10.7 Network Configuration
To enable Modbus RTU on the frequency converter, set the following parameters:
Parameter Number
Parameter name Setting
O-30 Protocol Modbus RTU
O-31 Address 1 - 247
O-32 Baud Rate 2400 - 115200
O-33 Parity/Stop bits Even parity, 1 stop bit (default)
10.8 Modbus RTU Message Framing Structure
10.8.1 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
Data byte Stop/
parity
Stop
Coding System 8-bit binary, hexadecimal 0-9, A-F. Two hexadecimal characters contained in each 8-bit field of the message
Bits Per Byte 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
Error Check Field Cyclical Redundancy Check (CRC)
10.8.2 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
Address Function Data CRC check End
T1-T2-T3-T4 8 bits 8 bits N x 8 bits 16 bits T1-T2-T3-T4
AF-650 GP Design Guide
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