IBM 727, Руководство по инструкции

Страница: 16 / 163

4. 6 VACUUM COLUMNS AND VACUUM-OPERATED SWITCHES
The vacuum columns are vertical columns of rectangular cross section. The in­
side dimensions are 2 -1 /2 " x .510 (+ .002", - .000"). The front face of the vacuum
column is transparent so that tape can be observed at all times. The top of the
vacuum column is open and the lower end is connected to a manifold leading to a
vacuum pump that maintains a vacuum of about eight inches of water. Tape is allowed
to hang in the columns in such a way that the oxide surface of the tape strikes the sides
of the column only at the edges of the sem icircular loop form ed. Because of this ac­
tion, vacuum is maintained below the loop and atmospheric pressure above it. This
difference in air pressure is sensed by vacuum-operated switches located behind each
column. Pressure is transmitted to each switch via a short tube emerging from a
hole in the column. Each column contains two such holes and associated switches;
one located about 1/3 of the column length from the top and the other about 1/3 of the
column length from the bottom.
The vacuum columns act as a storage area for the tape, allowing tape to be
moved across the head at random, without having to turn the reels simultaneously.
They also put a constant tension on the tape, preventing tape buckle at the head when
starting and stopping.
The tape reel drive clutches are controlled with relay circuitry to produce the
following forward motion results. As tape is pulled past the read-w rite head by the
drive capstan it is pulled out of the left column. When the loop of tape is pulled
above the upper vacuum switch in the column, the file reel forward (left down) clutch
is energized and the reel turns in a clockwise direction causing more tape to be fed
into the left column. As soon as the loop is again below the upper switch, the left
down clutch is de-energized and the brake applied. This action causes the loop to
oscillate about the upper switch as long as tape is moving past the read-w rite head.
At the same time, tape is continually being fed into the right column from the
read-w rite head area. When the loop of tape in this column falls below the lower
switch, the machine reel forward (right up) clutch is energized and the reel turns in
a clockwise direction to pull tape out of the column and wind it on the reel. As soon
as the tape is again above the lower switch, the right up clutch is de-energized and
the brake applied. This action results in the tape loop's oscillating about the lower
switch in the right column. Action of the reels and the tape in the columns is just the
reverse when tape is moving in the reverse direction.
Generally speaking, whenever the tape is between the two vacuum column switches
the associated reel is stopped (neglecting overshoot); when the loop is above the up­
per switch, more tape is fed into the column and, when it is below the lower switch,
tape is pulled from the column.
The vacuum column switch is shown in cross section in Figure 11. The p res­
ence of a vacuum causes the diaphram to move in a direction that transfers the con­
tacts of the switch. For greater reliability, two sets of contacts are used in parallel.
A bellows-type vacuum switch is mounted on the manifold between the two co l­
umns. It is used to prevent normal machine functions until the vacuum is maintained
at a certain level. This is named the vacuum safety switch.
16