Modbus Rtu; Prerequisite Knowledge; Overview; Frequency Converter With Modbus Rtu - Danfoss VLT AutomationDrive FC 360 Design Manual

RS485 Installation and Set-...

5.6 Modbus RTU

5.6.1 Prerequisite Knowledge

Danfoss assumes that the installed controller supports the
interfaces in this document, and strictly observes all
requirements and limitations stipulated in the controller
and frequency converter.
The built-in Modbus RTU (Remote Terminal Unit) is
designed to communicate with any controller that
supports the interfaces defined in this document. It is
5 5
assumed that the user has full knowledge of the
capabilities and limitations of the controller.

5.6.2 Overview

Regardless of the type of physical communication
networks, this section describes the process a controller
uses to request access to another device. This process
includes how the Modbus RTU responds to requests from
another device, and how errors are detected and reported.
It also establishes a common format for the layout and
contents of message fields.
During communications over a Modbus RTU network, the
protocol:
•
Determines how each controller learns its device
address.
•
Recognises a message addressed to it.
•
Determines which actions to take.
•
Extracts any data or other information contained
in the message.
If a reply is required, the controller constructs the reply
message and sends it.
Controllers communicate using a master-slave technique in
which only the master can initiate transactions (called
queries). 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 initiate a
broadcast message to all slaves. Slaves return a response
to queries that are addressed to them individually. No
responses are returned to broadcast queries from the
master. The Modbus RTU protocol establishes the format
for the master's query by providing the following
information:
•
The device (or broadcast) address.
•
A function code defining the requested action.
•
Any data to be sent.
•
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.
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Design Guide
If an error occurs in receipt of the message, or if the slave
is unable to perform the requested action, the slave
constructs an error message, and send it in response, or a
time-out occurs.
5.6.3 Frequency Converter with Modbus
The frequency converter communicates in Modbus RTU
format over the built-in RS485 interface. Modbus RTU
provides access to the control word and bus reference of
the frequency converter.
The control word allows the modbus master to control
several important functions of the frequency converter:
•
•
•
•
•
•
•
The bus reference is commonly used for speed control. It is
also possible to access the parameters, read their values,
and, where possible, write values to them. This permits a
range of control options, including controlling the setpoint
of the frequency converter when its internal PI controller is
used.

5.7 Network Configuration

To enable Modbus RTU on the frequency converter, set the
following parameters:
Parameter
8-30 Protocol
8-31 Address
8-32 Baud Rate
8-33 Parity / Stop Bits
Table 5.11 Network Configuration
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RTU
Start.
Various stops:
- 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 set-up.
Control the frequency converter's built-in relay.
Setting
Modbus RTU
1–247
2400–115200
Even parity, 1 stop bit (default)
MG06B402