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

RS-485 Installation and Set...
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
PKE
Illustration 7.10 Response from Master
7.5.2 Reading a Parameter Value
Read the value in 3-41 Ramp 1 Ramp Up Time
7 7
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
PKE
Illustration 7.11 Telegram
If the value in 3-41 Ramp 1 Ramp Up Time is 10 s, the
response from the follower to the master is:
1155 H
PKE
Illustration 7.12 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.

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 frequency converter.
86
0000 H 0000 H 03E8
PWE PWE
IND
high
0000 H 0000 H 0000
IND PWE
high
0000 H 0000 H 03E8
PWE
IND
high
®
VLT HVAC Basic Drive FC 101 Design Guide

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
H
Regardless of the type of physical communication
low
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 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.
The master can address individual followers, or can initiate
a broadcast message to all followers. Followers return a
H
response to queries that are addressed to them
individually. No responses are returned to broadcast
PWE
low
queries from the master. The Modbus RTU protocol
establishes the format for the master's query by providing
the device (or broadcast) address, a function code defining
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
requested action, the follower constructs an error message,
and send it in response, or a time-out occurs.
MG18C502 - Rev. 2013-09-06
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