What The User Should Already Know; Modbus Rtu Overview; Frequency Converter With Modbus Rtu; Network Configuration - Danfoss VLT HVAC Basic Drive FC 101 Design Manual

RS-485 Installation and Set...

7.6.2 What the User Should Already Know

The Modbus RTU (Remote Terminal Unit) is designed to
communicate with any controller that supports the
interfaces defined in this document. It is assumed that the
user has full knowledge of the capabilities and limitations
of the controller.

7.6.3 Modbus RTU Overview

Regardless of the type of physical communication
networks, the Modbus RTU Overview 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
•
Recognizes 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 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. No responses are 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 constructs an error message, and send it
in response, or a time-out occurs.
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VLT HVAC Basic Drive Design Guide
7.6.4 Frequency Converter with Modbus
The frequency converter communicates in Modbus RTU
format over the built-in RS-485 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.

7.7 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 7.11
7.8 Modbus RTU Message Framing
7.8.1 Frequency Converter with Modbus
The controllers are set up to communicate on the Modbus
network using RTU (Remote Terminal Unit) mode, with
each byte in a message containing 2 4-bit hexadecimal
characters. The format for each byte is shown in Table 7.12.
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MG18C402 - VLT is a registered Danfoss trademark
RTU
•
Start
•
Stop of the frequency converter in various 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 set-up
•
Control the frequency converter's built-in relay
Setting
Modbus RTU
1-247
2400-115200
Even parity, 1 stop bit (default)
Structure
RTU
7 7
83