Modbus Rtu Overview; Assumptions; What The User Should Already Know - Danfoss VLT HVAC Basic Drive FC 101 Design Manual

RS-485 Installation and Set...
Data value 1000, corresponding to 100 Hz, see
chapter 7.4.12 Conversion.
The message looks like this:
E19E H 0000 H 0000
PKE IND PWE
Figure 7.9 Message
NOTICE!
4-14 Motor Speed High Limit [Hz] is a single word, and the
parameter command for write in EEPROM is "E".
Parameter 4-14 is 19E in hexadecimal.
The response from the follower to the master is:
119E H 0000 H 0000
PWE
IND
PKE
Figure 7.10 Response from Master
7.5.2 Reading a Parameter Value
Read the value in 3-41 Ramp 1 Ramp Up Time
PKE=1155 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 0000 H 0000
PKE IND
Figure 7.11 Message
If the value in 3-41 Ramp 1 Ramp Up Time is 10 s, the
response from the follower to the master is:
Figure 7.12 Response
MG18C522
Design Guide
H 03E8 H
PWE
high low
H 03E8 H
PWE
low
high
H 0000 H
PWE PWE
high low
Danfoss A/S © Rev. 2014-01-14 All rights reserved.
3E8 Hex corresponds to 1000 decimal. The conversion
index for 3-41 Ramp 1 Ramp Up Time is -2, that is, 0.01.
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 document, and strictly observes all
requirements and limitations stipulated in the controller
and adjustable frequency drive.

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
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
•
Recognizes 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.
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