Re: Smartport driver for superdrive?

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

<pubpc1@library.ucla.edu> wrote:

> Several years ago, AE had a 1.6MB GCR driver for their HD 3.5"
> floppy drive.  Since the AE drive used the same mechanism as
> the Apple drive, how difficult would it be to create a 1.6MB
> driver for a superdrive connected to the smartport.

They might have used the same mechanism, but I doubt they use the same
interface board in the drive.  I think the AE drive must have some
hardware support within the drive to handle the 1.6MB format.

The IIgs's built-in IWM only supports two data rates: 4 microseconds per
bit cell (used with 5.25" drives and intelligent SmartPort devices) and
2 microseconds per bit cell (used with 3.5" drives).  To access a 1.6MB
3.5" drive directly would require a 1 microsecond bit cell.

Therefore, the AE drive must have buffered data within the drive and
transferred it to the computer at the standard speed (2 microseconds per
bit cell).

This assumes that the 1.6MB format is achieved by doubling the number of
sectors per track (16 to 24 sectors instead of 8 to 12), and therefore
doubles the bit rate.

If it works by halving the width of the tracks and retaining the same
number of sectors, then no buffering is required, but the drive needs to
support a weaker write current and smaller head step distance (3.5"
drive head stepping is controlled by the drive).  This is not likely,
because normal high-density disks go by the first method - twice as many
sectors per track, and different media would be required.

The SuperDrive card uses a SWIM chip.  I don't have any technical
information on this chip, but by observation I can tell that it supports
MFM data encoding with 1 or 2 microsecond bit cells, as well as GCR with
2 or 4 microsecond bit cells.  I don't know if it supports 1 microsecond
GCR (1.6 MB).

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