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How Do the CST Clock and
SynchLink Node Clock
Work Together?
64-bit clock
1756-SYNCH
SynchLink Node Clock
Configuring Time
Mastership Functionality
As stated earlier, the ControlLogix Coordinated System Time clock
(CST) is a 64-bit clock on the ControlLogix backplane. It is used as 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 2.4 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 2.4
Chassis A
Coordinated System Time
SynchLink beacon over the fiber optic link
You must configure each 1756-SYNCH module's role in Coordinated
System Time (CST) Mastership. For any SynchLink system, there can
only be one SynchLink CST Time Master for the system; the Time
Master can be a 1756-SYNCH module or a ControlLogix controller. All
other devices connected to the SynchLink must be configured as Time
Slaves. However, the 1756-SYNCH modules that are Time Slaves are
Chassis CST Masters that receive the SynchLink time from the fiber
optic cable and relay it to all devices in their local chassis.
For example, the 1756-SYNCH modules in Figure 2.4 can be configured as
follows:
• 1756-SYNCH in Chassis A - MUST be SynchLink CST Master;
CAN also be Chassis CST Master
• 1756-SYNCH in Chassis B - MUST be Chassis CST Master;
CANNOT be SynchLink CST Master
For a full explanation on how to configure CST and SynchLink Mastership,
see page 3-18.
Time Synchronization in the ControlLogix System
Chassis B
Coordinated System Time
1756-SYNCH
Clock
Publication 1756-UM521C-EN-P - July 2004
2-5
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