Danfoss VLT Series Design Manual page 147

VLT ® HVAC Drive Design Guide
The control word allows the modbus master to control several important functions of the adjust-
able frequency drive:
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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 op-
tions, including controlling the setpoint of the adjustable frequency drive when its internal PID
controller is used.
7.7. Network Configuration
To enable Modbus RTU on the VLT HVAC, set the following parameters:
Parameter
ber
8-30
8-31
8-32
8-33
7.8. Modbus RTU Message Framing Structure
7.8.1. VLT AQUA with Modbus RTU
The controllers are set up to communicate on the Modbus network in RTU (Remote Terminal Unit)
mode, with each 8-bit byte in a message containing two 4-bit hexadecimal characters. The format
for each byte is shown below.
Start bit
Start
Stop of the adjustable frequency drive 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 adjustable frequency drive's two built-in relays
Num- Parameter name
Protocol
Address
Baud Rate
Parity/Stop bits
MG.11.B4.22 - VLT ® is a registered Danfoss trademark.
7. RS-485 Installation and Set-up
Setting
Modbus RTU
1 - 247
2400 - 115200
Even parity, 1 stop bit (default)
Data bit Stop Stop
parity
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