From: Ethan Dicks (ethan.dicks_at_gmail.com)
Date: 2006-07-03 09:56:34
On 7/3/06, Spiro Trikaliotis <ml-cbmhackers@trikaliotis.net> wrote:
> I do not know if the "Commodore Shugart bus" drives are
> standards-compliant; anyway, the PC floppy drives are not. The only
> difference I can name for sure is that the Shugart bus allowed for up to
> four drives, while the PC's incarnation only allows for two of them.
>
> So, perhaps, Commodore was compliant, while the PC is/was not?
The 34-pin floppy interface (SA-400 drives and such) is a
reduced-format version of the 50-pin 8" drive bus that preceded it...
in fact with one or two possible signal exceptions (TG43), you can
make a passive cable that allows you to put an 8" drive on a
controller that was intended for 5.25" drives. FM-vs-MFM and the
analog electronics might or might not permit you to correctly
interpret what's on the disk, but the motor-on and step signals and
the like _are_ compatible. Back in the day of 8" and 5.25" drives,
there were 4 drive select options per drive (DS0-DS3) and your
controller might be able to talk to 2 drives or up to 4 drives.
What changed with PCs was the commercial expectation that people would
only ever need up to 2 floppy drives per machine. The "magic twist"
was a way of simplifying maintenance - flip the drive select part of
the cable around and you won't ever have to restrap a floppy drive -
just set it to DS1 (vs DS0) at the factory, and on a twist cable, it's
positional - the drive before the twist is B: and the drive after the
twist is A. The twist does, though, IIRC, prohibit a 4-drive cable.
If the controller _would_ talk to 4 drives (some would, but not many),
you could always make a straight cable with 5 connectors (controller
and 4 drives) and set the drive jumpers. That ended with 3.5" drives,
as most could only be selected to DS0 or DS1, then later, as
manufacturing costs plummeted, they couldn't be selected at all
without a soldering iron - they were DS1 and that's it.
Once people didn't "need" 4 drives, manufacturers started cutting
corners and now you can't _have_ 4 drives on a single controller. I'd
say you have to look up the specific drive to see what its options
are. Newer drives won't be as flexible.
-ethan
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