22 Oct
2005
22 Oct
'05
1:33 p.m.
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