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                3.1     GENERAL DESCRIPTION. . . . . . . . . . . . . . . . 3-1
                3.2     CONSTRUCTION . . . . . . . . . . . . . . . . . . . 3-1
                3.2.1     Double-Width CAMAC Module  . . . . . . . . . . . 3-1
                3.2.2     Front Panel with the following items:  . . . . . 3-1
                3.3     CAMAC COMMANDS . . . . . . . . . . . . . . . . . . 3-2
                3.4     X AND Q RESPONSE . . . . . . . . . . . . . . . . . 3-3
                3.5     MANUAL MODE  . . . . . . . . . . . . . . . . . . . 3-3
                3.6     DRAWING PACKAGE NUMBER . . . . . . . . . . . . . . 3-3
                3.7     RESPONSIBLE ENGINEERS  . . . . . . . . . . . . . . 3-3

                                      CHAPTER 3
                             BEAM POSITION MONITOR (BPM)

        3.1  GENERAL DESCRIPTION.

        The BPM is a CAMAC module that is used to  process  the  four  signals
        from  a directional-coupler type of beam position monitor, and provide
        digitized  outputs  of  the  vertical   difference,   the   horizontal
        difference  and the half sum signals.  This is done by first splitting
        each signal, one part of each going to a power combiner to  provide  a
        "fast sum" signal.  The other part is passed through a Gaussian filter
        and then to one of  two  hybrid  junctions,  to  obtain  the  required
        differences  and  a  "slow sum" signal.  These three signals then pass
        through variable attenuators to "track & hold" circuits, through  fast
        switches.   These  switches are closed by the simultaneous presence of
        the fast sum signal and an external beam  gate.   This  external  beam
        gate  allows  the  measurement  to be made on a particular bunch.  The
        track & hold circuit voltages are digitized by 10-bit ADCs, which  can
        be  read  through the CAMAC dataway.  The beam gate is supplied from a
        PDU via the upper backplane connector.
        The diode variable  attenuators  are  all  set  to  the  same  nominal
        attenuation  by  means  of  a  DAC.   The attenuators have a means for
        reversing the signal polarity, for operation with positrons.  The unit
        is  calibrated  by  applying the slow sum signal to the two difference
        channels, thus showing up any difference in gain  between  the  three.
        In  the  absence  of  beam, a special trigger can be used to close the
        track & hold switches, thus measuring the pedestal levels.

        3.2  CONSTRUCTION


        3.2.1  Double-Width CAMAC Module


        3.2.2  Front Panel with the following items:

              o  4 SMA sockets for signals from BPM
              o  1 Lemo input for test gate signal
        ______________________________
        For further details on the BPM, see SINGLE PASS COLLIDER MEMO CN-217.



        BEAM POSITION MONITOR (BPM)                                   Page 3-2

              o  4 Lemo outputs - Beam generated trigger
                                - Horizontal difference signal
                                - Sum signal
                                - Vertical difference signal
              o  4 LEDs - CAMAC X response
                         - Coversion being done
                         - Electron bunch - self triggering
                         - Positron bunch -   "    "    "
              o  Push-button for manual operation.

        3.3  CAMAC COMMANDS

              o  F0 A0,A1,A2:  READ ADCs
                 X is returned to the dataway and Q also, but  only  when  the
                 data from the ADCs is ready, that is, after the completion of
                 a conversion.  The command for a conversion is  generated  by
                 the  beam gate or, in its absence, by the beam derived "hold"
                 signal.
              o  F1 A0:  READ DAC
              o  F1 A1:  READ OFFSETS
                 The x and y offsets are two 8 bit words which are stored  for
                 eventual  corrections  of  the  measured  data.   The 8 least
                 significant bits contain the x offset,  the  y  offset  being
                 written in the next 8 higher order bits.
              o  F3 A0:  READ MODULE TYPE AND SERIAL NUMBER
                 The serial number is presented on the lower 10 bits  and  the
                 type code on the next 6 bits.
              o  F9 A0:  CLEAR MODULE
              o  F16 A0:  WRITE DAC
                 The attenuation (not the dB figure) is  approximately  linear
                 with the count number as follows:
                  o  0 Minimum attenuation for the e- bunch
                  o  127 Maximum attenuation for the e- bunch
                  o  128 Maximum attenuation for the e+ bunch
                  o  255 Minimum attenuation for the e+ bunch



        BEAM POSITION MONITOR (BPM)                                   Page 3-3

                 These values are not affected by an Initialize (Z and S2)  or
                 a Clear (C and S2).
              o  F24 A0:  CALIBRATE
                 A beam and a  beam  gate  are  required.   This  function  is
                 latched and is reset by an Initialize, a Clear, F9 or F26.
              o  F25 A0:  PEDESTAL
                 No beam required, only a beam  gate  or  a  substitute  gate.
                 This  function is latched and is reset in the same way as the
                 Calibrate mode.
              o  F26 A0:  CLEAR CALIBRATE AND PEDESTAL
              o  F27 A0:  BEAM GATE SUBSTITUTE ON
                 This function is reset by an Initialize, a Clear, F9 or F28.
              o  F28 A0:  BEAM GATE SUBSTITUTE OFF.

        3.4  X AND Q RESPONSE

        X and Q are returned true  for  all  the  above  commands;  Q  is  not
        returned  true, however, when the data is not ready.  The data is made
        not ready following an Initialize, a Clear, F9,  F0  A3  or  when  the
        substitute  gate  has  been  turned  on  by depressing the front panel
        push-button.

        3.5  MANUAL MODE

        This  feature  is  implemented  for  test  purposes.   Depressing  the
        push-button  switch  once sets the module with a substitute gate and a
        minimum attenuation for the e- bunches; depressing the button a second
        time sets the circuit in the same mode but for the e+ bunches; a third
        actuation of the button will return the module to its original  state.
        The manual mode is of course reset by any valid command.

        3.6  DRAWING PACKAGE NUMBER

        123-605

        3.7  RESPONSIBLE ENGINEERS

        G.  Oxoby, J.-L.  Pellegrin, and V.  Smith