Unsolicited message handling

In the world of PLCs and Communication Drivers, it is obvious that a PLC will know when a critical event has occurred before the Communication Driver will have a chance to poll for that data. Therefore, it would seem natural that if a critical event occurs, the PLC should have the capability to inform the Communication Driver immediately, without having to wait for the Communication Driver to poll it.

This is the role of an unsolicited message. Once a PLC has determined that a critical condition exists, it can generate a message immediately sent to the Communication Driver without a prior request from the Communication Driver. The unsolicited message implementation requires both the messaging instructions properly programmed in the PLC logic and the device group appropriately configured in the Communication Driver.

The Allen-Bradley processors, specifically the 1785 PLC-5 and PLC-5/250 (EI), are capable of producing unsolicited messages that the Communication Drivers can understand.

There are two types of unsolicited messages supported by the ABTCP Communication Driver:

• Peer-to-Peer: This method involves IP-address-to-IP-address communications.

  • Configure which computer (by its IP address) receives unsolicited messages in your client application.

  • Two instances of peer-to-peer unsolicited messages are generated by the Communication Driver:

  • If the value of "Update Interval" for a topic is 0 (zero), the server will poll this topic only once at the start. After that, only an unsolicited message will update the data.

  • If the value of "Update Interval" for a topic is >0 (zero), the server will update the data for a particular item immediately upon receiving an unsolicited message for the item. The Communication Driver will also update the data at every Update Interval.

• Client Messaging (also called general broadcast): This method involves a general broadcast of the unsolicited message onto the Ethernet network.

  • Only the first node on the network that is capable of receiving the message gets it.

  • If this option is not enabled, the Communication Driver will ignore all client messages sent out from the PLC.

  • If this option is enabled, the Communication Driver will attempt to respond to the PLC when it gets the first client message from the PLC. Note that if the response is accepted by the PLC, the PLC will send unsolicited messages exclusively to the Communication Driver thereafterwards.

  • If the value of "Update Interval" for a topic is 0 (zero), the server will poll this topic only once at the start. After that, only an unsolicited message will update the data.

  • If the value of "Update Interval" for a topic is >0 (zero), the server will update the data for a particular item immediately upon receiving an unsolicited message for the item. The Communication Driver will also update the data at every Update Interval.

    Note: For a more specific Allen Bradley definition of peer-to-peer and client-messaging unsolicited messaging, please refer to the Ethernet AB PLC-5 Family documentation.

Receive unsolicited messages

• Set a device group’s "Update Interval" to 0.

Access the settings for device groups

1. Click on the PLC’s name in the Configuration hierarchy of your Communication Driver.

2. Select the Device Groups tab of the configuration view pane at right.

3. Double-click the number in the Update Interval column of the desired device group and type the number 0 (zero).

Save the configuration change by clicking the Save icon Located at the upper-right corner of the configuration view pane.