$COMMAND

 In order to start the SSP  running  a  program,  the  master
 writes  a  command word to the location $COMMAND in the SSP,
 followed by an SSP GO to start the  SSP  running.   The  SSP
 checks the location $COMMAND to see which subroutine it must
 execute.

 The command word consists of a  4  character  ASCII  string.
 The  commands  that  have  been defined so far are listed in
 COMMANDS_UNDERSTOOD_BY_ALL_SSPS.


 $MASTER_ID

 This word is a 4 character ASCII string that is to be filled
 by  the  master  who wrote the command word in $COMMAND.  So
 far, the following IDs have been defined:


       o  VAX - ID of host computer

       o  MSTR - ID of MASTER_SSP

       o  TRIG - ID of TRIGGER_SSP


 Note that subsiduary SSPs that never act as  the  master  do
 not need a MASTER_ID.



 $TRIGGER

 This word is to be filled by the master.  Each bit  of  this
 word may be defined to represent a hardware configuration of
 the  experimental  trigger  external  to   the   SSP.    The
 definition of the bits are SSP_independent.



 $PR_EVENT

 This word is the number of the present event.  It is  to  be
 incremented  and  written by the master every time the EVENT
 command is executed.



 $SPILL

 This word is the spill number, and  is  to  written  by  the
 master  at  the beginning of each spill.  The begin of spill
 is signified to the SSP by sending the BEGIN_SPILL command.



 $RUN

 This word is the run number, and is to written by the master
 at the beginning of each run.  The begin of run is signified
 to the SSP by sending the START command.

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