Modbus Rtu; Modbus Rtu Overview - Danfoss VLT Micro Drive FC 51 Series Operating Instructions Manual

®
VLT Micro Drive FC 51 Operating Instructions

5 Modbus RTU

5.1 Modbus RTU Overview

5.1.1 Assumptions
These operating instructions assume that the installed controller supports the interfaces in this document and that all the requirements stipulated in the
controller, as well as the frequency converter, are strictly observed, along with all limitations therein.
5.1.2 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.
5.1.3 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 includes i.a. how it will respond to requests from another device, and how errors will be 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 will learn its device address, recognise a message
addressed to it, determine the kind of action to be taken, and extract any data or other information contained in the message. If a reply is required, the
controller will construct the reply message and send 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
will construct an error message and send it in response, or a time-out will occur.
5.1.4 Frequency Converter with Modbus RTU
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:
• 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
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.
®
MG.02.A4.02 - VLT is a registered Danfoss trademark
5 Modbus RTU
5
25