Rockwell Automation Logix5000 Controllers I/O and Tag Data Programming Manual - Manual (Page 12)

Chapter 1 Communicate with I/O modules
The Logix5000 controller uses connections to transmit I/O data.
Term Definition
Connection A communication link between two devices, such as between a controller and an I/O module, PanelView terminal, or another controller.
Connections are allocations of resources that provide more reliable communications between devices than unconnected messages. The number of
connections that a single controller can have is limited.
You indirectly determine the number of connections the controller uses by configuring the controller to communicate with other devices in the
system. The following types of communication use connections:
• I/O modules
• Produced and consumed tags
• Produced and consumed program parameters
• Certain types of Message (MSG) instructions (not all types use a connection)
Requested packet interval
(RPI)
The RPI specifies the period at which data updates over a connection. For example, an input module sends data to a controller at the RPI that you
assign to the module.
• Typically, you configure an RPI in milliseconds (ms). The range is 1 ms (1000 microseconds)… 536870.911 ms.
• If a ControlNet network connects the devices, the RPI reserves a slot in the stream of data flowing across the ControlNet network. The timing of
this slot may not coincide with the exact value of the RPI, but the control system guarantees that the data transfers at least as often as the RPI.
In Logix5000 controllers, I/O values update at a period that you configure in the
I/O configuration folder of the project. The values update asynchronous to the
execution of logic. At the specified interval, the controller updates a value
independently from the execution of logic.
ATTENTION: Make sure that data memory contains the appropriate values throughout a task’s execution. You
can duplicate or buffer data at the beginning of the scan to provide reference values for your logic.
• Programs within a task access input and output data directly from
controller-scoped memory.
• Logic within any task can change controller-scoped data.
• Data and I/O values are asynchronous and can change during the course of
a task’s execution.
• An input value referenced at the beginning of a task’s execution can be
different when referenced later.
• To prevent an input value from changing during a scan, copy the value to
another tag and use the data from there (buffer the values).
Tip: Starting with Logix Designer version 24, you can use program parameters to share data between
programs in much the same way as you have used controller-scoped tags. Input and Output program
parameters automatically buffer data, without using another program parameter or tag. For more
information on program parameters, refer to the
Logix5000 Controllers Program Parameters
Programming Manual , publication 1756-PM021 .
The communication format that you choose determines the data structure for the
tags that are associated with the module. Many I/O modules support different
Requested packet interval
Communication format
12 Rockwell Automation Publication 1756-PM004E-EN-P - October 2014