interrupt  on  the  input  data  lines  and  set  the  interrupt line.   The   interrupt   is   then   processed   by   the   CDS computer  and  the  input  buffer  is  closed. If  the  received  stop  code  is  a  picket  stop  code,  the DTS  simply  resets  itself.  If  the  stop  code  is  a  control station  stop  code,  the  DTS  will  compare  the  next  two frames  received  with  its  own  station  address  code. CDS   Computer   Output   (Transmit)   Data   Cycle The  output  data  cycle  is  initiated  when  the  DTS detects   its   own   station   address,   either   in   an interrogation  message  or  at  the  end  of  an  NCS  report and  interrogation  message.  When  the  DTS  recognizes its   own   station   address,   it   starts   to   transmit   the preamble.  During  the  first  frame  of  the  preamble,  the DTS  sets  the  prepare  to  transmit  interrupt  on  the  input data   lines.   The   computer   samples   the   interrupt   and sends  an  IDA  to  acknowledge  receipt  of  the  interrupt. The  DTS  finishes  sending  the  preamble  and  phase reference   frames   During   the   second   frame   of   the start  code,  the  DTS  sets  the  output  data  request  (ODR) active,   requesting   the   first   word   of   the   tactical   data. The   CDS   computer   responds   by   placing   24   bits   of data   on   the   lines   and   then   setting   the   output   data acknowledge  (ODA).  The  DTS  samples  the  data  and clears  the  ODR.  The  first  frame  of  data  is  processed for   transmission   and   the   ODR   line   is   then   set   to request  the  next  data  word. This  procedure  is  repeated  until  all  the  data  words have   been   transmitted.   Once   the   CDS   computer   has completed   sending   all   the   data   words,   it   will   not acknowledge   the   ODR   from   the   DTS.   If   the   CDS computer   has   not   acknowledged   an   ODR   from   the DTS  in  a  preset  amount  of  time,  the  DTS  will  clear the   ODR   line   and   generate   a   stop   code.   Upon transmission  of  the  two-frame  stop  code,  the  DTS  will return  to  the  receive  mode. Net   Control   Station   (NCS)   I/O   Operations The   station   acting   as   NCS   follows   the   same protocols   when   communicating   with   the   CDS computer.  Some  differences  exist  in  the  generation  of the   control   codes.   The   net   control   station   is responsible   for   interrogating   each   station.   Upon receipt  of  a  picket  stop  code,  the  DTS  checks  the  next station   address   and   sends   an   interrogation   message. After   the   interrogation   message   is   transmitted,   the DTS   waits   to   receive   a   start   code   from   the interrogated  station.  If  a  start  code  is  not  recognized after   15   frame   intervals,   the   station   will   be reinterrogated.   If   a   start   code   is   not   received   after another   15   frame   intervals,   the   address   control   unit will  advance  to  the  next  active  picket  address  and  the interrogation   process   is   repeated. The  other  major  difference  is  when  the  net  control station   has   completed   its   own   tactical   data transmission,   a   control   stop   code,   followed   by   the next   station   address,   is   transmitted.   Again,   if   a   start code   is   not   received   within   15   frame   intervals,   a second interrogation is sent. This second interrogation   is   a   normal   interrogation   message consisting  of  the  preamble,  phase  reference  frame,  and address   code. Modulator/Demodulator The   modulator/demodulator   function   of   the   DTS provides   the   digital   to   analog   and   analog   to   digital conversion.   During   data   transmission,   the   24-bit binary  data  word  is  expanded  to  30  bits  by  adding  the six  bits  for  error  detection  and  correction.  The  30  bits are  then  examined  in  pairs  to  determine  the  required phase   angle   shift   for   each   of   the   15   data-carrying tones  in  the  audio  package. At the frame boundary, the phase of each data tone is  shifted  with  respect  to  the  previous  frame.  Figure 4-19  shows  the  four  possible  phase  shifts.  A  sixteenth tone,  the  605-Hz  Doppler  correction  tone,  is  added  to the   tone   package.   The   Doppler   tone   is   not   phase modulated   and   is   used   to   correct   for   Doppler   shifts caused   by   the   relative   motion   between   the transmitting  station  and  the  receiving  station.  The  16 tones  are  combined  into  a  composite  audio  signal  and sent   to   the   radio   set.   The   radio   set   transmits   the composite   tone   package   on   the   carrier   frequency   in independent   sideband   form. 4 - 1 8