>> I'll also point out that NRZI requires a lot tighter physical tolerances
>> on the alignment of the tape head (the reason why many drives don't support
>> 800 BPI NRZI at all) than 1600 and 6250 BPI (which allow substantial skew
>> between the tape channels as part ofthe spec.) If at some point the head
>> in your transport had been replaced or knocked around without properly
>> being re-aligned you might see something like what you're seeing.
>Would you? Most tape head/path adjustments are only really critical for
>'interchanageability' -- so that tapes on one drive can be read on
>others. If the head is slightly skewed, it will write a skewed tape, but
>it will also be expecting to read a tape that's similarly skewed (and
>that's what it will get).
>
>Now, admittedly if they're way out it's not going to work. But then it
>probably wouldn't read a 1600bpi distribution either.
What you say above is certainly true for tape systems where the same
head is used for reading and writing, and most small tape drives fall
into this category. But most 7-track and 9-track drives have separate
read and write heads, and these are often out of skew with each other.
Because the read and write heads are often part of the same assembly,
the necessary deskewing can be a combination of both physical
adjustment (to get the read head correctly aligned) and electronic
timing adjustment (to get the write pulses aligned on tape relative to
the read head.) There may be some drives where the write head is
physically adjusted and the read timings are electronically adjusted,
but this makes the entire procedure much more difficult - what good
does a master skew tape do you in doing mechanical alignment of the
write heads??? :-)
For most drives with electronic deskewing (and the Kennedy 9100 and successors
certainly fall in this category) when you get a replacement head it comes
with a set of "starting" electronic deskewing settings. I'll gladly fax
you the section of a Kennedy maintenance manual that details the procedure,
if you're interested.
>I would cartainly look for other causes before tweaking head adjustment
>screws.
Absolutely. Barring physical damage (a good drop onto concrete from
a few feet is enough to whack other things out of alignment) I'd look for other
causes first. The NRZI data path is different than the PE data
path, for one thing.
--
Tim Shoppa Email: shoppa_at_trailing-edge.com
Trailing Edge Technology WWW: http://www.trailing-edge.com/
7328 Bradley Blvd Voice: 301-767-5917
Bethesda, MD, USA 20817 Fax: 301-767-5927
Received on Tue Jul 06 1999 - 16:44:10 BST