Message Frequency

Introduction

The term message frequency refers to how often a device transmits a particular type of message. Message frequency directly affects control network load and performance, as well as the CPU capacity of every network device that processes these messages.

Depending on your application requirements, real-time I/O data can be transmitted using implicit messaging as follows:

on a cyclic basis, at the request packet interval (RPI) rate, or
upon the occurrence of a change of state event

Cyclic Real-Time I/O Messaging

Much of the load on an Ethernet control network consists of cyclic real-time I/O data. Consequently, carefully consider how to set the RPI value for transmitting these messages:

A small RPI value results in more frequent, and more numerous, message transmissions. This increases network load, and may waste network resources and degrade system performance.
Conversely, a larger RPI value—for example, one that is equal (or nearly equal) to the frequency of your application’s need for new data—can result in your application not receiving the most current data. Also, if a connection is lost, the time to re-establish the connection will be relatively long, because the connection timeout is proportional to the RPI.

Schneider Electric recommends setting RPI to 50% of the actual frequency by which your application requires data for cyclic real-time I/O messaging.

NOTE: The I/O scanner can simultaneously communicate with different I/O adapters at different RPI rates. This enhances the ability of the PLC to control and monitor different devices with varying processing capacities.

Change of State I/O Messaging

For change of state triggered real-time I/O data messages:

output transmissions occur at the rate of the PLC controller application cycle time
input transmissions occur whenever an input event is detected by an input device

Consequently, for an I/O device with a rapid response and transmission time, using a direct connection to the I/O device may be more efficient than using a rack optimized connection. In this design, because only the single device input data is sent, the size of the frequently transmitted message is potentially much smaller than would be the case if the message contained data from every I/O device on the remote island.

NOTE: A change of state (versus cyclic) triggered real-time I/O message usually reduces network load. Configure the change of state message with a longer connection timeout value.

RSTP and IGMP Messaging

RSTP and IGMP messages usually consume a very small amount of network bandwidth. Set up the IGMP query period based on your application requirements.

Scheduling Certain Explicit Messages

Depending on your application requirements, you can also configure certain explicit messages to be transmitted either cyclically or upon the occurrence of a change of state event. For example, you can periodically monitor a device using SNMP query, Web pages, EtherNet/IP, and Modbus TCP. The cyclic period should be configured so that the total load consumed by explicit messaging does not exceed 10% of network capacity.