IBM 727 Manual Of Instruction Download Page 82

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"re se t write triggers" pulse could coincide with the fall of a write pulse. In this case

(Case B, Figure 30) assuming all ones were being written, at time T^

some of the

write  triggers  might be  turned  on  and  some  might  be  turned  off.  Then  these  triggers
would be  flipping out  of  step  with  each  other  for  the  rest  of  that  writing period.  This
condition  could  be  observed  at  the  write  trigger  neons  when  the  writing  is  stopped.
Usually,  however,  (as  in  Case  A)  all  triggers  are  either  on  or  off.  Therefore,
observing the  write  trigger  neons  shows  whether  the  triggers  are  operating properly.

1 9 .3 .3 Reading

To read a tape, set the read-w rite switch to READ and press the start button.

Tape moves forward in read status. The pre-am p output of the track selected by

the response select switch can be observed with a scope at the read bus hub. De
pressing the stop button stops the operation.

When in read status, the tape is read in start-stop fashion if the start-stop

switch is turned on. The start-stop frequency dial controls the frequency of starting
and stopping the tape. Unless the manual switch is used, tape cannot be read when

the tape indicator is on.

1 9 .3 .4 Auto Cycle

The auto cycle feature on the TDT may be used to check the following operations

of the tape drive: end-of-tape sensing, load-point sensing, high-speed rewind,

vacuum switches, and flapper valves. The function of the auto cycle feature is to

write  the  tape  in  a forward  direction  until  an  EOT  reflective  spot  is  sensed.  The  ma
chine  then rewinds  the  tape  back  to  the  load point.  Upon  reaching the  load  point,  the
tape  unit  starts  writing  again  and  the  cycle  is  repeated.

This operation is accomplished in the following manner; upon sensing the EOT

reflective spot, the signal "s e l and TI on" goes through section E of the response

select  switch  and back  to  the  tape  drive  as  "rew in d ."  This  signal  causes  the  tape
to  rewind,  and  the  signal  "select  unit  rewinding"  com es  from   the  tape  drive  through
section  D  of  the  response  select  switch  and  returns  to  the  tape  drive  as  "reset  T I,"

thus  turning off  the  tape  indicator.  Section  F  of  the  response  select  switch  discon
nects  the  "select  and  TI  on"  line  that normally  turns  off  the  go-fb  trigger.  When the
tape  is  rewound to  load point,  the  ready  line  com es  up  and  the  tape  drive  starts  writ

ing again.

To put the machine into the auto cycle operation, the response select switch is

set to AUTO CYCLE and the read-w rite switch is set to WRITE. P ressing the start

button then puts the tape drive into auto-cycle operation. By placing the EOT reflective

spot a few feet from the load point, the cycle can be made very short and the load

point and EOT photo cells and single shots can be checked. If the EOT reflective spot

is  placed farther  back  on the  tape  so  that  the  tape  drive  can  just  go  into  high-speed
rewind  upon  sensing the  spot,  and  the  load point  is  placed  just  before  this  area,  then
the  operation  of  the  vacuum  system  and  associated  switches  can be  observed  at  a

frequent rate.

With a full reel of tape on the tape drive and the EOT spot at the end of the tape,

the tape drive can be auto-cycled to check the rewinding of a full reel at high speed.