Chapter 9 Modbus Communications
Chapter 9 Modbus Communications
This chapter specifies the Modbus Communications protocol as RS-232 or RS-485 interface module is installed. Only
RTU mode is supported. Data is transmitted as eight-bit binary bytes with 1 start bit, 1 stop bit and optional parity
checking (None, Even or Odd). Baud rate may be set to 300, 600, 1200, 2400, 4800, 9600, 14400, 19200, 28800 and
38400.
Only function 03, 06 and 16 are available for this series of controllers.
The message formats for each function are described as follows:
Query ( from master )
Query
Response ( from slave )
Response
Slave address (0-255)
Function code (3)
Starting address of register Hi (0)
Starting address of register Lo (0-79,
128-131)
No. of words Hi (0)
No. of words Lo (1-79)
CRC16 Hi
CRC16 Lo
Byte count
Data 1 Hi
Data 1 Lo
Data 2 Hi
Data 2 Lo
CRC16 Hi
CRC16 Lo
Function 03: Read Holding Registers
Function 03: Read Holding Registers
Function 06: Preset single Register
Function 06: Preset single Register
Query ( from master )
Query
Response ( from slave )
Response
Slave address (0-255)
Function code (6)
Register address Hi (0)
Register address Lo (0-79, 128-131)
Data Hi
Data Lo
CRC16 Hi
CRC16 Lo
9 1 Functions Supported
9 1 Functions Supported
Query ( from master )
Query
Response ( from slave )
Response
Slave address (0-255)
Function code (16)
Starting address of register Hi (0)
Starting address of register Lo (0-79,128-131)
No. of words Hi (0)
No. of words Lo (1-79)
Byte count
(2-158)
Data 1 Hi
Data 1 Lo
Data 2 Hi
Data 2 Lo
CRC16 Hi
CRC16 Lo
CRC16 Hi
CRC16 Lo
Function 16: Preset Multiple Registers
Function 16: Preset Multiple Registers
110
UM25001H
