HOLD ENABLE:
double switch near FIFO's.
LEFT:  Output B - top=ignores Hold down=enable Hold
RIGHT: Output A - top=ignores Hold down=enable Hold

INPUT ENABLE:
This version of the merger will process data from ALL inputs anyhow,
but will wait for end_event only on those that have been enabled.
Central switch.
top          : input D : left=ignore right=wait for End_Event
2nd from top : input C : left=ignore right=wait for End_Event
3rd from top : input B : left=ignore right=wait for End_Event
bottom       : input A : left=ignore right=wait for End_Event

VME ADDRESS:
2 top switches near P1: set to all 0 (top)
switch near P2: use only 5 right switches to select Gegraphical
Address in 0-32 range. Set lefttmost 3 switches to 0 (top).
The 5 right switches are GA(4-3-2-1-0) from right to left, top=0, down=1:

 switch (left to right)   sw1 sw2 sw3  GA0 GA1 GA2 GA3 GA4
 value if top              0   0   0    0   0   0   0   0
 value if down             1   1   1    1   1   1   1   1