Examples; Writing A Parameter Value; Reading A Parameter Value; Modbus Rtu Overview - Danfoss VLT AQUA Drive FC 202 Design Manual

RS-485 Installation and Set...

7.5 Examples

7.5.1 Writing a Parameter Value

Change 4-14 Motor Speed High Limit [Hz] to 100 Hz.
Write the data in EEPROM.
PKE=E19E Hex - Write single word in 4-14 Motor Speed High
Limit [Hz]
IND=0000 Hex
PWE =0000 Hex
high
PWE =03E8 Hex - Data value 1,000, corresponding to 100
low
Hz, see chapter 7.4.12 Conversion.
E19E H
PKE
7 7
Illustration 7.12 Telegram
NOTICE
4-14 Motor Speed High Limit [Hz] is a single word, and the
parameter command for write in EEPROM is "E."
Parameter number 4–14 is 19E in hexadecimal.
119E H
PKE
Illustration 7.13 Response from Master to Follower

7.5.2 Reading a Parameter Value

Read the value in 3-41 Ramp 1 Ramp Up Time
PKE=1,155 Hex - Read parameter value in 3-41 Ramp 1
Ramp Up Time
IND=0000 Hex
PWE =0000 Hex
high
PWE =0000 Hex
low
1155 H
PKE
Illustration 7.14 Parameter Value
If the value in 3-41 Ramp 1 Ramp Up Time is 10 s, the
response from the follower to the master is:
154
0000 H 0000 H 03E8
IND PWE PWE
high
0000 H 0000 H 03E8
PWE
IND
high
0000 H 0000 H 0000
IND PWE
high
Danfoss A/S © Rev. 06/2014 All rights reserved.
Design Guide
1155
Illustration 7.15 Response from Follower to Master
3E8 Hex corresponds to 1000 decimal. The conversion
index for 3-41 Ramp 1 Ramp Up Time is -2.
3-41 Ramp 1 Ramp Up Time is of the type Unsigned 32.

7.6 Modbus RTU Overview

7.6.1 Assumptions

Danfoss assumes that the installed controller supports the
interfaces in this manual, and strictly observes all
H
requirements and limitations stipulated in the controller
and frequency converter.
low

7.6.2 Prerequisite Knowledge

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
reader has full knowledge of the capabilities and
limitations of the controller.
7.6.3 Modbus RTU Overview
H
PWE
low
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:
•
•
•
•
H
If a reply is required, the controller constructs the reply
PWE
low
message and sends it.
Controllers communicate using a master-follower
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 responding to the 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
H 0000 H 0000
PKE PWE
IND
high
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
H 03E8 H
PWE
low
MG20Z202