Pertec Tape Drive Interface Musings

Chuck Guzis cclist at
Wed Jun 10 11:37:57 CDT 2015

On 06/10/2015 09:12 AM, Mark J. Blair wrote:

> Ok, now three more questions come to mind:
> 1) Is it ever acceptable to mix densities on a single tape? I'm not
> sure that my Kennedy drive will even allow that, but I don't know if
> that is universal.

Acceptable?  I don't know about that.  Otherwise, it depends on the 
drive.  Some will allow a density change only at load point.

> 2) What's the scoop on a final record overlapping the EOT marker? Or
> even a new record starting after the EOT marker? I seem to recall
> reading about some applications that stuck data after the EOT, such
> as backup volume information.

Not that unusual--the EOT marker is mostly intended as a warning when 
writing.  Long-record tapes frequently write past the EOT.  On the old 
CDC vacuum-column (60x, 65x) drives, I'd sometimes add an extra BOT 
marker or two on a reel and start a new "logical tape" there.  You just 
hit the "LOAD" button again while the tape was already loaded and it 
would space down to the next BOT marker.  I don't know if anyone else's 
drives did that, though.  I had my pet deadstart tapes with several 
different versions of a system on them.

> 3) Did anybody ever go over to the dark side and implement copy
> protection on magtapes, say, by deliberately including a record with
> bad CRC that a normal driver+drive would not support writing? Or was
> that evil limited to the floppy disk world?

Half-inch tape comes from a more polite, gentler era.  Although, on the 
7-track drives, you could toggle the parity, which would confuse the 
heck out of someone trying to read the tape.


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