Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 20)

Publication AG-UM008C-EN-P - February 2005
1-2 Designing Communication
Choosing a Polling Mode
for DF1 Half-Duplex Master A master station can be configured to communicate with slave stations
in either Message-based polling mode or Standard polling mode. The
pros and cons of each polling mode are described below.
Message-Based Polling Mode
Message-based polling mode is best used in networks when
communication with the slave stations is not time critical and where
the user needs to be able to limit when and how often the master
station communicates with each slave station. It is NOT recommended
for systems that require time continuous communication between the
master and all the slave stations have MSG instructions in their
programs.
With Message-Based polling mode, the only time a master station
communicates with a slave station is when a message (MSG)
instruction in ladder logic is triggered to that particular slave station’s
address. This polling mode gives the user complete control (through
ladder logic) over when and how often to communicate with each
slave station.
If multiple MSG instructions are triggered simultaneously, they will be
executed in order, one at a time, to completion (i.e., the first MSG
queued up will be transmitted and completed to done or error before
the next queued up MSG is transmitted. Refer to appendix E for
sample application programs). Any time a message is triggered to a
slave station that can’t respond (for instance, if its modem fails), the
message will go through retries and timeouts that will slow down the
execution of all the other queued up messages. The minimum time to
message to every responding slave station increases linearly with the
number of slave stations that can’t respond.
If the Message-based selection is ‘don’t allow slaves to initiate
messages,’ then even if a slave station triggers and queues up a MSG
instruction in its ladder logic, the master station will not process it.
This mode is similar to how a master/slave network based on Modbus
protocol would work, since Modbus slave stations cannot ever initiate
a message.
If the Message-based selection is ‘allow slaves to initiate messages,’
when a slave station initiates a message to the master station (polled
report by exception messaging) or to another slave station
(slave-to-slave messaging), the MSG command packet will remain in
that slave station’s transmit queue until the master station triggers its
own MSG command packet to it (which could be seconds, minutes or
hours later, depending on the master’s ladder logic).