Modbus Rtu Protocol - Danfoss VLT Refrigeration Drive FC 103 Design Manual

System Integration
1155 H
PKE
Illustration 3.54 Parameter Value
3 3
If the value in parameter 3-41 Ramp 1 Ramp Up Time is 10
s, the response from the slave to the master is
1155 H
PKE
Illustration 3.55 Response from Slave
3E8 hex corresponds to 1000 decimal. The conversion
index for yparameter 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.

3.9.9 Modbus RTU Protocol

3.9.9.1 Assumptions
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
supports the interfaces defined in this manual. It is
assumed that the user has full knowledge of the
capabilities and limitations of the controller.
3.9.9.2 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.
•
Recognises 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.
78
0000 H 0000 H 0000
IND PWE
high
0000 H 0000 H 03E8
PWE
IND
high
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®
VLT Refrigeration Drive FC 103
Controllers communicate using a master/slave technique in
which only the master can initiate transactions (called
H
queries). Slaves respond by supplying the requested data
PWE
low
to the master, or by taking the action requested in the
query.
The master can address individual slaves, or initiate a
broadcast message 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. The Modbus RTU protocol establishes the format
for the master's query by providing:
H
•
PWE
low
•
•
•
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
returns an error message. Alternatively, a timeout occurs.
3.9.9.3 Frequency Converter with Modbus
The frequency converter communicates in Modbus RTU
format over the built-in RS485 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:
•
•
•
•
•
•
•
The bus reference is commonly used for speed control. It is
also possible to access the parameters, read their values,
and, where possible, write values to them. This permits a
range of control options, including controlling the setpoint
of the frequency converter when its internal PI controller is
used.
The device (or broadcast) address.
A function code defining the requested action.
Any data to be sent.
An error-checking field.
RTU
Start
Stop of the frequency converter in various ways:
- Coast stop
- Quick stop
- DC brake stop
- Normal (ramp) stop
Reset after a fault trip
Run at various preset speeds
Run in reverse
Change the active set-up
Control the frequency converter's built-in relay
MG16G202