PC floppy cable twists...

[Tony Duell]

> Story 1 is that the original IBM power supply was only 63 watts, and didn't
> have enough power to run two drive motors at once.  However, the
> "straight-through" interface only had one motor-control line, but several (3
> or 4) drive select lines.  By inserting the twist, one of the extra drive
> select lines became an independent motor on-off control line for the 2nd
> drive.

The factual part of that is most certainly true (the original PSU was 
63W, the twist does connect the motor-on lines for the 2 drives to 
different pins on the controller)

> 
> Story 2 is that for service reasons, IBM did not want to have to deal with
> drive jumpering, they wanted every drive to be the same so that drives could
> be simply swapped with no fuss if one failed.

While this is probably not the primary reeason for the twist, it was 
certainly a bonus :-)

> 
> I worked for Zenith [Data Systems] at the time, and Zenith [different
> division, but we had some contact after the IBM PC was introduced] made the
> original power supplies for IBM [as well as the power supply for my Z-100].

And yes, the IBM PC. PC/XT and PortahlePC power supplies are certainly 
Zenith. The ICs have Zenith house numbers on them (although it's bl**dy 
obvious what they really are). The AT PSU, at least the ones we got over 
here, were not always Zenith, though.

FWIW, the PortablePC monitor is also a Zenith unit.

> We were always told that story 1 was correct.  Also, the logic behind story
> 2 doesn't address the fact that drive terminators still had to be
> configured.  So I say it was because of the drive motor power requirements.
> That's my story, and I'm sticking to it.

I think you're right, but have no real proof

-tony