Danfoss VLT Midi Drive FC 280 Design Guide

5.6 Modbus RTU
5.6.1 Prerequisite Knowledge
Danfoss assumes that the installed controller supports the
interfaces in this manual, 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 dened in this manual. It is
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 telegram elds.
During communications over a Modbus RTU network, the
protocol:
•
Determines how each controller learns its device
address.
•
Recognizes a telegram addressed to it.
•
Determines which actions to take.
•
Extracts any data or other information contained
in the telegram.
If a reply is required, the controller constructs the reply
telegram 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 acting as requested in the query.
The master can address individual slaves, or initiate a
broadcast telegram 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 query by providing the following information:
•
The device (or broadcast) address.
•
A function code dening the requested action.
•
Any data to be sent.
•
An error-checking eld.
The response telegram of the slave device is also
constructed using Modbus protocol. It contains elds
conrming the action taken, any data to be returned, and
an error-checking eld. If an error occurs in receipt of the
telegram, or if the slave is unable to perform the requested
action, the slave constructs and sends an error message.
Alternatively, a timeout occurs.
5.6.3 Frequency Converter with Modbus
RTU
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:
•
Start.
•
Various stops:
- Coast stop.
- Quick stop.
- DC brake stop.
- Normal (ramp) stop.
•
Reset after a fault trip.
•
Run at various preset speeds.
•
Run in reverse.
•
Change the active set-up.
•
Control built-in relay 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. Accessing the
parameters
oers a range of control options, including
controlling the setpoint of the frequency converter when
its internal PI controller is used.
5.7
Network Conguration
To enable Modbus RTU on the frequency converter, set the
following parameters:
Parameter Setting
Parameter 8-30 Protocol Modbus RTU
Parameter 8-31 Address 1–247
Parameter 8-32 Baud Rate 2400–115200
Parameter 8-33 Parity / Stop Bits Even parity, 1 stop bit
(default)
Table 5.11 Network Conguration
RS485 Installation and Set-...
VLT
®
Midi Drive FC 280
60 Danfoss A/S © 03/2016 All rights reserved. MG07B102
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Danfoss VLT Midi Drive FC 280 Specifications

General

Brand	Danfoss
Model	VLT Midi Drive FC 280
Category	DC Drives
Language	English