Rockwell Automation 1756-OF8I ControlLogix Eight-channel Isolated Analog I/O Modules - Manual (Page 14)

14 Rockwell Automation Publication 1756-UM540A-EN-P - May 2014
Chapter 1 Isolated Analog I/O Module Operation in the ControlLogix System
Input Module Operation In traditional I/O systems, controllers periodically poll input modules to obtain
their input status.
In the ControlLogix system, the controller does not poll the isolated analog input
modules. Instead, the modules broadcast their input data, that is, channel and
status data, to their backplane periodically.
Requested Packet Interval (RPI)
The RPI is a configurable parameter that defines a specific period of time at
which the module broadcasts input data to the backplane. Valid RPI values are
1…750 ms. The default value is 100 ms.
You set the RPI value at initial module configuration and adjust it as necessary
only when the controller is in Program mode.
At the RPI, the following events occur.
1. The module scans its channels for input data.
2. The module broadcasts the data to its backplane.
IMPORTANT Other ControlLogix analog input modules offer the Real Time Sample (RTS)
parameter that determines when channel data is scanned and stored on the
module’s on-board memory until broadcast to the chassis backplane.
The 1756-IF8I and 1756-IRT8I modules do not offer the RTS parameter.
With these modules, the channel sampling rate is exclusively determined by
the RPI value.
On-Board Memory
Status Data
Channel Data
Channel Data
Channel Data
Channel Data
Channel Data
Channel Data
Ch 0
Ch 1
Ch 2
Ch 3
Ch 4
Ch 5
Timestamp
1
2
Channel Data
Channel Data Ch 6
Ch 7