IBM 727 Manual Of Instruction Download

Page: 37 / 163

11.4 TAPE INDICATOR 75. 03

The tape indicator is a trigger that may be used to indicate certain conditions

sensed within the tape unit. It may be controlled externally or internally. The trig
ger is turned on internally by sensing the reflective spot at the end of a reel of tape,

when writing. The rear photo-cell output is AC coupled to an inverter to pull over a

20ms SST . The plus output of the S S j is combined with write status in AND circuit
B29, turning on the trigger. The trigger may also be turned on by bringing up "turn

on TI" from the control unit if the select line is plus.

The right output of the trigger is combined again with select and becom es "select

and  TI  on"  which  is  sent back  to  the  external  control  unit.  Also,  the  tape  indicator
light  is  turned  on.  "Turn  off  tape  indicator"  is  combined with  select  and used to  turn
off  the  tape  indicator  trigger  if  it  is  on.  This  causes  the  "select  and  TI  off"  line  to
come  up to  provide  this  indication to  the  external  control  unit.

11. 5

TAPE BREAK INDICATION 75.03

If the machine is rewinding at high speed and a break occu rs, provision must be

made  to  stop  the  unit.  This  is  done  by  using the  same  photo  cell  that  is  used for  end-
of-tape  indication.  A  light  source  is  brought  up  from   underneath the  tape.  It  shines
on the  cell  if  the  tape  breaks.  The  photo  cell  output  is  combined with the  "rewind
interlock status" line  in  75.03  and the  output  is  used to  pick  a  tape break  relay  (R41)
in  75.09. 03.  This  is  a latch pick  relay  that  remains  latched  until  an  unload  condi
tion  is  initiated.  To  stop the  reel  motion,  a R41-1  point  opens  in the  circuit  of R l,
a  run relay.  When the  run  relays  drop  out,  the  m otors  stop  running  and the  brake  is
applied.

Depressing the unload button picks R41LT (75. 09.03) through R22-6 and R41-2.

This places the machine back in normal status.

11. 6

STARTING FROM LOAD POINT 75. 04

To read or write at the beginning of the tape, the start line is brought up as a

result of the go line from the external control. This line is sent to 75. 04 and com

bined with "at load point" in AND circuit G20 to produce a line called "drop load
point r e la y ." This picks R19 in 75.09.03. The R 1 9 -ln /o points close to pick the

latch trip coil of R42. As a result of this, "at load point" drops and "select and at
load point" drops. Therefore, unless the tape is rewound, no more load point con

siderations are made.

11.7 START, STOP, AND REVERSE CONTROL 75.05.01

There are two ways of bringing up the start line, resulting in a start position of

the moving co il. The go line, brought up externally, combined with select and ready
raises the start line, and drops the stop line. Also, raising the manual start line,

combined with "not arriving at load point" raises the start line. This is the combina

tion used while rewinding and unloading.

There are two ways of bringing up the reverse line, conditioning the machine to

move tape in a reverse direction. The backward line., brought up externally, com

bined with "select and ready" raises the reverse line.