11.7 Network Configuration
To enable Modbus RTU on the adjustable frequency drive, set the following parameters:
Parameter Number
Parameter name
Setting
8-30
Protocol
Modbus RTU
8-31
Address
1 - 247
8-32
Baud Rate
2400 - 115200
8-33
Parity/Stop bits
Even parity, 1 stop bit (default)
11.8 Modbus RTU Message Framing Structure
11.8.1 Adjustable Frequency Drive 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)
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 recognize
when the message is completed. Partial messages are detected, and errors are set as a result. Characters for transmission must be in hexadecimal 00
to FF format in each field. The adjustable frequency drive continuously monitors the network bus, also during ‘silent’ intervals. When the first field (the
address field) is received, each adjustable frequency drive 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
FC 300 Design Guide
11 RS-485 Installation and Set-up
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