2711PC-T6C20D8 A-B  communication module

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.

This second use is based on the supposition that if a slave station didn’t respond the last time it was polled (which was just a few seconds ago, if that long), then chances are it won’t be able to receive and respond to a MSG instruction now, and so it would most likely just end up going through the maximum number of retries and timeouts before completing in error (which slows down both the poll scan and any other messaging going on). Using this technique, the minimum time to message to every responding slave station actually decreases as the number of slave stations that can’t respond increases.

Polled report-by-exception lets a slave station initiate data transfer to its master station, freeing the master station from having to constantly read blocks of data from each slave station to determine if any slave input or data changes have occurred. Instead, through user programming, the slave station monitors its own inputs for a change of state or data, which triggers a block of data to be written to the master station when the master station polls the slave.

If your SCADA application is time-critical and any two or more of the following apply, then you can benefit from polled report-by-exception messaging: • communication channel is slow (2400 bps or less) • average number of words of data to monitor in each slave station is greater than five • number of slave stations is greater than ten

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