3-1
ENGLISH
Description
MODBUS protocol manual
3. DESCRIPTION
MODBUS ASCII/RTU is a master-slave communication protocol, able to support up to 247 slaves
connected in a bus or a star network.
The protocol uses a simplex connection on a single line. In this way, the communication messages
move on a single line in two opposite directions.
MODBUS TCP is a variant of the MODBUS family. Specifically, it covers the use of MODBUS messaging
in an “Intranet” or “Internet” environment using the TCP/IP protocol on a fixed port 502.
Master-slave messages can be:
Reading ($03)• : the communication is between the master and a single slave. It allows to read
information about the queried instrument
Writing ($10)• : the communication is between the master and a single slave. It allows to
change the instrument settings
Broadcast (not available for MODBUS TCP)• : the communication is between the master and all
the connected slaves. It is always a write command ($10) and required logical number $00
In a multi-point type connection (MODBUS ASCII/RTU), slave address (called also logical number)
allows to identify each instrument during the communication. Each instrument is preset with a
default slave address (01) and the user can change it.
In case of MODBUS TCP, slave address is replaced by a single byte, the Unit identifier.
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