14 Mar
2005
14 Mar
'05
3:30 p.m.
The one and only standard the document gives for pin 2
is speed change.
All of the original AT controllers required two speed drives. Later
controllers added the 300K transfers.
According to the IBM Techref, the IBM PC/AT controller (which was
actually a Western Digital card, of course) only ever used the 360rpm
speed of the high-capacity (1.2M) drive. Pin 2 was a write-current select.
I have never seen a PC/AT system which slowed the 1.2M drive down to read
360K disks.
And as Pete said, there is certainly a need to reduce the write current
to write to the 360K media. There's no way the drive can sense the type
of media (there's no extra hole/notch as in 3.5" disks), that information
has to come from the controller. And pin 2 was used to select the write
current.
-tony