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IBM 2401 Magnetic Tape Unit
The IBM 2401 Magnetic Tape Unit provides magnetic tape
input and output capabilities for the 1800 system. A
maximum of two 2401
's
(model 1, 2, or 3) can be
connected to the 1800 system.
A limited number of IBM 2404 Magnetic Tape Units
were used on 1800 systems. The 2401 and 2402 are
functionally
the same. Therefore, the descriptions in this
section apply to both units.
The 2401 operates under data channel control and
functions as both an input
and
output device. It performs
reading, writing, and control functions as directed by the
stored program.
The recording medium for the 2401 is 1/2 inch wide
magnetic tape. Magnetic tape is wound on plastic reels
10-1/2
inches
in diameter. A full reel contains approxi-
mately 2,400
feet
of usable tape, but lengths as short as
50 feet can be used. A miniature reel, 8-1/2 inches in
diameter, is available and holds 1,200 feet of tape.
Writing and Reading Magnetic Tape
During reading or writing operations, magnetic tape moves
from
the file reel through the left vacuum column, across
the read/write head, and through the right vacuum column
to the machine reel (Figure 29). The loop in each vacuum
column acts as a buffer for sudden start and stop motion
of
the tape.
Tape may be backspaced over a tape block or
rewound to beginning of the reel. Reading, but not writing,
may occur while tape
is
moving backward.
Information is written by magnetizing small discrete
areas in parallel channels called tracks. Information is read
by sensing the presence or absence of the magnetized spots.
During a write operation, the tape passes an erase head
prior to reaching the read/write head. Information pre-
viously written on the tape is erased by the erase head
during write operations.
File Protection Device
Since information
is
erased during write operations, a
file protection device is provided to prevent accidental
erasure. A plastic ring (Figure 30) fits in a circular groove
molded in the back (machine side) of the tape reel. This
ring must be in place to enable write operations. When
the ring is removed, writing is suppressed and only reading
can take
place;
the file is protected from accidental writing,
which could erase valuable information.
76
IBM 1800 Operating Procedures
File
Reel
I
Reel
Release
Drive
Capstan
Read/Write Head
Assembly
2401 Models 2 and 3
Tape Drives
Figure 29. 2401 Tape Feed Path
Figure 30. File Protection Ring
Machine
Reel
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