GE AF-600 FP Design Manual page 118

6.7 Network Configuration
To enable Modbus RTU on the frequency converter, set the following parameters:
Parameter Number
O-30
O-31
O-32
O-33
6.8 Modbus RTU Message Framing Structure
6.8.1 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
Error Check Field
6.8.2 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
T1-T2-T3-T4 8 bits
Parameter name Setting
Protocol Modbus RTU
Address 1 - 247
Baud Rate 2400 - 115200
Parity/Stop bits Even parity, 1 stop bit (default)
Data byte
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)
Function
8 bits
parity
Data CRC check
N x 8 bits 16 bits
AF-600 FP Design Guide
Stop/ Stop
End
T1-T2-T3-T4
117
6