Rockwell Automation Publication 1756-UM540E-EN-P - December 2017
15
Analog I/O Module Operation in the ControlLogix System
Chapter 1
If controller configuration refers to a chassis slot in the system, the controller
periodically checks for the presence of a device there. If a device is detected, the
controller sends the configuration, and one of the following occurs:
•
If the configuration is appropriate to the module detected, a connection is
made and operation begins.
•
If the configuration is not appropriate to the module detected, the data is
rejected and the Logix Designer application indicates that an error
occurred.
The configuration can be inappropriate for any of a number of reasons.
For example, the configuration or a module can include a mismatch in
electronic keying that prevents normal operation.
The controller maintains and monitors its connection with a module. Any break
in the connection, for example, the removal of the module from the chassis while
under power, causes a fault.
The Logix Designer application indicates that the fault occurred in the fault
status bits associated with the module. The Logix Designer application monitors
the fault status bits to annunciate the failures of a module.
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 analog input
modules. Instead, the modules broadcast their input data–channel and status
data–to their backplane periodically.
Requested Packet Interval (RPI)
The RPI is a configurable parameter that defines a specific period when the
module broadcasts input data to the backplane. Valid RPI values are 1…750 ms.
The default value is 100 ms for inputs and 10 ms for outputs.
Set the RPI value at initial module configuration and adjust it as necessary only
when the controller is in Program mode.
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, 1756-IRT8I, 1756-IR12, and 1756-IT16 modules
do not offer
the RTS parameter
. With these modules, the channel sampling rate is
exclusively determined by the RPI value.
