IBM 5-1/4" Diskette Drive Adapter Hardware Reference Manual page 25

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Adapter Outputs
-Drive Select A and B
-Motor Enable A and B
-Step
r ' \
-Direction
-Head Select
-Write Data
r ' \
- Write Enable
(Driver: 7438): These two lines are
used by drives A and B to de gate all
drivers to the adapter and receivers
from the attachment (except 'motor
enable') when the line associated
with a drive is inactive.
(Driver: 7438): The drive associated
with each of these lines must control
its spindle motor such that it starts
when the line becomes active and
stops when the line becomes
inactive.
(Driver: 7438): The selected drive
moves the read/write head one
cylinder in or out per the direction
line for each pulse present on this
line.
(Driver: 7438): For each recognized
pulse of the 'step' line, the
read/write head moves one cylinder
toward the spindle if this line is
active, and away from the spindle if
inactive.
(Driver: 7438): Head 1 (upper
head) will be selected when this line
is active (low).
(Driver: 7438): For each
inactive-to-active transition of this
line while 'write enable' is active, the
selected drive causes a flux change
to be stored on the diskette.
(Driver: 7348): The drive disables
write current in the head unless this
line is active.
Diskette Adapter 21