Modbus Rtu Message Framing Structure - Danfoss FC 300 Design Manual

11 RS-485 Installation and Set-up
11.7 Network Configuration
To enable Modbus RTU on the frequency converter, set the following parameters:
Parameter Number
8-30
8-31
8-32
8-33

11.8 Modbus RTU Message Framing Structure

11.8.1 Frequency Converter with Modbus RTU
The controllers are set up to communicate on the Modbus network using RTU (Remote Terminal Unit) mode, with each 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
11
Error Check Field
11.8.2 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
broadcast messages. No response is permitted for broadcast messages. A typical message frame is shown below.
Typical Modbus RTU Message Structure
Start
T1-T2-T3-T4
240
Parameter name
Protocol
Address
Baud Rate
Parity/Stop bits
Data byte
8-bit binary, hexadecimal 0-9, A-F. Two hexadecimal characters contained in each 8-bit field of the
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 8 bits
MG.33.BC.02 - VLT
Setting
Modbus RTU
1 - 247
2400 - 115200
Even parity, 1 stop bit (default)
message
Data CRC check
N x 8 bits
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FC 300 Design Guide
Stop/ Stop
parity
End
16 bits T1-T2-T3-T4