Campbell Scientific CR1000 Measurement and Control System - Manual (Page 325)

Section 8. Operation
325
Figure 104: Current limiting resistor in a rain gage circuit
8.1.10.3 RS-232 Sensors
RS-232 sensor cable lengths should be limited to 50 feet.
8.1.10.4 SDI-12 Sensors
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.
8.1.11 Synchronizing Measurements
Timing of a measurement is usually controlled relative to the CR1000 clock.
When sensors in a sensor network are measured by a single CR1000,
measurement times are synchronized, often within a few milliseconds, depending
on sensor number and measurement type. Large numbers of sensors, cable length
restrictions, or long distances between measurement sites may require use of
multiple CR1000s. Techniques outlined below enable network administrators to
synchronize CR1000 clocks and measurements in a CR1000 network.
Care should be taken when a clock-change operation is planned. Any time the
CR1000 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. 77)
– when reliable telecommunications are common to all
CR1000s 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
telecommunications 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. An easy way to keep a PC
clock accurate is to utilize public domain software available at
http://www.nist.gov/pml/div688/grp40/its.cfm.
2. Digital trigger – a digital trigger, rather than a clock, can provide the
synchronization signal. When cabling can be run from CR1000 to CR1000,