Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 33)

Publication AG-UM008C-EN-P - February 2005
Designing Communication 1-15
Communication Scheme
Design Using
Message-Based Mode In message-based communication mode, the master station sends
solicited messages (messages programmed via ladder logic) to a
specific slave station when the master requires information. In this
mode, the communication link is inactive until the master station has a
message to send to a slave station. Figure 1.7 explains the
communication sequence that occurs.
Figure 1.7
Message-Based Communication
41186
Master
Station
Modem
Slave
Station 1 Slave
Station 2
Modem Modem 1. Message (via MSG instruction) sent
to a specific slave station
(eg., slave station 1).
2. Slave station receives message and
sends an acknowledgment back (ACK).
3. Master station waits a user-defined time ‘Reply
Message Wait’ parameter before polling the
station for a reply.
4. Slave station forms a reply message to the
master station’s enquiry.
5. Master station polls slave station for its reply.
6. Slave station sends its reply message.
7. Master station receives reply and sends an
acknowledgement back (ACK).