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11.10
WRITE ECHO
For checking purposes, a write echo pulse is developed and sent back to the
external circuitry.
This pulse is taken at the cathode of the 1 ^ ^ . It is AC coupled
to an Ig to feed a K p
which in turn brings up the w rite-echo bus.
q
veloped in the following manner.
Assume that the right side of the
one half of the read-write head. When the write triggers flip, the right side of the
*WR is cut
in the half of the read-write head which was conducting.
an EMF to be developed on the other half of the read-write head.
this EMF is negative at the plate of the left side.
the left sidë from conducting until the EMF has been dissipated.
cathodes of the I^y^, the voltage swings downward as long as no conduction is taking
place in either side of the IwR*
voltage again rises to its original value.
coupled to the Ig.
The duration of the pulse is from 8 to 12 usee, with a nominal
value of 10 usee.
11.11
WRITING A CHECK CHARACTER (LRCC)
Since the write trigger in any particular channel is turned alternately off and on
to write 1 bits, the state of the trigger at the end of a block of information indicates
whether an odd or even number of l fs has been written. If an odd number of l fs has
been written, the write trigger is on. If an even number of l fs has been written, the
write trigger is off.
If, after writing a block of information, all the write triggers are reset, only
those which are on will be turned off, thereby writing a 1 on the tape.
written as a result of resetting the write triggers is usually called the longitudinal
redundancy check character.
from the tape, to indicate dropping or picking up a 1 bit in any channel.
A line usually referred to as "write check character" is routed to the machine
from the external circuitry and enters 75.07 where it causes a -12 clamp and reset
line to rise to ground potential.
11.12
ERASING
The erase head is energized through an I£R on 75.07 whenever the write-status
line is up, causing the complete width of the tape to be saturated in one direction.
11.13
REWIND 75.08
Rewinding may be initiated in two ways.
line externally.
This is combined in 75.08 with "select and ready" to eventually
form "rewind PU re la y ." In 75.09.03, this line picks R40 to intiate a rewind opera
tion. When in the rewind operation, R36 is up and a line called "rewind status" is
brought up (75. 09.03).
"se le ct" to form "select and réwind status." This result is sent back to the control
Unit to indicate the condition.
is conducting and current is flowing through
sic* e tries to con(Iuct. However, a back EMF is developed
When the left side begins to conduct, the cathode
Thus, a negative pulse is developed and
The LRCC can be used when reading this information
This is the Z reset for the write triggers.
One way is to bring up a start-rewind
This line is combined in AND circuit A17 in 75. 08 with
39
The pulse is de
This collapsing field causes
The polarity of
This negative polarity then prevents
Therefore, at the
The character
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