FUNCTION BEAM_CROSSING_PERIOD(DELTA_TIME, DELTA_BEAM_CROSSING)
C----------------------------------------------------------------------
C-
C-   Purpose and Methods : To find the exact beam crossing period, given the
C-      number of beam crossings in an amount of time. It picks a period of
C-      3.5us or 5us depending on which is closer to the observed time. It is
C-      possible to add other possible beam crossing periods. Once a beam
C-      crossing period has been established, three consecutive readings of a
C-      different beam crossing period is required before this function will
C-      reflect the change.
C-      
C-      +-----------------------------------------------------------+
C-      |HARDCODED TO 3.5 MICROSECOND BEAM CROSSING RATE 29-AUG-1994|
C-      +-----------------------------------------------------------+
C-      
C-   Returned value  : The beam crossing period in seconds.
C-   Inputs  : DELTA_TIME       The elapsed time in quanta of 100 ns
C-             DELTA_BEAM_CROSSINGS
C-                              The number of beam crossings in the elapsed
C-                              period of time.
C-   Outputs : none
C-   Controls: none
C-
C-   Created  10-JAN-1991   MICHIGAN STATE UNIVERSITY, TRIGGER CONTROL SOFTWARE
C-   Updated  10-AUG-1994   Philippe Laurens,Freddie Landry
C-                          DELTA_TIME changed from INT to REAL
C-                          hardwired a period of 3.5 microseconds
C-
C----------------------------------------------------------------------
      IMPLICIT NONE
C
      REAL BEAM_CROSSING_PERIOD
      REAL DELTA_TIME
      INTEGER DELTA_BEAM_CROSSING
C
      BEAM_CROSSING_PERIOD = .349314470E-5
C
C       Array of possible beam crossing periods. They must be in increasing
C       order. NUM_PERIODS must be equal to the number of entries in PERIODS
C
C      REAL PERIODS(2)
C      DATA PERIODS / .349314470E-5, .523971706E-5 /
C      SAVE PERIODS
C
C      INTEGER NUM_PERIODS
C      PARAMETER (NUM_PERIODS = 2)
C      INTEGER NUM_CONSECUTIVE
C      PARAMETER (NUM_CONSECUTIVE = 2) ! This is actually number of
C                                      ! consecutive readings - 1
C
C      REAL LAST_PERIODS(NUM_CONSECUTIVE)
C      SAVE LAST_PERIODS
C      REAL LAST_PERIOD
C      SAVE LAST_PERIOD
C      LOGICAL FIRST
C      DATA FIRST / .TRUE. /
C      SAVE FIRST
C
C      REAL MEASURED_PERIOD
C      INTEGER COUNT
C
C      IF ((DELTA_BEAM_CROSSING .GT. 0) .AND. 
C     &  (DELTA_TIME .GT. 0)) THEN
C       MEASURED_PERIOD = FLOAT(DELTA_TIME) *100E-9 / 
C    &    FLOAT(DELTA_BEAM_CROSSING)
C      ELSE
C        BEAM_CROSSING_PERIOD = 0.
C        GOTO 999
C      ENDIF
C
C       Find how many packets there appear to be
C
C
C      BEAM_CROSSING_PERIOD = 0.
C      IF (MEASURED_PERIOD .LT. PERIODS(1)) THEN
C        BEAM_CROSSING_PERIOD = PERIODS(1)
C      ELSEIF (MEASURED_PERIOD .GT. PERIODS(NUM_PERIODS)) THEN
C        BEAM_CROSSING_PERIOD = PERIODS(NUM_PERIODS)
C      ELSE
C        DO COUNT = 2, NUM_PERIODS
C          IF (MEASURED_PERIOD .LT. PERIODS(COUNT)) THEN
C            IF ((MEASURED_PERIOD - PERIODS(COUNT-1)) .LT.
C     &        (PERIODS(COUNT) - MEASURED_PERIOD)) THEN
C              BEAM_CROSSING_PERIOD = PERIODS(COUNT-1)
C            ELSE
C              BEAM_CROSSING_PERIOD = PERIODS(COUNT)
C            ENDIF
C          ENDIF
C        ENDDO
C      ENDIF
C
C The first time through, make it look like this beam crossing period has been
C here forever.
C
C      IF (FIRST .EQV. .TRUE.) THEN
C        DO COUNT = 1, NUM_CONSECUTIVE
C          LAST_PERIODS(COUNT) = BEAM_CROSSING_PERIOD
C        END DO
C        LAST_PERIOD = BEAM_CROSSING_PERIOD
C        FIRST = .FALSE.
C      ELSE
C
C After first time, check to see if the previous two readings were the same.
C
C        DO COUNT = 1, NUM_CONSECUTIVE
C          IF (LAST_PERIODS(COUNT) .NE. BEAM_CROSSING_PERIOD) GOTO 200
C        END DO
C        LAST_PERIOD = BEAM_CROSSING_PERIOD
C        GOTO 999 ! If all are the same, then return
C
C  200   CONTINUE
C        DO COUNT = 2, NUM_CONSECUTIVE
C          LAST_PERIODS(COUNT-1) = LAST_PERIODS(COUNT)
C        END DO
C        LAST_PERIODS(NUM_CONSECUTIVE) = BEAM_CROSSING_PERIOD
C        BEAM_CROSSING_PERIOD = LAST_PERIOD
C      ENDIF
C
C
C----------------------------------------------------------------------
  999 RETURN
      END
C
C
C
      FUNCTION OLD_BEAM_CROSSING_PERIOD(A1, A2)
C----------------------------------------------------------------------
C-
C-   Purpose and Methods : Fake crossing period calculation. 
C-
C-   Returned value  : The beam crossing period in seconds.
C-   Inputs  : none 
C-   Outputs : none
C-   Controls: none
C-
C-   Created  10-JAN-1991   MICHIGAN STATE UNIVERSITY, TRIGGER CONTROL SOFTWARE
C-
C----------------------------------------------------------------------
      IMPLICIT NONE
      REAL OLD_BEAM_CROSSING_PERIOD
      INTEGER A1, A2
C
      OLD_BEAM_CROSSING_PERIOD = 1 / 286275.0
C----------------------------------------------------------------------
  999 RETURN
      END