Ref: 19880002
Title: Why Tape Interchange Test Not Needed on Exabyte Tape Drives
Date: 4/9/90

Copyright 3Com Corporation, 1991.  All rights reserved.

A tape interchange test makes sure that a tape drive's heads are set
the same, because any difference will cause a problem reading a tape
written to by another tape drive.  This is vital for QIC drives such
as Wangtek, Tandberg, and Archive tape drives.  They write tracks from
one end of the tape to the other.  The head must follow these tracks to
read the data.  If the head is mechanically "off track" in either
direction by too much, the data can be missed.  Some of the better QIC
drives can step their heads to one side or the other to follow the tracks,
but there are limits in how far they can go.  If the drive's head is too
far off track, or the data on the tape was previously written too far off
track by a bad drive, the data cannot be recovered.

Helical scan drives, such as the Exabyte tape drive, can adjust so that
a tape interchange test is not needed.  Helical scan drive tracks are
written diagonally in stripes across the tape, from the bottom of the tape
to the top.  At the bottom of each stripe there is a "servo zone" that is
written immediately before the data portion of every stripe.  The servo zone
acts like a sync pulse for the drive's electronics while it is reading the
stripe.  If the head starts to move off track in either direction, the drive
slows or speeds the tape accordingly, to keep the head on track, somewhat
like the way better QIC drives move the head to compensate for being off
track.  The difference is, the helical scan drive effectively moves the
track to compensate for being off the head, since the read/write head is in
a fixed plane and cannot drift to either side.

