IBM 727, Руководство по инструкции

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Raising the manual reverse line raises the reverse line. This is the method used
while rewinding.
11. 8 READ PRE-AMPLIFIER 75. 06
To read from tape, a tap is taken off the read-write head in 75. 07 and tied to an
amplifier circuit 75.06. As the bits are read they are amplified in the AT circuit and
the output is fed to a K x o This circuit is controlled by ’’select, ready, and read. ”
If the line is down (-30) the K x o *s biased below cut-off and the signal from the AT
does not get through. However, if the line is up (+10), the signal passes through and
emerges on the read-sync bus to the external circuitry. There are seven such am­
plifier circuits.
The component circuit diagram of the AT is on C. 04. 04. The input to the first
stage comes from the read head, and either a plus pulse or a minus pulse represents
a one bit. The input pulses should be 35 to 40mv peak to peak.
The output of the cathode follower K x o should be 20 volts, peak to peak. This
level should be set with the potentiometer controlling the feed back from the third
stage of the AT to the first.
11. 9 WRITING ON TAPE (ONLY ONE BIT POSITION CONSIDERED)
Reviewing briefly the operation of storing information on tape, remember that
the tape is magnetically saturated in a predetermined direction by passing current
through the read-write head in one direction. The flow of current is reversed under
control of information lines entering from the control circuitry. Therefore, current
is always flowing through the read-write coil in one direction or the other. The c ir ­
cuit for this is shown on 75. 07 and C. 01. 07.
The right and left outputs of a write trigger are fed to an 1^ ^ . The plate c ir ­
cuits of this I^y£ are the two coils making up the R /W head. Because one of the two
outputs of the write trigger is always up, either one side or the other of the I ^
r
is
conducting, causing current to flow in the associated coil.
The write trigger is alternately turned on and off from the negative shift output
of a diode gate. This diode gate is conditioned by the write bus that enters from the
external control. The level of this bus represents the information to be written on
the tape. The other input to the diode gate contains timed pulses that actually flip
the trigger. These pulses are routed from 75. 02 where they are a combination of
’’select, ready, and w rite ,’’ and write pulses generated externally. The externally
generated pulses are 67 usee apart and last for 8 .4 usee in most system s.
The I^y^ is further controlled by an I
q
in the cathode circuit. The I
q
is allowed
to conduct only when the write-status line is up. This line was generated on 75. 02
as the right output of the re ad-write-status trigger conditioned by ’’not file protection. ”
It is at plate level, so a divider is used on the input to the I
q
on 75. 07. When the
write-status line is down (indicating read status) the I
q
is cut off, preventing the
IwR ^rom conducting on either side.
Remember that there are seven circuits such as the one described above, one for
each of the seven channels.
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