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
Regardless of the type of physical networks over which they communicate. It describes the process
a controller uses to request access to another device, how it will respond to requests from another
device, and how errors will be 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
will learn its device address, recognise a message addressed to it, determine the kind of action to
be taken, and extract any data or other information contained in the message. If a reply is re-
quired, the controller will construct the reply message and send 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 re-
sponses 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 will occur.
7.6.4. VLT HVAC with Modbus RTU
The VLT HVAC 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 fre-
quency converter:
•
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 a variety of preset speeds
•
Run in reverse
•
Change the active set-up
•
Control the frequency converter’s two built-in relays
The Bus Reference is commonly used for speed control. It is also possible to access the parame-
ters, 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 PID con-
troller is used.
7. RS-485 Installation and Set-up
VLT
®
HVAC Drive Design Guide
134
MG.11.B2.02 - VLT
®
is a registered Danfoss trademark
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