Modbus Rtu Overview - Danfoss VLT AQUA Drive Design Manual

VLT ® AQUA Drive Design Guide

7.6. Modbus RTU Overview

7.6.1. Assumptions
This instruction manual assumes that the installed controller supports the interfaces in this docu-
ment and that all the requirements stipulated in the controller, as well as the adjustable frequency
drive, are strictly observed, along with all limitations therein.
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 includes, among other
things, 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.
While communicating over a Modbus RTU network, the protocol determines how each controller
will learn its device address, recognize 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 taking the action requested in the query.
The master can address individual slaves or initiate a broadcast message to all slaves. Slaves
return a message (called a response) to queries that are addressed to them individually. No re-
sponses 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 timeout will occur.
MG.20.N2.22 - VLT ® is a registered Danfoss trademark.
7. RS-485 Installation and Set-up
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