Frequency Converter With Modbus Rtu; Network Configuration; Modbus Rtu Message Framing Structure; Introduction - Danfoss VLT Micro Drive FC 51 Design Manual

RS485 Installation and Set-...
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.
8 8
The Modbus RTU protocol establishes the format for the
master 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 response telegram of the slave device 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
telegram, or if the slave is unable to perform the requested
action, the slave constructs and sends an error message.
Alternatively, a timeout occurs.
8.6.4 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:
-
-
-
-
•
Reset after a fault trip.
72
®
VLT
Coast stop.
Quick stop.
DC brake stop.
Normal (ramp) stop.
Danfoss A/S © 01/2016 All rights reserved.
Micro Drive FC 51
•
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 offers a range of control options, including
controlling the setpoint of the frequency converter when
its internal PI controller is used.

8.7 Network Configuration

To enable Modbus RTU on the frequency converter, set the
following parameters:
Parameter
Parameter 8-30 Protocol
Parameter 8-31 Address
Parameter 8-32 Baud Rate
Parameter 8-33 Parity / Stop Bits
Table 8.11 Network Configuration
8.8 Modbus RTU Message Framing
Structure

8.8.1 Introduction

The controllers are set up to communicate on the Modbus
network using RTU (remote terminal unit) mode, with each
byte in a telegram containing 2 4-bit hexadecimal
characters. The format for each byte is shown in Table 8.12.
Start
bit
Table 8.12 Format for Each Byte
Coding system
Bits per byte
Error check field
Table 8.13 Byte Details
Setting
Modbus RTU
1–247
2400–115200
Even parity, 1 stop bit
(default)
Data byte Stop/
parity
8-bit binary, hexadecimal 0–9, A–F.
2 hexadecimal characters contained in each
8-bit field of the telegram.
•
1 start bit.
•
8 data bits, least significant bit sent first.
•
1 bit for even/odd parity; no bit for no
parity.
•
1 stop bit if parity is used; 2 bits if no
parity.
Cyclic redundancy check (CRC).
Stop
MG02K402