Jerome H. Fine jhfinedp3k at
Fri Oct 5 08:52:13 CDT 2007

 >Jason T wrote:

>I just received my shipment of items from this auction:
>which should have been a load of recyclable TK50 media.  Instead of
>CompacTape, however, they are all CompacTape III.  I think these were
>TK85 2.6gb media.   Does anyone know if they will still be writable
>with the standard format in a TK50?  Or do I have a bunch of useless
>DLT I tapes?
Jerome Fine replies:

I notice that there have been a number of replies to this
question and, if I might summarize what I have read, it seems
as if the answer depends on if the responder could (or could
not) use the (I believe you have received) the DLT III tapes
(since as far as I know DLT I and DLT II tapes were actually
the CompacTape I, for the TK50, and CompacTape II, for the

FIRST:  In the 1990's I used BOTH CompacTape (by default it
was CompacTape I) and CompacTape II media without any problems
in both TK50 and TK70 drives - although when I wanted to
switch between which drive would WRITE the tape, I always did
a BULK  ERASE.  If the tape was completely unused (very rare
for me an I only ever had about a dozen), both TK50 and TK70
drives could be used.

SECOND:  I always verified the contents of any backup files
written to the tape and over a period of about 10 years, I
can't remember having had more than isolated problems with
the verify operation which consisted of a completely separate
comparison AFTER all the backup files had been written by
the TK70 and the tape switched to WRITE  PROTECT.

THIRD:  A memorex site (long since disappeared) provided
all the physical and magnetic properties of the DLT line
of tape media starting with CompacTape I, CompacTape II,
DLT III, DLT IV, etc.  The physical and magnetic properties
of BOTH CompacTape I and CompacTape II were IDENTICAL
(which reflected my experience in using them on either
TK50 or TK70 - but with BULK  ERASE in between).  The
DLT III tapes had a much higher magnetic rating and
I seem to remember reading that they can't be used with
either the TK50 or TK70 drives - even if you can overcome
the small physical differences.  And, of course, the
CompacTape media can't be used where DLT III media are

The following point is based on my use of the TK70 drive
and media when they were used to hold 32 MB backup files
which were written under RT-11 by BUP using a PDP-11/73.
I only used the TK70 for this one purpose (well about
99.9% of the time).  THe disk drives were 600 MB ESDI
Hitachi and the disk controller was the RQD11-EC.

FOURTH:  While many individuals over the years have
attempted to justify the higher price of CompacTape II
media over CompacTape I media by saying that the quality
control was better, etc., my experience over those 10 years
when I used TK70 drives and CompacTape I media after a
bulk erase (90% were used media from a TK50) was more
than satisfactory.  The TK50 could write a maximum of
about 50 MB on a tape while a TK70 could write over
256 MB on a tape.  IN  ADDITION, the TK70 wrote the
same file about in about 1/3 of the time on a PDP-11/73,
BUT  FOR  ALL  PRACTICAL  PURPOSES, it was impossible
to verify the file using a TK50 except to restore the
backup file to an unused RT-11 disk partition and then
compare the "restored" RT-11 disk partition against
the original RT-11 partition.  This was because the
TK50 could not be kept in streaming mode while the
comparison was being made even though with the identical
hardware and software (except for the TK70 drive and
controller of course) the TK70 was able to perform
the VERIFY operation in about the same time period
(about 7 minutes on a TK70 vs about 19 minutes on a
TK50 - if I remember correctly) as it originally took
to write the backup file under RT-11 using BUP.

By the way, I have a large number of CompacTape I media which
I no longer will be using, but they will need to be bulk erased
since they have all been initialized in a TK70.  Assuming they
are still usable, I will look into selling them.  I am in
Toronto (M2R 3G3).

Sincerely yours,

Jerome Fine
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