Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 21)

Publication AG-UM008C-EN-P - February 2005
Designing Communication 1-3
Standard Polling Mode
Standard polling mode is strongly recommended for larger systems
that require time critical communication between the master and all
the slave stations, or for any system where slave station-initiated
messages are going to be used (this includes slave programming over
the network, since this uses the same mechanism that slave-to-slave
messaging uses). The Active Node Table automatically keeps track of
which slaves are (and are not) communicating. Standard polling mode
should NOT be used in cases where the user needs to be able to limit
when and how often the master station communicates with each slave
station.
Standard polling mode causes the master station to continuously send
one or more 4-byte poll packets to each slave station address
configured by the user in the poll list(s) in round robin fashion. As
soon as the end of the polling list is reached, the master station
immediately goes back and starts polling slave stations from the top of
the polling list over again. This is independent and asynchronous to
any MSG instructions that might be triggered in the master station
ladder logic. In fact, this polling continues even while the master
station is in program mode. Refer to chapter 3 of the DF1 Protocol and
Command Set Reference Manual, publication 1770-RM516, for
additional information.
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