Campbell CR6 Series, Руководство оператора

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Section 8. Operation
471
or powering the sensor with a second 12 Vdc power supply placed near the
sensor.
8.1.9 Synchronizing Measurements — Details
Related Topics:
• Synchronizing Measurements — Overview
(p. 85)
•
Synchronizing Measurements — Details
(p. 471)
Timing of a measurement is usually controlled relative to the CR6 clock.
8.1.9.1 Synchronizing Measurement in the CR6 — Details
When sensors in a sensor network are measured by a single CR6, measurement
times are synchronized, often within a few milliseconds, depending on sensor
number and measurement type.
Measurements are sychnronized in the CR6 by the task sequencer. See Execution
and Task Priority
(p. 210).
8.1.9.2 Synchronizing Measurements in a Datalogger Network — Details
Large numbers of sensors, cable length restrictions, or long distances between
measurement sites may require use of multiple CR6s.
Techniques outlined below enable network administrators to synchronize CR6
clocks and measurements in a CR6 network.
Care should be taken when a clock-change operation is planned. Any time the
CR6 clock is changed, the deviation of the new time from the old time may be
sufficient to cause a skipped record in data tables. Any command used to
synchronize clocks should be executed after any CallTable() instructions and
timed so as to execute well clear of data output intervals.
Techniques to synchronize measurements across a network include:
1. LoggerNet
(p. 97)
– when reliable comms are common to all CR6s in a network,
the LoggerNet automated clock check provides a simple time synchronization
function. Accuracy is limited by the system clock on the PC running the
LoggerNet
server. Precision is limited by network transmission latencies.
LoggerNet compensates for latencies in many comms systems and can achieve
synchronies of <100 ms deviation. Errors of 2 to 3 second may be seen on
very busy RF connections or long distance internet connections.
Note Common PC clocks are notoriously inaccurate. Information
available at http://www.nist.gov/pml/div688/grp40/its.cfm gives some
good pointers on keeping PC clocks accurate.
2. Digital trigger — a digital trigger, rather than a clock, can provide the
synchronization signal. When cabling can be run from CR6 to CR6, each CR6
can catch the rising edge of a digital pulse from the master CR6 and