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MG.10.P2.22
Modbus
RTU
Overview
The common language used by all Modicon
controllers is the Modbus RTU protocol. This
protocol defines a message structure that
controllers will recognize and use, regardless
of the type of networks over which they
communicate. It describes the process a
controller uses to request access to another
device, how it will respond to requests from
the other devices, and how errors will be
detected and reported. It establishes a
common format for the layout and contents
of message fields.
During communications on a Modbus RTU
network, the protocol determines how each
controller will know its device address,
recognize a message addressed to it,
determine the kind of action to be taken,
and extract any data or other information
contained in the message. If a reply is
required, the controller will construct the
reply message and send it.
Controllers communicate using a master-
slave technique in which only one device
(the master) can initiate transactions (called
‘queries’). The other devices (slaves) respond
by supplying the requested data to the
master, or by taking the action requested in
the query.
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 RTU 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 in
response.
