ARMY TM 5-6675-238-14 MARINE CORPS TM 08839A-14/1 arm. The corrected output voltage is within the range of 0 to + 28V and is used to drive supply reel motor B2 and  takeup  reel  motor  B3.  In  the  reverse  mode  of operation, the power supplied to the spooling motors is reversed. The output from each motor driver control plug-in circuit card is reversed, step motor B1 windings are energized in reverse, and the motor advances in a clockwise  direction  causing  tape  advance  from  right  to left. f. Power  Supply.   Input  ac  voltage  to  the  power supply  is  applied   to  a  step-down  transformer.  The outputs  of  the  transformer  are  full-wave  rectified  by diode  bridge  rectifiers.  The  rectified  voltages  are  regu- lated to produce two + 5V and + 28V output voltages. 6-5.1. Tape Reader, Part No. 877406-2, Function. A functional description of the tape reader is contained in the following paragraphs. The tape reader is function- ally  illustrated  in  figure  FO-16.1. a. Parallel  Mode.  After  all  four  S1  switches  (acces- sible through rear of tape reader chassis) have been set to parallel mode of operation, the tape may be loaded and power turned on. The reader ready lamp will light after a delay of approximately 2 seconds. This delay is required to allow all slack to be removed from the tape before drive commands are applied. When the ready lamp is continuously lighted, the reader is ready for a run  command. b. Run  Operation.  A  command  for  run  operation will  allow  data  to  be  read  and  processed  to  the  I/O connector at a data rate dependent upon the position of the  FAST/SLOW  switch  and  the  SPL/LOOP  switch. When   the   SPL/LOOP   switch   is   set   to   the   LOOP position, the data process rate will be up to 200 charac- ters  per  second,  and  is  independent  of  the  FAST/ SLOW switch setting. When the SPL/LOOP switch is set to the SPL position, the data process rate will be up to 600 characters per second when the FAST/SLOW switch is set to the FAST position, and 125 characters per second when the FAST/SLOW switch is set to the SLOW   position. c.   Read   Direction   Change.   When   a   direction change is sensed by the reader during a run operation, a small  time  delay  is  initiated.  During^-this delay the data buffer which may contain as many as 180 characters is emptied. The data buffer is then reloaded with data in the  new  direction  selected.  While  the  data  buffer  is being emptied and then reloaded, the microprocessor keeps  track  of  the  data  load  pointer  which  is  incre- mented or decremented accordingly, ensuring that no character  is  dropped.  In  spool  mode,  the  time  delay involved  in  a  direction  change  depends  on  the  data buffer content and the spool inertia load. d. Stop-On-Character  Timing.  During run opera- tion,  valid  data  is  presented  approximately  100  mi- croseconds  before  the  sprocket  signal  becomes  true (positive). The leading edge of the sprocket signal is used  to  latch  data.  To  stop-on-character,  a  run  com- mand must be removed within 750 microseconds after the sprocket signal becomes true. A run command not removed within 750 microseconds will cause the next character to be sent. e. Loop Mode.  The loop mode allows a short loop of  tape  to  be  read.  When  this  mode  is  selected,  the stepper motor is energized during a run operation while the spooling motors are disabled. Receiving a run com- mand will cause data to be transferred at a rate of up to 200  characters  per  second.  For  each  read  direction change, a time delay of approximately 0.5 seconds is observed. During this delay, the sprocket signal is kept alive. f. Data  Handling.  Two  8-bit  registers  (data  fetch pointer and data load pointer) inside the microprocessor are  used  to  maintain  data  flow  continuity.  The  data fetch pointer register keeps track of the memory loca- tion where data was last fetched. Depending upon the reading  direction,  the  register  content  will  either  be incremented  or  decremented.  The  data  load  pointer register  maintains  the  memory  location  of  the  last character saved. The data buffer is full when it contains 180 characters. When the microprocessor senses a full data buffer, the spool motor drive signal is disabled and the dynamic brakes are applied which brings the motors to a smooth stop. When the data buffer holds 25 or less characters, the microprocessor generates a move com- mand.  This  command  moves  the  tape  at  a  predeter- mined speed, filling the buffer. The speed is controlled by the program in the microprocessor. g. Interrupt  Driven  Logic.  Drive  commands  applied to the reader are processed on a priority interrupt basis. An interrupt polling routine is set up to establish a run right as having a higher priority than a run left opera- tion. Whenever a system interrupt is sensed, the micro- processor  scans  drive  lines  according  to  priority  to determine  which  causes  the  interrupt.  This  information is then saved in the flag register which is constantly updated  for  future  references  with  every  interrupt  serv- iced. h. Sprocket Interrupt.  The sprocket signal derived from the moving tape is sensed by the microprocessor as   an   external   interrupt.   The   leading   edge   of   the sprocket signal sets a flip-flop, acknowledging the inter- rupt. Data appearing at the data bus is then read and stored in the data buffer at the memory location speci- fied by data load pointer register. For every sprocket signal detected, the data load pointer register is either incremented or decremented, depending upon the read direction. Upon exit from the sprocket subroutine, the flip-flop  is  reset  to  accommodate  the  next  sprocket interrupt. To improve data integrity, every datum will be  read  twice  and  compared  for  every  sprocket  de- tected. Data will be stored only if comparison is good. Change 1 6-15