Campbell Scientific CR3000 Micrologger - Manual (Page 331)

Section 8. Operation
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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. 76)
– when reliable telecommunications 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
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 CR3000 to CR3000,
each CR3000 can catch the rising edge of a digital pulse from the Master
CR3000 and synchronize measurements or other functions, using the
WaitDigTrig() instructions, independent of CR3000 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 CR3000 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 CR3000 clock has a resolution of 10 ms,
which is the resolution used by PakBus clock-sync functions. In networks
without routers, repeaters, or retries, the communication time will cause an
additional error (typically a few 10s of milliseconds). PakBus clock
commands set the time at the end of a scan to minimize the chance of skipping
a record to a data table. This is not the same clock check process used by
LoggerNet as it does not use average round trip calculations to try to account
for network connection latency.
4. An RF401 radio network has an advantage over Ethernet in that
ClockReport() can be broadcast to all dataloggers in the network
simultaneiously. Each will set its clock with a single PakBus broadcast from
the master. Each datalogger in the network must be programmed with a
PakBusClock() instruction.
Note Use of PakBus clock functions re-synchronizes the Scan() instruction. Use
should not exceed once per minute. CR3000 clocks drift at a slow enough rate
that a ClockReport() once per minute should be sufficient to keep clocks within
30 ms of each other.