Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 23)

Publication AG-UM008C-EN-P - February 2005
Designing Communication 1-5
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
About Slave-to-Slave Messaging
Most SCADA half-duplex protocols do not allow one slave station to
talk to another slave station, except through special
application-specific code, which requires processing overhead in the
master station. However, Allen-Bradley’s DF1 half-duplex protocol
implements slave-to-slave communications as a feature of the protocol
within the master station, without any additional application code or
extra processing overhead. Refer to chapter 3 of the DF1 Protocol and
Command Set Reference Manual, publication 1770-RM516, for
additional information.
If one slave station has a message to send to another, it simply
includes the destination slave station’s address in the message
instruction’s destination field in place of the master station’s address
when responding to a poll. The master station checks the destination
station address in every packet header it receives from any slave
station. If the address does not match its own station address, the
entire message is forwarded back onto the telemetry network to the
appropriate slave station, without any further processing.
IMPORTANT
Slave stations using 1747-KE interfaces can respond
to slave-to-slave messages but cannot initiate
slave-to-slave messages.