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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. The pulse is de

veloped in the following manner.

Assume that the right side of the

is conducting and current is flowing through

one half of the read-w rite head. When the write triggers flip, the right side of the

*WR is cut

sic*e tries to con(Iuct. However, a back EMF is developed

in the  half  of  the  read-w rite  head which was  conducting.  This  collapsing field  causes
an  EMF  to  be  developed  on the  other  half  of the  read-w rite  head.  The  polarity  of
this  EMF  is  negative  at  the  plate  of  the  left  side.  This  negative  polarity  then prevents
the  left  sidë  from   conducting  until  the  EMF  has  been  dissipated.  Therefore,  at  the
cathodes  of  the  I^y^,  the  voltage  swings  downward  as  long  as  no  conduction  is  taking
place  in  either  side  of  the  IwR*  When the  left  side  begins  to  conduct,  the  cathode
voltage  again  rises  to  its  original  value.  Thus,  a negative  pulse  is  developed  and
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.  The  character
written  as  a result  of  resetting the  write  triggers  is  usually  called the  longitudinal
redundancy  check  character.  The  LRCC  can be  used when  reading this  information
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. This is the Z reset for the write triggers.

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. One way is to bring up a start-rewind

line  externally.  This  is  combined  in  75.08  with  "select  and  ready"  to  eventually
form   "rewind  PU  r e 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). This line is combined in AND circuit A17 in 75. 08 with

"s e le ct" to form "select and réwind status." This result is sent back to the control
Unit to indicate the condition.