IBM 709 General Information Manual page 28

Figure 54. Reflective Spots on Tape
loop of tape is held in the vacuum columns and acts
as a buffer for this motion. As tape is drawn from
one column, it is replenished from the reel above it.
As it is fed into the opposite column, the associated
reel takes up the slack.
The read-write head assembly consists of two parts,
the lower of which is stationary, and the upper part
moves up and down under control of the load and un-
load keys. Tape threading must be done with the tape
Left Ptoloy _ L - - - r - T
..... mbly
1
'~-U
Read-Write Head
As.ernbly
Figure 55. Schematic, Tape Feed
Right Prolay
Assembly
unit in an unload status. (Changing reels or thread-
ing tape takes approximately two minutes.)
Rewind is under the control of a stored program in-
struction or may be initiated by depressing the load-
rewind key on the unit. If more than one-half inch
of tape has been wound on the machine reel, the unit
automatically performs a high-speed rewind (500
inches per second). The head is raised, tape is pulled
from the vacuum columns and the tape is moved in
a reverse direction until less than one-half inch of
tape remains on the machine reel. When this occurs,
the tape is lowered into the columns, the head as-
sembly is lowered and the backward movement is
reduced to the normal operating speed of the unit.
During the high-speed rewind, tape is passed be-
tween a light source and a photo cell. If the tape
breaks, the light strikes the photo cell causing tape
motion to immediately stop.
The
IBM
729 I Magnetic Tape Unit is used with the
709 systems; 729 II and 729 IV units are used with
the 7090 system (Figure 56) .
Each unit is equipped with a selector dial to be set
from 0 through 9 by the operator. The selected num-
ber then becomes the "address" of the unit. Instruc-
tions from the stored program condition or alert the
unit for use by specifying this address.
The 729 I reads and writes at a density of 200 char-
acters per inch of tape. This means that a 100-char-
acter record would occupy one-half inch of tape. Tape
is moved past the read-write head at a speed of 75
inches per second, thereby making the rate of read-
ing or writing 15,000 characters per second or 67
microseconds per character.
Input-Output Components 27