Rockwell Automation 1756-OF8I ControlLogix Eight-channel Isolated Analog I/O Modules - Manual (Page 17)

Rockwell Automation Publication 1756-UM540A-EN-P - May 2014 17
Isolated Analog I/O Module Operation in the ControlLogix System Chapter 1
Output Module Operation The RPI defines when a controller sends data to the isolated analog output
module and when the output module echoes data. The controller sends data to
an output module only at the RPI.
When an output module receives new data from the controller, the module
multicasts or ‘echoes’ a data value that corresponds to the signal present at its
terminals to the rest of the control system. This feature, called
Data Echo , occurs
whether the output module resides in the local or remote chassis.
Depending on the value of the RPI, with respect to the length of the controller
program scan, the output module can receive and ‘echo’ data multiple times
during one program scan.
When the RPI is less than the program scan length, the module’s output channels
can change values multiple times during a single program scan. The controller
does not depend on reaching the end of the program to send data.
Output Modules in a Local Chassis
When an output module resides in a local chassis, as shown on page 11 , it receives
data almost immediately after the owner-controller sends it.
Output Modules in a Remote Chassis
When an output module resides in a remote chassis, as shown on page 12 , and is
connected to the local chassis via an EtherNet/IP network, the following events
occur for the controller to send data to the output module.
1. The controller broadcasts data to its local chassis at one of the
following events:
•
RPI value
• A programmed Immediate Output (IOT) instruction is executed.
An IOT sends data immediately and resets the RPI timer.
2. The 1756 ControlLogix EtherNet/IP communication module in the local
chassis broadcasts the data over the EtherNet/IP network.
3. After receiving the output data, the 1756 ControlLogix EtherNet/IP
communication in the remote chassis broadcasts the data to its backplane,
that is, the remote chassis.
4. The output module receives the data almost immediately after it is
broadcast to the remote chassis backplane.