Guralp Systems CMG-3TB - Manual (Page 18)

CMG-3TB
digitizer. In systems using downhole digitizers, this is replaced
by a 10-way mil-spec serial connector for attaching to a Data
Communications Module (DCM), modem or other
communications link.
•
The CONTROL connector is a 26-way mil-spec plug intended
for connecting to an external controller or Handheld Control
Unit, with the same pin out as the RECORDER connector.
•
The POWER connector is a 10-way mil-spec plug, which should
be connected to a source of 12 – 30 V DC power, for supplying to
the borehole instrumentation. When operating the hole lock,
you should connect the Holelock Control Unit to this connector.
Because of the high voltages employed, the hole lock circuitry is
entirely isolated from the rest of the electrical systems in the
sensor and surface unit; it is not usual to power the sensor
whilst using the hole lock.
For deep-borehole installations (over 50 m) we recommend that you
use a breakout box with internal line drivers, to ensure that logic
signals are reliably transmitted to the sensor. Contact Güralp Systems
for advice.
Note: The breakout box looks very similar to other Güralp breakout
boxes. However, its internal wiring is different from that used for some
other instruments. For this reason, if you are using several instrument
types, you should mark each breakout box clearly so that it is always
used with the correct instrument.
Calibration
To calibrate the instrument, the Calibration enable line must be
activated. This operates a relay which allows a calibration signal to
flow through the transducer feedback coil. This provides an extra force
acting on the sensor masses, producing a corresponding deflection in
the output signal, which can be analysed by a control computer to
extract the seismometer's response characteristics.
Most Güralp instruments are manufactured with active-low
Calibration enable lines. However, instruments with active-high
calibration can be manufactured on request.
Mass locking and unlocking
The 3TB is delivered with its sensor masses locked, so that they will
not be damaged in transit. You should lock the masses whenever you
18 Issue C