ARMY   TM   5-6675-238-14 MARINE   CORPS   TM   08839A-14/1 utilized.   The   actual   conversion   is   accomplished   by   a universal    asynchronous    receiver/transmitter    (UART) module.   The   UART   communicates   with   the   computer via   the   computer   data   bus.   The   teletypewriter   is   con- trolled  from  the  SPU  using  the  computer  DMA  channel in   a   programmed   input/output   mode.   To   complete   a single   programmed   input/output   operation   to   the   tele- typewriter,   two   input/output   instructions   are   required in   the   program.   The   first   instruction   of   the   air   will ) always   be   an   output   to   device   address   (DA   15   (17 octal).   The   data   associated   with   the   first   instruction must  be  a  command  word  and  will  be  strobed  into  the UART   control   register.   This   control   word   initiates   the UART  send/receive  cycle,  and  also  enables  the  service interrupts   if   desired.   The   second   instruction   of   a   pair initiates   the   data   transfer   and   is   either   an   input   or output,  depending  on  the  command  previously  loaded. (1)   The   only   operation   that   can   be   performed without   executing   two   input/output   instructions   is   INP MSR   (status   register   interrogate),   This   is   accomplished by  executing  an  INP  DA  15  at  a  time  when  the  SPU  is conditioned  to  expect  the  first  of  a  pair  of  input/output instructions. (2)   To   determine   whether   any   given   input/output instruction  is  the  first  or  second  of  a  pair,  a  flip-flop  is utilized.   When   the   flip-flop   is   set,   it   indicates   that   the current  instruction  is  the  first  of  a  pair.  Reset  indicates second  of  a  pair.  The  conditions  for  control  of  the  first- second  flip-flop  are  as  follows:  set  if  either  the  SPU  has just   gone   through   a   power   turnon,   or   the   immediately preceding   input/output   instruction   was   an   input   (any type),   or   the   immediately   preceding   input/output   in- struction  was  not  the  first  of  a  pair;  reset  if  the  immedi- ately  preceding  input/output  instruction  was  the  first  of a  pair.  Clocking  of  the  first-second  flip-flop  takes  place at  address  time  of  the  current  input/output  instruction. (3)   Typical   programming   sequence   for   the   tele- typewriter  status  operation  is  as  follows: Instruction Function LDA   DATA   Load   accumulator   with   ASCII   charac- ter OUT  17 Transfer   ASCII   character   to   UART, send   to   teletypewriter   register (5)   After   the   second   OUT   17   instruction,   the UART   will   be   activated   to   accomplish   the   parallel-to- serial   conversion   necessary   to   have   the   teletypewriter print  or  punch  the  character  which  was  sent  to  it  by  the computer. (6)   For   receiving   a   character   from   the   teletype- writer,  the  following  sequence  is  used: Instruction Function LDA   CMD   IN   Load   either   5000   or   105000   into accumulator OUT   17 Put   accumulator   contents   into UART  control INP  17 Read  the  UART  receive  from  tele- typewriter   register   contents Upon   completion   of   the   above   sequence,   the   ASCII character   in   the   UART   control   register   is   received   by the   computer. (7)   Each   of   the   above   sequences   depends   on   the receipt   of   a   service   interrupt   by   the   computer.   To assure   that   the   SPU   service   interrupt   is   enabled,   the following  sequence  is  followed: Instruction Function LDA   CMD   Command   is   a   value   that   enables   the SPU  service  interrupt  circuit OUT  17 Actuates  the  SPU  interrupt  enable (8)   The   SPU   status   register   bits   that   are   checked are: INP   17 Get  teletypewriter  status  from  SPU Bit Definition (4)   For   sending   a   character   to   the   teletypewriter, the  following  sequence  is  used: Instruction Function LDA  CMD  Load  either  a  3  or  100003  into  accu- mulator OUT  17 Transfer   accumulator   contents   to UART  control  register o Peripheral   input   /output   opera- tion   complete 1 Teletypewriter  on-line 2 Teletypewriter   input   ready 3 Teletypewriter   transmitter   not busy 6-9