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    User name gilman

    Log entry time 10:25:43 on October 23,1998

    Entry number 3310

    keyword=helicity documentation, as of 10/22/98

    Updated documentation for helicity circuitry for FPP/GDH. R Gilman 10/23/98
    This can be accessed also as
    http://www.jlab.org/~gilman/helicity_circuitry_102298.txt

    ----- Helicity scheme -----

    In original FPP runs, we used "toggle" mode, with 30 Hz of pulses,
    33.3 ms + alternating with 33.3 ms minus. From the source we got 2
    helicity correlated logical pulse signals, + and -, that were
    complements of each other, and we sent both downstairs.

    During GDH, we are running in pseudorandom mode. We get pairs of 1 s
    long pulses. The second of each pair is the complement of the first,
    but the phase of the first is chosen pseudorandomly. Thus the first
    pulse of a pair may be the same as the second of the previou pair,
    and apparent pulse widths vary from 1 to 2 s. We get two signals
    from MCC, a correlated signal plus a clock signal. We use the correlated
    signal to generate an h+ signal and the complement h- signal that we send
    downstairs.

    ----- Cables to downstairs, hadron arm -----

    The h+ and h- signals go downstairs to the equipment aisle through
    patch 1H75B03, numbers 17+18.

    The signals are connected with 4 foot BNC cables to the patch to the hadron
    arm "nose", through numbers 9+10.

    At the back of the detector stack, 10 foot BNC cables bring the signals to
    patch 161B, numbers D5+D6, which takes them upstairs to the top level
    of the stack.

    ----- Circuit in hadron arm -----

    The signals sent downstairs are complements, and do not blank off the
    first ~200 us, to give time for the helicity to stabilize and the electrons
    to reach the hall. This blank-off period is introduced in electronics in the
    hadron detector stack.

    For both h+ and h- signals, the circuitry looks like the following...
    (Note: diagram adjusted for going from fixed width to variable width font...)
    signal
    |
    |
    linear fan out-----\
    | |
    | | GO output
    Phillips gate+delay | from TS
    200 us delay | |
    200 us long gate output | |
    | | |
    | | assorted level
    | | shifting
    | | |
    | | |
    | | |
    veto input input b input c
    LeCroy 465 coincidence module
    linear output logical output (100 ns wide)
    | | |
    | | - unused -
    | |
    gate% linear
    for scalers _fan out_
    _/ | | \_
    _/ | | \_
    _/ / | \_
    _/ / | \_
    _/ / | \
    _/ ADC for backup ADC LeCroy level shifter&,^
    | event for event channel 6+, 10- for
    | helicity helicity input to scalers --
    | tagging* tagging* level shifter output
    | is fanned out through LeCroy ECL
    to electron arm delay box to inputs to the MLU/TS
    scaler and the helicity gated versions
    of it

    % Since the scaler gate is an AND of run and good helicity, the helicity
    gated scalers should reflect
    * For the event data, the long helicity gates are:
    h+ gate into channel 15 of the S1 and gas Cerenkov 4413 discriminators
    h- gate into channel 16 of the S1 and gas Cerenkov 4413 discriminators
    Experimentally we find that ~50% of events are +, ~50% are minus, a
    small fraction are neither, and perhaps 1 in 10^6 events are both
    helicities, a situation which should not happen.
    & We originally used scaler channels 9 and 10, but this was shifted to
    6 and 10 sometime. When GDH added the additional V-to-F scalers?
    ^ The scaler inputs used to be the short logical outputs of the
    LeCroy 465 coincidence module; we do not know when this was changed.
    I will attempt to change this back when there is time to check it out.
    
    ----- Cables to electron arm -----

    To enable independent DAQ for hadron and electron arms, we need to bring
    the first-200-us-blanked-off signals to the electron arm. The ~1 us
    delay from hadron to electron arms should not matter.
    The cabling is as follows:

    From the top level of the stack, the cables go to the back of the stack
    through patch panel 161B, numbers D7+D8.

    Two ~10 foot cables then connect the helicity signals to the "hadron nose"
    patch panel, cables 1 + 2.

    Two short BNC cables connect the "hadron nose" channels 1 + 2 in the equipment
    aisle to "electron nose" channels 3 + 4.

    In the back of the electron spectrometer, we cable the "electron nose" to
    patch panel Alfred D 13+16 to take the signals to the upper electronics
    level. As of 10/22/98, we have seen that we get good complementary,
    200-us-blanked-off signals in the electron arm; we have not identified
    into which two ADCs to send the signals.