440F-M1224BYNN A-B  programmable controller module

When a MSG instruction is triggered while the master station is in run mode, the master station will transmit the message packet just after it finishes polling the current slave station in the poll list and before it starts polling the next slave station in the poll list (no matter where in the poll list it is currently at). If multiple MSG instructions have been triggered simultaneously, at least four message packets may be sent out between two slave station polls. Each of these messages will have an opportunity to complete when the master polls the slave station that was addressed in the message packet as it comes to it in the poll list. If each of the transmitted message packets is addressed to a different slave station, the order of completion will be based upon which slave station address comes up next in the poll list, not the order that the MSG instructions were executed and transmitted in. When a slave station receives a poll packet from the master station, if it has one or more message packets queued up to transmit (either replies to a command received earlier or MSG commands triggered locally in ladder logic), the slave station will transmit the first message packet in the transmit queue. If the standard mode selection is ‘single message per poll scan,’ then the master station will then go to the next station in the poll list. If the standard mode selection is ‘multiple messages per poll scan,’ the master station will continue to poll this slave station until its transmit queue is empty. The master station knows the slave station has no message packets queued up to transmit when the slave station responds to the master poll packet with a 2-byte poll response. Every time a slave station responds or doesn’t respond to its poll packet, the master station automatically updates its active node list (again, even if it’s in program mode). In this list, one bit is assigned to each possible slave station address (0 to 254). If a slave station doesn’t respond when it is polled, its active node list bit is cleared. If it does respond when it is polled, its active node bit is set. Besides being an excellent online troubleshooting tool, two common uses of the active node list are to report good/bad communication status for all slave stations to an operator interface connected to the master station for monitoring, alarming and logging purposes, and to precondition MSG instructions to each particular slave.

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