INTRODUCTION


 Since most of the SSPs in the BNL787 system control  FASTBUS
 crates containing different types of slave modules, the code
 loaded into each SSP is likely to be very different.  It  is
 therefore  necessary  to  adopt  some  standard  features to
 enable efficient accessing of the data from all the SSPs  in
 the  system.   The  standard  features  described  here  are
 defined locations in SSP data memory.

 This document is intended to be a clarification of  the  use
 of the defined locations.

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 SYSTEM_OPERATION


 The BNL787 FASTBUS system consists of a  number  of  FASTBUS
 crates  (or CRATE SEGMENTS), each of which contains at least
 one SSP  which  acts  as  the  master  of  the  crate.   The
 remainder  of  the  crate  will  contain  slave  modules  of


 different kinds that collect the data from the experiment.

 The SSPs are linked together by a CABLE SEGMENT.  One of the
 crates  is  designated  the "MASTER CRATE" and this contains
 the "MASTER_SSP".  The "MASTER CRATE" will also contains  an
 interface to the host computer (CFI interface).

 Each SSP is responsible for the initialization and set up of
 the  modules  in  the  same  crate,  and for reading out and
 storing data from their local crate segment on  each  event.
 The  MASTER_SSP  will then read the data from the other SSPs
 via the cable segment, and will assemble and store  it.   It
 will then be read out by the host.

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 DATA_FLOW


 The flow would be as follows:

 The master sends a command INIT to all subsiduary SSPs.   On
 receipt  of  this command, all SSPs should call a subroutine
 INITIALIZE.   Initialization  includes  filling  the  BNL787
 defined  locations  for  crate  contents  (see $CRATE).  The
 slave modules should be made ready for data taking.  Further
 actions are SSP_dependent, and may include filling of a YBOS
 bank containing initial set-up values of the slave  modules.
 If  so,  the address and length of the bank should be loaded
 into the SSP_dependent  defined  locations  $SLAVE_REGS  and
 $LSLAVE_REGS.


 After the conclusion of Initialize the SSP halts.

 The master sends a command  STAR  to  all  subsiduary  SSPs.
 This  denotes  the  start  of  a  run.   On  receipt of this
 command, all SSPs  should  call  a  subroutine  START.   The
 action  of this subroutine is SSP_dependent.  It may include
 initializing any counters or histograms for the  whole  run.
 After the conclusion of START, the SSP halts.

 The master sends a command BSPI to all  subsiduary  SSPs  to
 denote begin of spill.  On receipt of this command, all SSPs
 should call a subroutine BEGIN_SPILL.  The  action  of  this
 subroutine  is  SSP_dependent.   It  may  include  resetting
 pointers   to   the   data   and   address   buffers    (see
 BUFFER_LOCATIONS_AND_SIZES),   and   initialization  of  any
 counters for the spill.

 If a front panel SSP trigger  is  being  used,  the  routine
 BEGIN_SPILL  will write the command EVNT to its own $COMMAND
 location and enable front panel triggers.  It then sets soft
 halt,  and waits for an SSP front panel trigger.  Otherwise,
 BEGIN_SPILL will simply halt in the same way  as  the  other
 subroutines.

 On receipt of a hardware trigger to  denote  an  event,  the
 master  SSP  will  cause  the subsiduary SSPs to execute the
 subroutine EVENT,  either  by  sending  a  SSP  front  panel
 trigger, or by writing the command EVNT.

 On receipt of the command EVNT, a subroutine EVENT  will  be
 called.  This will read the data from the slave modules into
 a data buffer, where it is  stored  in  YBOS  format.   EVNT
 commands  will  be  given repeatedly until either the end of
 spill occurs, or the buffer becomes  full.   If  the  buffer
 becomes full, a special return code is sent (see $RET_CODE).
 On concluding subroutine EVENT, the SSP will halt unless SSP
 front  panel triggers are enabled, in which case it will set
 the soft halt.

 The master will signal the  end  of  spill  by  sending  the
 command  ESPI.   On  receipt  of  this command, a subroutine
 END_SPILL will  be  called.   The  action  of  END_SPILL  is
 SSP_dependent,  and may include processing the raw data.  On
 concluding subroutine END_SPILL, the SSP will halt.

 At this point, the master may read all  the  pieces  of  the
 event  from the subsiduary SSPs, and combine them in buffer.
 The commands BSPI,EVNT,ESPI would then be repeated until the
 end of run.

 The commands PAUS and CONT are dummies presently.


 BUFFER_FORMAT


 All the SSPs in the system must collect data from the slaves
 in  their  crate  segment,  and  produce  a  data buffer and
 address buffer that can be read out by the master at the end
 of  spill.   The  data  buffer  may contain raw or processed
 data.  The gathering of the data and filling of the  address
 and  data  buffers is SSP_dependent, but the final format of
 the contents of the data and event  buffers  should  be  the
 same for all SSPs.

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