Input-Output Components; Magnetic Tape Storage - IBM 709 General Information Manual

Input-Output Components
Magnetic Tape Storage
The principal input-output medium for the computer
is magnetic tape. The tape is used, in addition to in-
put-output functions, for storage of intermediate re-
sults. It is also used as permanent storage for large
files of data. Magnetic tapes may be re-used many
times, because old data are automatically erased as
new data are recorded.
The tape is a plastic material that is coated on one
side with a metallic oxide. It is one-half inch wide
and is packaged on reels with tape lengths as great as
2,400 feet. The tape can be easily magnetized and
yet retains the magnetized "spots" when put in static
storage for indefinite periods of time.
The basic electronic principle of magnetic tape re-
cording, as used in data processing systems, is similar
to that of a home tape recorder. Instead of recording
music or voice, business and scientific data are re-
corded in the form of magnetized spots. A schematic
of the recording head together with a section of mag-
netic tape is shown in Figure 47.
When electrical current flows through the record-
ing head coil, the magnetic oxide particles on the tape
are oriented in one direction. If the current in the
coil reverses its direction, the particles on the tape will
be oriented in the opposite direction. This is called
reversing the polarity of the magnetic flux. Each such
change of polarity is given the value of 1. If no
change occurs, the value is given as O.
Now, if the tape is moved past the recording head
and the current in the coil is alternated at time inter-
vals, writing on magnetic tape is accomplished.
To read the tape, the process is reversed. As the
magnetized "spots" pass the recording head, small
voltages appear in the recording coil. These voltages
are amplified (as in core storage) and are then used
by the computer.
The advantages of tape storage are shown in Fig-
ure 48.
R/W HEAD
Figure 47. Magnetic Tape Recording
24
IBM 709-7090
Master
Records
Low Cost •••
Compact •••
One foot of tape wi th its
compact starage has a re-
cording density of 200 or
556 and
m~y
contain 14,400
or 40,030 binary digits.
Fast •••
Reading and writing speed
of 90,000 ta 375,000 binary
digits per second.
Variable Record Size •••
From one to several thousand
alphamerical characters per
record:
Recording Density 556 = 4,620
records of 3000 characters to
37,380 six-character records
on each reel of tape.
Updated
Records
Working
Records
Permanent •••
Reels may
be used many
times for many
different jobs.
One reel of tape has
a capacity equal to
several thousand IBM
cards.
Figure 48. Magnetic Tape Advantages