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3.1.3 Client / server model
Modbus Client
Modbus Server
Request Indication
Response
Confirmation
Figure 2: Client / server model
The Modbus messaging service implements client/server communica-
tion between devices connected by means of a TCP/IP network. Mod-
bus TCP does not use an object dictionary.
The client/server model is based on 4 types of messages:
•
Request: Message sent by the client to initiate a transaction.
•
Indication: Request as received by the server.
•
Response: Response message to the request sent by the server.
•
Confirmation: Response as received by the client.
A communication cycle consists of the request from the client (request
from the fieldbus master) and a response from the server (response
from the fieldbus slave). Modbus request and Modbus response have
the same structure. If an error occurs on receipt of the Modbus
request or if the slave cannot execute the action, the slave sends an
error message in the Modbus response.
The product analyzes the Modus requests received. Depending on the
Modbus request, the product triggers actions or provides requested
data.
3.1.4 Network service SNMP
The Internet community has developed the SNMP standard "Simple
Network Management Protocol" to support the management of differ-
ent network devices by means of a single system.
The Network Management System can exchange data with SNMP
devices. The tasks of the network management system comprise
monitoring, control and configuration of network components as well
as error detection and error messaging.
SNMP agent ConneXview The product supports SNMP version 1.0. An SNMP agent must be
used to monitor a network with SNMP. Schneider Electric offers the
tool ConneXview for such purposes.
3 Basics
LXM32M
16 Modbus-TCP module
0198441113843, V1.01, 01.2012