•
•
•
•
•
•
•
•
e3
0b
a0
94
.1
0
1155
0000
0000
0000
H
H
H
H
PKE
IND
PWE
PWEhigh
low
Illustration 75: 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 shown in the following
illustration.
e3
0b
a2
67
.1
0
1155
0000
0000
03E8
H
H
H
H
PKE
IND
PWE
PWEhigh
low
Illustration 76: 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.
8.6 Modbus RTU
8.6.1 Prerequisite Knowledge
Danfoss assumes that the installed controller supports the interfaces in this manual, and strictly observes all requirements and limi-
tations 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 de-
fined in this manual. It is assumed that the user has full knowledge of the capabilities and limitations of the controller.
8.6.2 Modbus RTU Overview
Regardless of the type of physical communication networks, this section describes the process that a controller uses to request ac-
cess 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.
8.6.3 Drive with Modbus RTU
The drive communicates in Modbus RTU format over the built-in RS485 interface. Modbus RTU provides access to the control word
and bus reference of the drive.
The control word allows the Modbus master to control several important functions of the drive:
AJ363928382091en-000101 / 130R0983 | 111
Danfoss A/S © 2021.04
RS485 Installation and Set-up
VLT® Flow Drive FC 111
Design Guide
