Modbus Rtu Overview; Assumptions; What The User Should Already Know - Danfoss VLT DriveMotor FCP 106 Programming Manual

RS-485 Installation and Set...
1155 H 0000 H 0000
PKE IND
Illustration 3.13 Telegram
If the value in parameter 3-41 Ramp 1 Ramp Up Time is 10
s, the response from the follower to the master is:
1155 H 0000 H 0000
PKE
IND
Illustration 3.14 Response
3E8 hex corresponds to 1000 decimal. The conversion
index for parameter 3-41 Ramp 1 Ramp Up Time is -2, that
is, 0.01.
Parameter 3-41 Ramp 1 Ramp Up Time is of the type
Unsigned 32.
®
VLT DriveMotor FCP 106 and FCM 106 Programming Guide
H 0000 H
PWE PWE
high low
H 03E8 H
PWE PWE
high
low
MG03N102 - Rev. 2013-12-02

3.6 Modbus RTU Overview

3.6.1 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 frequency converter.

3.6.2 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.
3.6.3 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
•
Recognises 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.
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
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