IBM 709 General Information Manual page 27

Tape Mark
Check Character
Figure 52. Record and File Gaps on Magnetic Tape
As information is recorded on tape, the number of
1 's in each track across the tape is automatically
counted and a check bit (1) is placed in the seventh,
or C, track. When the tape is read, the check bit is
recomputed and compared against the recorded check
bit. Any discrepancy will signal an error in tape op-
eration.
In addition to this vertical checking of each column,
a horizontal count of each track containing l's is taken
and recorded at the end of the record as a separate
column called "longitudinal redundancy check char-
acter" (LRCR).
A word or group of words, recorded consecutively
on tape, is referred to as a record. The number of
words making up a record is unrestricted within the
storage limitations of the system itself. If more than
one record is placed on the same tape, a
%
inch sec-
tion of blank tape called a record gap is placed be-
tween them. Records may also be grouped together
forming files. These files are separated by other blank
sections of tape called file gaps.
This file gap consists of a section of blank tape
and includes a special character called a tape mark.
This character and/or gap defines the difference be-
tween the records and files on the tape (Figure 52).
The number of words in a record and the number
of records in a file are variable and are determined by
the stored program and the limitations of the system.
When data are to be recorded on tape, a full word
of 36 bits is taken from storage and is subdivided into
Figure 53. File Protect Ring
26 IBM 709·7090
groups of six bits each. Each group, with its associated
check bit, is recorded in one column across the tape.
Tapes are written or read in the forward direction
only. However, the same tape may be written, back-
spaced, and then read or rewritten if desired. Back-
spacing is halted by the record gap or file gap; thus,
one backspace instruction results in moving the tape
in a backward direction to the next record or file gap.
Because the writing operation automatically erases
any previous information recorded on the tape, a file
protection device is provided to prevent accidental
erasure of information. A circular groove is molded
around the center of each tape reel to fit a demount-
able plastic ring. Without the ring in place, writing
is suspended and only reading may occur. The reel
in this condition is protected. When the ring is in
place, either writing or reading may be performed
(Figure 53).
Like movie film, the tape must have a short length
of blank space at the beginning and at the end of the
reel that can be threaded through the feeding mechan-
ism of the tape unit. Reflective spots of aluminum
foil, placed on the tape by the operator at any de-
sired distance from the ends of the tape, are photo-
electrically sensed to indicate the physical end of tape
and the starting point for recording (Figure 54) . Dur-
ing writing operation, the reflective spot signals that
the end of the reel has been sensed.
Figure 55 shows schematically the position of the
tape reels in relation to the read-write head, feed roll-
ers, and the vacuum columns. While reading or writ-
ing, tape is transported from the file reel (left side)
past the recording head to the machine reel (right
side) .
Since it is impossible to start and stop high-speed
motion of the tape without some slack in the tape, a