Rockwell Automation Publication 1756-UM542A-EN-P - September 2014 29
Time Synchronization Chapter 4
Time Adjustments
To keep the time of the 1756HP-TIME module as smooth as possible, the time is
adjusted in small increments (ramp) until it is equal to the NTP source. If the
time of the 1756HP-TIME module drifts more than 1 second off of the source
time, the 1756HP-TIME module jumps to the time given by the NTP source.
Loss of Communication
If the 1756HP-TIME module loses its connection to the NTP source (if the
source does not reply after the update interval time has expired) then the PPS and
SYNC indicators illuminate red, the TimeValid bit is set to FALSE, and the
1756HP-TIME module runs on its internal oscillator until the NTP source
becomes available again. The 1756HP-TIME module tries to reconnect to the
NTP source every 10 seconds after a loss of communication.
IRIG-B
The 1756HP-TIME module can be used as an IRIG-B master clock (outputs the
IRIG-B signal) or an IRIG-B slave clock (receives the IRIG-B signal from
another master clock). The time is transmitted over a coaxial cable, which plugs
into the BNC connector at the bottom of the 1756HP-TIME module.
1756HP-TIME Module as an IRIG-B Master
When the 1756HP-TIME module is an IRIG-B master clock, it sends the
current time over the IRIG-B network and synchronizes slave clocks to
approximately 1ms.
Note that the 1756HP-TIME module does not send the current year over the
IRIG
-B network.
The accuracy of the NTP time depends on the quality and reliability of the
Ethernet network. If the update time is set too high, the clock can drift and
jump. Rockwell Automation recommends a value of 30 seconds or less for
optimal accuracy.
NTP time as a source is accurate to 10 ms; therefore, the synchronization
accuracy of the output 1588 PTP is limited to 10 ms.
The 1756HP-TIME module currently supports the IRIG-B-122 format.
Allen-Bradley Motors
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