Rockwell Automation 5720 DriveLogix System - Manual (Page 204)

Publication 20D-UM002C-EN-P - November 2003
9-10 Communicating with Devices on a Serial Link
the serial port of the controller for either the DF1 master or DF1 slave
protocol (both half-duplex).
Master/slave communication methods
A master station can communicate with a slave station in two ways:
Name: This method: Benefits:
standard
communication
mode Initiates polling packets to slave stations
according to their position in the polling
array(s).
Polling packets are formed based on the
contents of the normal poll array and the
priority poll array.
This communication method is most often used for
point-to-multipoint configurations.
This method provides these capabilities:
• slave stations can send messages to the master station
(polled report-by-exception)
• slave stations can send messages to each other via the
master
• master maintains an active station array
The poll array resides in a user-designated data file. You can
configure the master:
• to send messages during its turn in the poll array
or
• for between-station polls (master transmits any message
that it needs to send before polling the next slave station)
In either case, configure the master to receive multiple messages
or a single message per scan from each slave station.
message-based
communication
mode initiates communication to slave stations
using only user-programmed message (MSG)
instructions.
Each request for data from a slave station
must be programmed via a MSG instruction.
The master polls the slave station for a reply
to the message after waiting a
user-configured period of time. The waiting
period gives the slave station time to
formulate a reply and prepare the reply for
transmission. After all of the messages in the
master’s message-out queue are transmitted,
the slave-to-slave queue is checked for
messages to send.
If your application uses satellite transmission or public
switched-telephone-network transmission, consider choosing
message-based communication. Communication to a slave
station can be initiated on an as-needed basis.
Also choose this method if you need to communicate with
non-intelligent remote terminal units (RTUs).