Rockwell Automation 1756-XXXX ControlLogix Analog I/O Modules - Manual (Page 29)

Publication 1756-UM009C-EN-P - December 2010 29
Analog I/O Operation in the ControlLogix System Chapter 2
Output Module Operation The RPI parameter governs exactly when an analog output module receives
data from the owner-controller and when the output module echoes data. An
owner-controller sends data to an analog output module only at the period
specified in the RPI. Data is not sent to the module at the end of the
controller’s program scan.
When an analog output module receives new data from an owner-controller
(that is, every RPI), the module automatically multicasts or ‘echoes’ a data
value that corresponds to the analog signal present at the output terminals to
the rest of the control system. This feature, called Output Data Echo, occurs
whether the output module is local or remote.
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 controller effectively
allows the module’s output channels to change values multiple times during a
single program scan because the output module is not dependent on reaching
the end of the program to send data.
Output Modules in
a Local Chassis When specifying an RPI value for an analog output module, you instruct the
controller when to broadcast the output data to the module. If the module
resides in the same chassis as the owner-controller, the module receives the
data almost immediately after the controller sends it.
40949
Data sent from owner at the RPI
Owner-controller Output module