Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 26)

Publication AG-UM008C-EN-P - February 2005
1-8 Designing Communication
Communication Scheme
Design Using
Standard-Mode Standard-communication mode for an Allen-Bradley master station
uses centralized polling to gather data from slave stations. A master
station using this communication technique asks (polls) individual
slave stations if they have any information to send. All stations on the
link ‘hear’ the master station’s requests, but only the slave station to
which a request is addressed replies. PLC-5, Logix and RSLinx master
stations poll slave stations based on an ordered list (polling list)
configured by the system designer. SLC 500 and MicroLogix master
stations poll slave stations sequentially in a range of addresses
configured by the system designer. Figure 1.1 shows how a slave
station gets polled and how it responds.
A master station polls the slave stations in the order the slave stations
appear on the list. Slave stations send either a data packet or a packet
indicating that the station has no data to send.
Figure 1.1 Slave Station Polling and Response
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Polling List
Polling List
Master
Station
Master
Station
slave
station 1 slave
station 2 slave
station 3
slave
station 1 slave
station 2 slave
station 3
Modem
Stn 1
Stn 2
Stn 3
1. Master station polls a slave station for data.
2. If the slave station has data to send, then it sends a
data packet. If there is no data to send then it sends
an end of transmission packet (DLE EOT).
1. Master station polls the next slave station for data.
2. If the slave station has data to send, then it sends a
data packet. If there is no data to send then it sends
an end of transmission packet (DLE EOT).
3. Master station continues to poll each slave station
in the polling list. When the end of the list is
reached, the master station then moves back to the
beginning of the list and starts the polling sequence
over again.
Stn 1
Stn 2
Stn 3
Poll to slave
Poll to slave
Modem Modem Modem
Modem Modem Modem
Modem
Return Data
Packet or DLE
EOT to Master
Return Data
Packet or DLE
EOT to Master