IBM 727, Руководство по инструкции

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When switch 1 is closed at time T^, current generator E-^ (Figures 3 a and 3b)
cause the current 1^ to flow through the coil as indicated and a flux path is set up as
shown, using the right hand rule. Because this flux path extends into the magnetic
material on the tape, the domains in the material line up in the direction of the flux
path. If the tape is moving at a constant speed of 75 in c h e s /s e c ., all the area pass­
ing under the write head becom es magnetized in the same direction.
If, at time T2, switch 2 is closed and switch 1 is open, current generator E2
causes current I2 to flow through the opposite write coil as shown. This causes the
flux path to be reversed and the domains in the magnetic material line up in the
opposite direction. Because the switching time is very short, the tape moves only a
very short distance during the time the reversal takes place. This process consti­
tutes writing a binary one bit on the tape. If, at a time T3, another reversal is made,
then another binary one has been written.
If, at time T4, no reversal is made, then a binary zero has been written. Thus,
observe that if a reversal in flux is made at any time, a binary one bit is written and,
if no reversal is made, a binary zero bit is written.
Figure 3c illustrates the magnetic material on the tape as being a series of tiny
bar magnets placed end to end. Where the change in flux occu rs, there are like
poles; where no change occu rs, there appears to be a long magnet.
The reading of a binary one is simple in principle. The reading circuits make
use of the fundamental fact that voltage is induced in a coil whenever there is a time
rate of change of the flux linking the turns of the co il. This principle can be stated
in the following fundamental equation: e = Nd^/dt.
As the tape is passed over the head, a voltage is induced in the windings on the
head when a reversal of flux pattern is encountered. This voltage is taken from one
coil and routed to amplifiers and storing circuits.
A binary one is sensed as the presence of a voltage pulse at the terminals of the
coil. (This voltage pulse is produced by a flux change in either direction.) The
absence of a voltage pulse (no change in flux) indicates a binary zero.
The information stored on the tape may be erased by saturating the magnetic
material all in the same direction. This is done by a separate erase coil and head
described in section 4 .4 .2 .
The advantages of the NRZI system over a pulse system are:
a. High inherent density. Binary ones can be written closer together than in a
pulse type system 0
b. High output when reading. A maximum change of flux occurs from minus to
plus saturation.
c. Simplified erasing technique. Erasing to saturation is simpler than erasing
to zero flux.
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