Campbell Scientific CR1000 Measurement and Control System - Manual (Page 326)

Section 8. Operation
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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
).
3. PakBus 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.
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. CR1000 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.
With any synching method, care should be taken as to when and how things are
executed. Nudging the clock can cause skipped scans or skipped records if the
change is made at the wrong time or changed by too much.
5. GPS – clocks in CR1000s can be synchronized to within about 10 ms of each
other using the GPS() instruction. CR1000s built since October of 2008
(serial numbers ≥ 20409) can be synchronized within a few microseconds of
each other and within ≈200 µs of UTC. While a GPS signal is available, the
CR1000 essentially uses the GPS as its continuous clock source, so the
chances of jumps in system time and skipped records are minimized.
6. Ethernet – any CR1000 with a network connection (internet, GPRS, private
network) can synchronize its clock relative to Coordinated Universal Time
(UTC) using the NetworkTimeProtocol() instruction. Precisions are usually
maintained to within 10 ms. The NTP server could be another logger or any
NTP server (such as an email server or nist.gov). Try to use a local server —
something where communication latency is low, or, at least, consistent. Also,
try not to execute the NetworkTimeProtocol() at the top of a scan; try to ask
for the server time between even seconds.
8.2 Measurement and Control Peripherals
Peripheral devices expand the CR1000 input / output capacity. Classes of
peripherals are discussed below according to use. Some peripherals are designed
as SDM (synchronous devices for measurement) devices. SDM devices are
intelligent peripherals that receive instruction from and send data to the CR1000