Danfoss VLT AutomationDrive FC 301 Series Design Manual page 198

RS-485 Installation and Set...
12.9 Modbus RTU Overview
12.9.1 Assumptions
Danfoss assumes that the installed controller supports the
interfaces in this document, and strictly observes all
requirements and limitations stipulated in the controller
and adjustable frequency drive.
12.9.2 What the User Should Already Know
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
assumed that the user has full knowledge of the
capabilities and limitations of the controller.
12.9.3 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
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•
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-follower
technique in which only the master can initiate
transactions (called queries). Followers respond by
supplying the requested data to the master, or by taking
the action requested in the query.
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Design Guide
The master can address individual followers, or initiate a
broadcast message to all followers. Followers 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 device (or broadcast) address, a function code defining
the requested action, any data to be sent, and an error-
checking field. The follower'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 follower is unable to perform the
requested action, the follower constructs an error message,
and send it in response, or a timeout occurs.
12.9.4 Adjustable Frequency Drive with
The adjustable frequency drive 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 adjustable frequency drive.
The control word allows the Modbus master to control
several important functions of the adjustable frequency
drive:
•
•
•
•
•
•
•
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 adjustable frequency drive when its internal PI
controller is used.
Rev. 2014-04-04 All rights reserved.
Modbus RTU
Start
Stop of the adjustable frequency drive 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 adjustable frequency drive's built-in
relay
MG33BF22