Campbell Scientific CR800 and CR850 Measurement and Control Systems - Manual (Page 310)

Section 8. Operation
310
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 CR800 clock.
When sensors in a sensor network are measured by a single CR800, 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 CR800s. Techniques outlined below enable network administrators to
synchronize CR800 clocks and measurements in a CR800 network.
Care should be taken when a clock-change operation is planned. Any time the
CR800 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. 76)
– when reliable telecommunications are common to all
CR800s 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 CR800 to CR800, each
CR800 can catch the rising edge of a digital pulse from the Master CR800 and
synchronize measurements or other functions, using the WaitDigTrig()
instructions, independent of CR800 clocks or data time stamps. When
programs are running in pipeline mode, measurements can be synchronized to
within a few microseconds (see WaitDigTrig Scans
).
3. PakBus commands – the CR800 is a PakBus device, so it is capable of being a
node in a PakBus network. Node clocks in a PakBus network are
synchronized using the SendGetVariable(), ClockReport(), or
PakBusClock() commands. The CR800 clock has a resolution of 10 ms,