9-Track 1/2" Tape Drive Recommendations?
cclist at sydex.com
Sat Aug 22 13:43:11 CDT 2015
On 08/22/2015 10:07 AM, Jay Jaeger wrote:
> One other type: spring arm, where the tape reels do all the tape
> moving, and a speed sensor, rather than a traditional capstan. The
> tension arm is part of the servo system to which the reel motors
> respond. In this case the reel motors and tension arm are anything
> but independent.
Historically, the two are very different in application. If you've got
a COBOL handy on your tape system, try running the 1974 Navy Audit
Tests, once the set of benchmarks by which CODASYL compliance of a
vendor's COBOL was judged. (Al, do you have a copy of those on bitsavers?)
Very short tape records were written and read back in one of the tests
(I forget which one; it's been 40 years after all). Vacuum-column tape
drives made the most itneresting noises as they went through their
start-top tape motion. You could actually play melodies on some of
them, simply by varying the block length.
Vacuum-column drives, if you will, are inertia-trading devices. The reel
motors are powerful in a 300 ips drive; they have to be. Practically
speaking, it's not realistic to expect the system composed of the mass
of the tape reels and the motors behind them to go from 0 to 300 ips in
a fraction of a second--nor, to do it accurately. So, the reel motors
handle the approximate movement of the high-inertia reel system and only
maintain a few feet of loose tape, held in the vacuum columns, said
short length massing almost nothing. The capstan(s) can then manage the
quick tape movements quite nicely. The nature of the capstan mechanism
varied between manufacturers--some used a pinch-roller sort of affair;
the others used perforated capstans whose selection of nothing, vacuum
or positive pressure could be managed quite nicely by a voice-coil valve.
The point is that during the 50s and much of the 60s, diskless/drumless
systems were not uncommon (anyone remember S/360 TOS?) and tapes were
used as working storage. Sometime during the 70s, with the
proliferation of disk storage, tapes became relegated to archival or
offline storage, not working storage. Pick up a copy of Flores or Knuth
on sorting and you can see how important tapes were for handling and
manipulating large amounts of information.
Comes the minicomputer and you begin to see spring-arm and direct servo
drive units aimed toward the archival use of tapes--that is, a tape held
information to be copied to a disk, so start-stop on a dime wasn't
necessary. If one overshot a record, the drive/formatter only needed to
bring the tape to a halt and read backwards until the gap before the
desired block was reached, then back up another block or so and get a
running start at the target.
This change in role probably is what governed the brain-dead treatment
of tape by later operating systems such as UNIX and PRIMOS, to name a
couple. Until the advent of cheap cartridge tapes, almost all
microcomputer OSes were utterly ignorant of the existence of tapes.
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