9-Track 1/2" Tape Drive Recommendations?

Brent Hilpert hilpert at cs.ubc.ca
Sat Aug 22 05:32:54 CDT 2015

On 2015-Aug-21, at 10:20 PM, Ali wrote:
> Jon,
> ...
>> You have to decide, spring-arm, vacuum-column or streaming, and then
>> most dual density drives were either 800/1600 or 1600/6250.  Few would
>> do all three.  (Also there's 3200 BPI, identical to 1600 PE mode, just
>> double the clock, but it is a fairly rare setup.)
> Thanks for the info. I had seen (on YouTube) videos of the vacuum-column and
> streaming drives. The spring arm I am not familiar with. Is it just an arm
> that produces tension on the tape keeping it in place?

A brief conceptual overview:

Vacuum-column and spring-arm drives comprise 3 servo systems:
	- each reel motor is in a closed servo loop with position sensors in its associated vacuum column or spring arm.
	- the capstan motor, driving the exposed tape between the reels, is in a servo loop with a tape-speed sensor,
	  and externally controlled by commands for forward/reverse and speed selection.

The 3 servo systems are electrically independent (in control terms and leaving aside power supplies).
The connection between them is the tape.

The vacuum-columns or spring-arms serve to implement a length of 'buffer' tape between each reel and the heads and capstan.
The point of the buffer area is it is a very small mass of tape so movement of the tape over the heads can be started and stopped (by the capstan) very quickly. That is, it mechanically isolates the tape movement over the heads from the large mass of tape in the reels.

Each reel servo acts on its own to simply try to keep the tape in the middle of its vacuum-column or its spring-arm in the center of swing.
(It can be fun to play with the tape or arms while the drive is at idle and see how the reel motors respond, although one has to be cautious and delicate about doing so as the reel motors pack quite a whallop; and do so with a dud tape, not something that matters.)

When the capstan receives a command signal and starts moving, the tape in between the buffer areas starts moving.
The capstan speed servo acts to maintain constant linear tape speed over the heads.
The moving tape upsets both vac-cols or spring-arms in the buffer areas in opposite directions, the reel servos now act to correct this upset.

For example, when forward motion is called for, the capstan pulls tape out of the supply-side buffer (out of the vac-col or pulling the spring-arm tighter).
The sensors in the supply vac-col or spring-arm direct the supply-reel motor to rotate to supply more tape into the buffer area.
At the same time of course, tape is sent into the take-up buffer (into the vac-col or releasing tension on the spring-arm) and the take-up sensors and reel motor respond to rotate the take-up reel to pick up tape out of the buffer area.

Pardon the elaboration, I just find it a very elegant design. When you see these drives in full-blown action, without knowing, you might think the control system to achieve what they do must be horribly complex, but with the 3 independent servo loops acting in coordination it's actually quite simple.

(We'll leave auto-loading out of the discussion).

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