Seagate ST31250N Product Manual page 29

Barracuda 2LP Product Manual, Rev. D 19
You may operate the drive with a rotational skew when synchronized. The
rotational skew is applied in the retarded direction (lagging the synchronized-
spindle master control). A rotational offset of up to 255/256 of a revolution
lagging may be selected. Select the amount of offset by using the Mode
Select command, Rigid Disc Drive Geometry page (page 04h), byte 18 (see
the SCSI-2 Interface Product Manual ). The value in byte 18 (0–FFh) is the
numerator of a fractional multiplier that has 256 as the denominator. For
example, 40h selects 40h/FFh or 1/4 of a revolution lagging skew, 80h
selects 1/2 of a revolution lagging skew, etc. Since the drive supports all
offset values from 0 to 255, values sent by the initiator are not rounded off.
The drive's translation of the digital offset values to physical rotational offsets
results in offset values whose phase error lies within the ± 20 microseconds
phase error with respect to the supplied 120 Hz reference signal.
The drive does not have the capability to adjust the rotational offset value
requested by the initiator to a physical offset in the drive that corresponds
in any way to sector boundaries or changes in ZBR zones. The initiator must
formulate these boundaries or changes, if required, to calculate the value of
offset it sends to the drive.