Campbell CR3000 Micrologger, Руководство оператора

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Section 8. Operation
422
8.1.8.4 RS-232 Sensor Cabling
RS-232 sensor cable lengths should be limited to 50 feet.
8.1.8.5 SDI-12 Sensor Cabling
The SDI-12 standard allows cable lengths of up to 200 feet. Campbell Scientific
does not recommend SDI-12 sensor lead lengths greater than 200 feet; however,
longer lead lengths can sometimes be accommodated by increasing the wire gage
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. 79)
• Synchronizing Measurements — Details
(p. 422)
8.1.9.1 Synchronizing Measurement in the CR3000 — Details
Measurements are sychnronized in the CR3000 by the task sequencer. See
Execution and Task Priority
(p. 160).
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 CR3000s.
Techniques outlined below enable network administrators to synchronize CR3000
clocks and measurements in a CR3000 network.
Care should be taken when a clock-change operation is planned. Any time the
CR3000 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. 90)
– when reliable comms are common to all CR3000s 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.