Reading A Parameter Value; Modbus Rtu Overview; Assumptions; What The User Should Already Know - Danfoss VLT AutomationDrive FC 301 Design Manual

RS-485 Installation and Set...
PWELOW = 03E8 Hex - Data value 1000, corresponding to
100 Hz, see chapter 12.7.12 Conversion.
The telegram looks like this:
E19E H
PKE
Illustration 12.12 Write Data in EEPROM
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.
The response from the follower to the master is:
119E H
PKE
Illustration 12.13 Response from Follower

12.8.2 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
12 12
IND = 0000 Hex
PWEHIGH = 0000 Hex
PWELOW = 0000 Hex
1155 H
PKE
Illustration 12.14 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
1155
PKE
Illustration 12.15 Response from Follower
3E8 Hex corresponds to 1000 decimal. The conversion
index for 3-41 Ramp 1 Ramp Up Time is -2, i.e. 0.01.
3-41 Ramp 1 Ramp Up Time is of the type Unsigned 32.
182
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
H 0000 H 0000 H 03E8
PWE
IND
high
®
VLT AutomationDrive FC 301/FC 302 Design Guide, 0.25-75 kW

12.9 Modbus RTU Overview

12.9.1 Assumptions

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

12.9.2 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.
12.9.3 Modbus RTU Overview
Regardless of the type of physical communication
networks, the Modbus RTU Overview describes the process
H
a controller uses to request access to another device. This
PWE
low
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:
•
•
•
•
If a reply is required, the controller constructs the reply
message and sends it.
H
Controllers communicate using a master-follower
technique in which only the master can initiate
PWE
low
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
H
establishes the format for the master's query by providing
PWE the device (or broadcast) address, a function code defining
low
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
MG33BF02 - Rev. 2013-12-20
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