Rockwell Automation 1756-XXXX ControlLogix Digital I/O Modules - Manual (Page 30)

30 Rockwell Automation Publication 1756-UM058G-EN-P - November 2012
Chapter 2 Digital I/O Operation in the ControlLogix System
Worst Case RPI Multicast Scenario
In the worst case scenario, the module performs an RPI multicast just after the
reserved network slot has passed. In this case, the owner-controller will not
receive data until the next available network slot.
When selecting values for the remotely located module’s RPI, system throughput
is optimized when its RPI value is a power of two times the current NUT
running on the ControlNet network.
For example, the following table shows recommended RPI values for a system by
using a NUT of 5 ms.
Remote Input Modules Connected via the EtherNet/IP Network
When remote digital input modules are connected to the owner-controller via an
EtherNet/IP network, data is transferred to the owner-controller at these times:
• At the RPI, the module produces data within its own chassis.
• At the COS (if enabled), the 1756 EtherNet/IP communication module
in the remote chassis immediately sends the module’s data over the
network to the owner-controller as long as it has not sent data within a
timeframe that is one-quarter the value of the digital input module’s RPI.
This prevents flooding the network with data.
For example, if a digital input module uses an RPI = 100 ms, the
EtherNet/IP module sends module data immediately on receiving it if
another data packet was not sent within the last 25 ms.
For more information about specifying an RPI rate, see the Logix5000
Controllers Design Considerations Reference Manual,
publication
1756-RM094 .
IMPORTANT Enabling the COS feature on an input module in a remote chassis allows the
module to multicast data at both the RPI rate and when the input changes
state. This helps to reduce the worst case time.
Table 2 - Recommended RPI Values for System by Using NUT of 5 ms
NUT=5 ms x2 0 x2 1 x2 2 x2 3 x2 4 x2 5 x2 6 x2 7
Optimal RPI
Values (ms) 5 ms 10 ms 20 ms 40 ms 80 ms 160 ms 320 ms 640 ms