Manual

27
Modbus and Ethernet Modbus TCP/IP protocols
English
W916556680111
General
Modbus protocol
The splitter box and the compact stations constitute a Modbus network operating
on the client/server principle.
The splitter box is the client, and the compact stations are the servers.
MODBUS Addressing rules
Each server has its own address that enables it to know that the client is
addressing it.
The MODBUS addressing space comprises 256 different addresses:
NB: With the Modbus TCP/IP Ethernet protocol, the 255 address is the
address of the Modbus client.
Modes of Modbus addressing rules
Modbus presents 2 modes of addressing rules:
The Unicast mode:
In that mode, the client addresses an individual server. After receiving and
processing of the request, the server returns a reply message to the client .
A Modbus transaction is constituted of 2 messages: the client request and the
server reply.
Each server must have a unique address (from 1 to 247). For the Ositrack
compact station, this address is comprised between 1 and 15.
On the Modbus network, each server must have a different address.
For further details concerning the address setting of a station, refer to the user
manual of the compact station (DIA4ED3051001).
The broadcast mode:
In broadcast mode, the client sends a request to all servers, but contrary to
the individual mode, the latters will not return a reply. The broadcast requests
can be only writing commands. The 0 address is reserved to this use.
Warning: As the servers do not return any answer; the only way of being sure
that they all took into account the command is to send a reading of checking
server by server.
0 1...247 248...255
Broadcast address server individual addresses Reserved
Modbus and Ethernet Modbus TCP/IP protocols