Drive With Modbus Rtu - Danfoss iC2 Application Manual

Micro frequency converters

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Application Guide | iC2-Micro Frequency Converters

Danfoss assumes that the installed controller supports the interfaces in this manual, and strictly observes all requirements and

limitations stipulated in the controller and drive. The built-in Modbus RTU (remote terminal unit) is designed to communicate with

any controller that supports the interfaces defined in this guide. It is assumed that the user has full knowledge of the capabilities and

limitations of the controller.

Modbus RTU overview

Regardless of the type of physical communication networks, this section describes the process that 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 telegram fields.

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/follower

technique in which only the master can initiate transactions (called queries). Followers respond by supplying the requested data to

the master, or by acting as requested in the query. The master can address individual followers, or initiate a broadcast telegram 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 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 follower 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 follower is unable to perform

the requested action, the follower constructs and sends an error message. Alternatively, a timeout occurs.