Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 22)

Publication AG-UM008C-EN-P - February 2005
1-4 Designing Communication
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.
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.
About Polled Report-by-Exception
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.
IMPORTANT
In order to remotely monitor and program the slave
stations over the half-duplex network while the
master station is configured for Standard polling
mode, the programming computer DF1 slave driver
(Rockwell Software RSLinx) station address must be
included in the master station poll list.