Vlt Hvac With Modbus Rtu; Modbus Rtu Message Framing Structure - Danfoss VLT Series Design Manual

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VLT HVAC Drive Design Guide
7.7. Network Configuration

7.7.1. VLT HVAC with Modbus RTU

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 HVAC with Modbus RTU
The controllers are set up to communicate on the Modbus network using 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
Coding System
Bits Per Byte
Error Check Field
7.8.2. Modbus RTU Message Structure
The transmitting device places a Modbus RTU message into a frame with a known beginning and
ending point. This allows receiving devices to begin at the start of the message, read the address
portion, determine which device is addressed (or all devices, if the message is broadcast), and to
recognise when the message is completed. Partial messages are detected and errors set as a
result. Characters for transmission must be in hexadecimal 00 to FF format in each field. The
frequency converter continuously monitors the network bus, also during 'silent' intervals. When
the first field (the address field) is received, each frequency converter or device decodes it to
determine which device is being addressed. Modbus RTU messages addressed to zero are broad-
cast messages. No response is permitted for broadcast messages. A typical message frame is
shown below.
Start
T1-T2-T3-T4 8 bits
Num- Parameter name
Protocol
Address
Baud Rate
Parity/Stop bits
Data bit
8-bit binary, hexadecimal 0-9, A-F. Two hexadecimal characters
contained in each 8-bit field of the message
1 start bit
8 data bits, least significant bit sent first
1 bit for even/odd parity; no bit for no parity
1 stop bit if parity is used; 2 bits if no parity
Cyclical Redundancy Check (CRC)
Address Function
8 bits
Typical Modbus RTU Message Structure
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MG.11.B2.02 - 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 CRC check
N x 8 bits 16 bits
Stop/ Stop
parity
End
T1-T2-T3-T4
135
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