Modbus Rtu Overview; Prerequisite Knowledge; What The User Should Already Know; Overview - Danfoss VLT Compressor Drive CDS 803 Design Manual

RS485 Installation and Set-...
NOTICE
Parameter 4-14 Motor Speed High Limit [Hz] is a single
word, and the parameter command for write in EEPROM
is E. Parameter 4-14 Motor Speed High Limit [Hz] is 19E in
hexadecimal.
The response from the slave to the master is shown in
Illustration 7.11.
119E H
PKE
Illustration 7.11 Response from Master
7.5.2 Reading a Parameter Value
7 7
Read the value in parameter 3-41 Ramp 1 Ramp Up Time.
PKE = 1155 hex - Read parameter value in
parameter 3-41 Ramp 1 Ramp Up Time:
•
IND = 0000 hex.
•
PWE
•
PWE
1155 H
PKE
Illustration 7.12 Telegram
If the value in parameter 3-41 Ramp 1 Ramp Up Time is 10
s, the response from the slave to the master is shown in
Illustration 7.13.
1155 H
PKE
Illustration 7.13 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.
56
0000 H 0000 H 03E8
IND PWE
high
= 0000 hex.
HIGH
= 0000 hex.
LOW
0000 H 0000 H 0000
IND PWE
high
0000 H 0000 H 03E8
PWE
IND
high
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VLT Compressor Drive CDS 803

7.6 Modbus RTU Overview

7.6.1 Prerequisite Knowledge

Danfoss assumes that the installed controller supports the
interfaces in this manual, and strictly observes all
requirements and limitations stipulated in the controller
and frequency converter.
The built-in Modbus RTU (remote terminal unit) is
designed to communicate with any controller that
H
supports the interfaces defined in this manual. It is
assumed that the user has full knowledge of the
PWE
low
capabilities and limitations of the controller.

7.6.2 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 manual. It is
assumed that the user has full knowledge of the
capabilities and limitations of the controller.

7.6.3 Overview

Regardless of the type of physical communication
networks, this section describes the process a controller
uses to request access to another device. This process
includes how the Modbus RTU responds to requests from
H
another device, and how errors are detected and reported.
PWE
low It also establishes a common format for the layout and
contents of telegram fields.
During communications over a Modbus RTU network, the
protocol:
•
•
•
H
•
PWE
low
If a reply is required, the controller constructs the reply
telegram and sends it.
Controllers communicate using a master/slave technique in
which only the master can initiate transactions (called
queries). Slaves respond by supplying the requested data
to the master, or by acting as requested in the query.
The master can address individual slaves, or initiate a
broadcast telegram to all slaves. Slaves return a response
to queries that are addressed to them individually. No
responses are returned to broadcast queries from the
master.
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.
MG18N202