Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 154)

Publication 1785-UM012D-EN-P - July 2005
10-4 Communicating with Devices on a Serial Link
Master Station to Remote Station Communication Methods
A PLC-5 master station can communicate with remote stations in two ways:
Method Option Name Principal Benefits
initiating polling packets to remote stations
according to their position on a polling list
Polling packets are formed independently
of any user-programming.
standard
communication mode This is the communication mode used most
often in point-to-multipoint configurations.
Provides for these capabilities:
remote stations can send messages to the
master station (polled report-by-exception)
remote stations can send messages
to each other
lets the master station maintain an active
node table
The poll list resides in a user designated and
accessible integer-type data file. You can:
include the master on the poll list
configure the master for between-station polls
(master transmits any message that it needs to
send before polling the next remote station)
have the master both in the poll list and
configured for between-station polls
initiating communication to remote stations
using only user-programmed message
(MSG) instructions
Each request for data from a remote station
must be programmed via a message
instruction.
The master polls the remote station for a
reply to the message after waiting a
user-configured period of time. The waiting
period gives the remote 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 remote-to-remote queue is
checked for messages to send.
message-based
communication mode If your application uses satellite transmission or
public switched telephone network
transmission, consider choosing
message-based. Communication to a remote
station can be initiated on an as needed basis.
Or choose this method if you need to
communicate with non-intelligent remote
terminal units (RTUs).