Publication 1763-RM001C-EN-P - October 2009
508
Protocol Configuration
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!
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 it currently
is in the poll list). 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 in which the MSG
instructions were executed and transmitted.
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 fails to respond to its poll packet,
the master station “automatically” updates its Active Node Table (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 does not
respond when it is polled, its Active Node Table bit is cleared. If it does
respond when it is polled, its Active Node Table bit is set. Besides being
an excellent online troubleshooting tool, two common uses of the Active
Node Table are to report good/bad communication status for all slave
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