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5 Modbus protocol overview
5.1 Overview
MODBUS RTU is a non-proprietary serial communications protocol that is widely used in
the process control industry. The protocol was developed by Modicon for PLC
communications and later released for public use.
This protocol is available in all major Human Machine Interface (HMI) software packages
and terminals. Many of the major controller and PLC manufacturers also offer MODBUS
protocol as a standard or optional protocol in their instrumentation.
The hardware over which MODBUS RTU communications are performed is not defined
by the protocol. MODBUS RTU is supported on RS-232, RS-422, RS-485, Ethernet and
other electrical standards. It should be noted that MODBUS RTU, MODBUS ASCII and
MODBUS Plus are unique communication formats, and are not compatible with each
other. This document will discuss MODBUS RTU only.
5.1.1
Transactions on Modbus Networks
Modbus protocol uses a master–slave technique, in which only one device (the master)
can initiate transactions (called 'queries'). The other devices (the slaves) respond by
supplying the requested data to the master, or by taking the action requested in the
query. Typical master devices include host processors and programming panels. Typical
slaves include programmable controllers.
The master can address individual slaves, or can initiate a broadcast message to all
slaves.
Slaves return a message (called a 'response') to queries that are addressed to them
individually. Responses are not returned to broadcast queries from the master.
The Modbus protocol establishes the format for the master's query by placing into it the
device (or broadcast) address, a function code defining the requested action, any data
to be sent, and an error–checking field. The slave's response message is also constructed
using Modbus protocol. It contains fields confirming the action taken, any data to be
returned, and an error–checking field. If an error occurred in receipt of the message, or
if the slave is unable to perform the requested action, the slave will construct an error
message and send it as its response.
5.1.2
Serial Transmission Mode
The transmission mode defines the bit contents of message fields transmitted serially
on the networks. It determines how information will be packed into the message fields
and decoded.
Modbus defines two transmission modes: ASCII or RTU.
Only RTU mode will be used here. The mode and serial parameters must be the same for
all devices on a Modbus network.
RS485 ç Modbus protocol overview 13
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