Rockwell Automation Allen-Bradley SynchLink 1756-DM Series Design Manual page 17

64-bit clock
1756-SYNCH
SynchLink Node Clock
Time Synchronization in a Distributed Control System
The CST mechanism synchronizes ControlLogix chassis in a
distributed control system. In this case, SynchLink transfers the CST
value from the CST master chassis to CST slave chassis. At the time of
the publication, each chassis must be equipped with at least a
controller and a SynchLink module to synchronize time.
How Do the CST Clock and SynchLink Node Clock Work Together?
The CST is the main time reference for all modules plugged into a
ControlLogix chassis. The SynchLink node clock is used to establish
the time reference on the SynchLink fiber.
Figure 1.4 illustrates how the SynchLink node clock and the CST time
reference are coordinated in a system. In this example, the SynchLink
modules in chassis A & B synchronize the CST clock in chassis B with
the CST reference in chassis A.
Chassis A is the time master for the system. When the SynchLink
module strobes the beacon signal onto the fiber optic link, it also
transmits the CST time reference value that tells the downstream node
what time it is as the beacon occurs. The downstream chassis receives
the CST time reference and synchronizes its time with the CST
reference value on the beacon signal.
Figure 1.4
Chassis A
Coordinated System Time
SynchLink beacon over
the fiber optic link
The SynchLink modules in Figure 1.4 must be correctly configured
(via RSLogix 5000) to make sure the system is synchronized. For more
information on how to configure ControlLogix SynchLink modules,
see the ControlLogix SynchLink module user manual, publication
1756-UM521.
Chassis B
Coordinated System Time
1756-SYNCH
Clock
Publication 1756-TD008A-EN-P - August 2002
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