Rockwell Automation 1756-SYNCH ControlLogix SynchLink Module User Manual - Manual (Page 22)

Publication 1756-UM521C-EN-P - July 2004
2-2 Time Synchronization in the ControlLogix System
Using the Coordinated
System Time (CST) The Coordinated System Time (CST) is the clocking mechanism used
to achieve time synchronization in a ControlLogix chassis. The
ControlLogix Coordinated System Time (CST) clock is a 64-bit clock
on the backplane of the ControlLogix chassis. It has a 1μS resolution
and is used as the main time reference for all modules plugged into a
chassis backplane.
For more information on how the ControlLogix CST affects the
operation of other ControlLogix products, see the ControlLogix System
User Manual, publication 1756-UM001.
Time Synchronization in a
Distributed Control System The same CST mechanism described above is also used to
synchronize ControlLogix chassis in a distributed control system. In
such a system, SynchLink transfers the CST value from the CST Master
chassis to CST Slave chassis.
Each chassis must be equipped with a SynchLink module and,
depending on what version of RSLogix 5000 the system is using, each
chassis may or may not require that a controller reside in remote
chassis. This distributed control system is identified as a SynchLink
system.
The 1756-SYNCH module provides synchronization of CSTs between
ControlLogix chassis provides with no more than +/-5µs drift
between chassis.
Time Synchronization in the
SynchLink System Time synchronization within a SynchLink system is required to:
• transfer a CST value from the CST Master chassis to CST Slave
chassis.
• transfer motion and drive control data.
• support time synchronization between ControlLogix chassis and
non-ControlLogix products (e.g. PowerFlex 700S products).
The SynchLink Node Clock is integral to all devices that contain the
SynchLink circuitry. It is the clocking mechanism on the fiber optic
side of the SynchLink system. This clock has a resolution of 1μS.
During system configuration, you establish one SynchLink node clock
as the master system clock on the SynchLink fiber. By design, the
ControlLogix chassis that is configured as the SynchLink master also
acts as the CST master of the system. In this manner, one SynchLink
node acts as a Time Master for the entire system. This chapter gives
more detail on how this functionality is accomplished.