Section 8. Operation
8.1.9 Synchronizing Measurements
Related Topics:
•
Synchronizing Measurements — Overview
(p. 74)
•
Synchronizing Measurements — Details
(p. 365)
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
– 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
telecommunication 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 CR1000 to CR1000,
each CR1000 can catch the rising edge of a digital pulse from the master
CR1000 and synchronize measurements or other functions, using the
WaitDigTrig()
instructions, independent of CR1000 clocks or data time
stamps. When programs are running in pipeline mode, measurements can be
synchronized to within a few microseconds (see
WaitDigTrig Scans
(p. 157)
).
3.
PakBus
(p. 88)
commands — the CR1000 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 CR1000 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.
365
Summary of Contents for CR1000
Page 2: ......
Page 4: ......
Page 6: ......
Page 32: ......
Page 36: ......
Page 38: ......
Page 40: ......
Page 60: ...Section 4 System Quickstart Figure 16 PC200W View Line Graph 60 ...
Page 96: ......
Page 98: ...98 ...
Page 302: ......
Page 453: ...Section 8 Operation Figure 115 Using the Keyboard Display 453 ...
Page 456: ...Section 8 Operation Figure 118 Real Time Custom 456 ...
Page 457: ...Section 8 Operation 8 8 1 3 Final Memory Tables Figure 119 Final Memory Tables 457 ...
Page 458: ...Section 8 Operation 8 8 2 Run Stop Program Figure 120 Run Stop Program 458 ...
Page 460: ...Section 8 Operation Figure 122 File Edit 460 ...
Page 461: ...Section 8 Operation 8 8 4 PCCard Memory Card Display Figure 123 PCCard CF Card Display 461 ...
Page 478: ......
Page 506: ......
Page 536: ......
Page 636: ......
Page 642: ......
Page 644: ......
Page 676: ......
Page 677: ......
