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Message Request Register with the Modicon PLC and the OIT Series.
Tags: Networking
The Maple Systems Operator Interface Terminal (OIT) allows a Modicon PLC to call up a message that is stored in the OIT by using a “Message Request Register”.

A 4XXXX Holding Register can be used by the PLC to tell the OIT to display one of its messages. For example, register 40500 is the default Message Request Register set in the configuration menu during the OIT setup. When the OIT is attached to the PLC and operating, it will continuously monitor holding register 40500 for a number.

If the PLC wants to display Message #1 of the OIT then it would simply put the number 1 into holding register 40500. The OIT would find the number 1 in register 40500, (the Message Request Register - also known as the mailbox) and quickly display message #1.
Content Created by David Franzwa