Re: Apple External 3.5" drive

[dempson@atlantis.actrix.gen.nz (David Empson)]

In article <3lkmer$rfg@tuba.cit.cornell.edu>,
Jay Krell <jmk3@crux4.cit.cornell.edu> wrote:
> 
> I've read that it is at least pin compatible, but that's not enough
> I guess. I'd imagine, but I'm rather hardware ignorant, that
> a SWIM might work as an IWM if you swapped them.

OK, I might be wrong about needing a higher clock speed.

The IWM will either run at roughly 8 MHz (Mac) or 7 MHz (Apple II)
depending on a configuration bit.  Internal circuitry is used to
generate timing from this reference signal. 

An Apple II 5.25" drive or a SmartPort drive (UniDisk 3.5) requires 4
microsecond per bit timing.  An Apple 3.5 Drive requires 2
microseconds per bit.

An MFM 3.5" drive requires 1 microsecond per bit resolution at 720k
(bit cells are 2 microseconds wide, but spacing has a resolution of 1
microsecond), or 0.5 microseconds per bit at 1440k.

All of these timings could be generated from the standard 8 MHz or 7
MHz clock speed.

Implication: a SWIM could be a complete superset of an IWM without
requiring any hardware changes.  Extra configuration registers would
be required to implement MFM and additional bit timings.

> The reference to their pin compatiblity even mentioned something
> about the Disk ][ circuitry being in the SWIM.

Well, the IWM is the Disk ][ _controller_ in a chip, with extra
features added.  The SWIM is an improved version of the IWM.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand