably boils down to which was the idiomatic way
to do sequential logic when the respective devices were designed. I wonder how
one might design a floppy interface today. I suspect that it'd end up as a pair
of TMDS links with some complex high-level multiplexing scheme.

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