PC floppy cable twists...

Chuck Guzis cclist at sydex.com
Thu Oct 20 12:18:44 CDT 2005

On 10/20/2005 at 5:48 PM Jules Richardson wrote:

>... discussion about this on another list got me curious - what *was* 
>the point of that cable twist in a (IBM clone) PC floppy cable, when 
>every other system on the planet was using straight-through cables?

It's actually pretty simple--there's only a single "motor on" line for the
standard floppy pinout.  IBM wanted to control drive motors individually.
The only ways to do that are with individual drive cables or using the
"twist" (or some modification of it) so that the individual drive motor
enables are given different conductors in the cable.

Other systems using "straight through" cables switched all drive motors on
and off--and many consequently used drives with head-load solenoids.  IBM's
solution let them economize a bit on power-supply design and use a cheaper
(and quieter) drive without the head-load mechanism.  FWIW, IBM makes no
mention of terminator removal or addition and I don't even recall if the
standard 5.25" drive allowed one to play with termination.

On one of the ISA floppy/hard controllers--I think it was DTC--you could
add a third drive to the string using a special cable and a drive jumpered
for DS0.  (I'd have to check my doc files to get the rest of the details on
the cable construction).


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