10 Publication 1756-PM004C-EN-P - October 2009
Chapter 1 Communicate with I/O Modules
Requested Packet Interval
The Logix5000 controller uses connections to transmit I/O data.
In Logix5000 controllers, I/O values update at a period that you configure via
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.
• Programs within a task access input and output data directly from
controller-scoped memory.
• Logic within any task can modify 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).
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
• 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 0.2 ms (200 microseconds)
…750 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.
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.
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