Modbus Rtu Overview; Assumptions; What The User Should Already Know - Danfoss VLT FC 360 Design Manual

RS-485 Installation and Set...
NOTE
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 slave to the master is:
119E H 0000 H 0000
PWE
PKE IND
Illustration 3.9 Response from Master
3.5.2 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
Illustration 3.10 Telegram
If the value in 3-41 Ramp 1 Ramp up Time is 10 s, the
response from the slave to the master is:
1155 H 0000 H 0000
PKE
IND
Illustration 3.11 Response
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.

3.6 Modbus RTU Overview

3.6.1 Assumptions

Danfoss assumes that the installed controller supports the
interfaces in this document, and strictly observe all
requirements and limitations stipulated in the controller
and frequency converter.
®
VLT AutomationDrive FC 360 Design Guide
H 03E8 H
PWE
low
high
H 0000 H
PWE PWE
high low
H 03E8 H
PWE PWE
high
low
®
MG06B202 - VLT is a registered Danfoss trademark

3.6.2 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.
3.6.3 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-slave 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 taking the action requested in 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
queries from the master. The Modbus RTU protocol
establishes the format for the master's query by placing
into it the device (or broadcast) address, a function code
defining the requested action, any data to be sent, and an
error-checking field. The slave'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 slave is unable to perform the requested
action, the slave constructs an error message, and send it
in response, or a time-out occurs.
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