Reading A Parameter Value; Modbus Rtu Overview; Assumptions; What The User Should Already Know - Danfoss VLT HVAC 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
number 4-14 is 19E in hexadecimal.
The response from the slave to the master will be:
119E H 0000 H 0000
PWE
PKE IND

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
PWEHIGH = 0000 Hex
PWELOW = 0000 Hex
1155 H 0000 H 0000
PKE IND
If the value in 3-41 Ramp 1 Ramp Up Time is 10 s, the
response from the slave to the master will be:
1155 H 0000 H 0000
PKE
IND
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.

7.6 Modbus RTU Overview

7.6.1 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.

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.
®
VLT HVAC Drive Design Guide
H 03E8 H
PWE
low
high
H 0000 H
PWE PWE
high low
H 03E8 H
PWE PWE
high low
®
MG11BB02 - VLT is a registered Danfoss trademark
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-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 will construct an error message, and send
it in response, or a time-out occurs.
7.6.4 Frequency Converter with Modbus
RTU
The frequency converter communicates in Modbus RTU
format over the built-in RS-485 interface. Modbus RTU
provides access to the Control Word and Bus Reference of
the frequency converter.
The Control Word allows the Modbus master to control
several important functions of the frequency converter:
•
Start
•
Stop of the frequency converter in various ways:
Coast stop
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