Rockwell Automation Allen-Bradley SynchLink 1756-DM Series Design Manual page 42

4-4
Publication 1756-TD008A-EN-P - August 2002
Coordinated System Time
The 1756-DMxxx drive modules use the CST differently than the
1756-SYNCH module. You should be aware of two significant
differences before implementing these modules in a control scheme.
First, the 1756-DMxxx drive modules are intelligent processing
modules that regulate Reliance Electric Distributed Power type drives.
Each 1756-DMxxx drive module has integrated Velocity and Position
Loops (VPL) that are a part of the module's firmware for controlling
the drives. As with any digital control scheme, the control loops are
scanned on a periodic basis, as measured against a given clock in the
system. In the case of the 1756-DMxxx drive modules, the clocking
mechanism is the CST clock on the ControlLogix backplane.
Since the 1756-DMxxx modules use the CST clock, as multiple
modules are placed in a single chassis, their VPLs are automatically
synchronized because they share the same clock.
CST/ SynchLink Time Relay
The second difference between the 1756-SYNCH and the 1756-DMxxx
drive modules is that the 1756-DMxxx drive modules do not act as
time relays. With this module, the SynchLink connection is for high
speed data transfer only. This also means that if a distributed drive
control system is installed where there are multiple 1756-DMxxx drive
modules distributed across multiple chassis, 1756-SYNCH modules
must also be installed in those chassis if tight synchronization between
drives is important from chassis to chassis.
Axis Support
The 1756-DMxxx drive modules do not support the motion engine or
profiler. They do not produce or consume axis data. They cannot be
configured with communications protocols which include axis data
types.