5
Modbus RTU
Overview
The common language used by all Modicon
controllers is the Modbus RTU (Remote
Terminal Unit) 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 occurs 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, or a timeout will occur.
TR1 Series
VFD with
Modbus RTU
Option
Overview
The TR1 Series variable frequency drive
communicates in Modbus RTU format over
an EIA-485 (formerly RS-485) network.
Modbus RTU allows access to the drive
Control Word and Bus Reference.
The Control Word allows Modbus to control
several important functions of the drive:
• Start
• Stop (in several ways):
Coast stop
Quick stop
DC Brake stop
Normal (ramp) stop
• Reset after a fault trip
• Run at a variety of preset speeds
• Run in reverse
• Change the active setup
• Control the drive's two built-in relays
The Bus Reference is commonly used for
speed control.
It is also possible to access the drive
parameters, read their values, and, where
possible, write values to them. This permits
a range of control possibilities, including
controlling the drive setpoint when its internal
PID controller is used.
Summary of Contents for Mobrus RTU TR1 Series
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