Technical data

150 • MODBUS Functions
Description of the MODBUS Functions
WAGO-I/O-SYSTEM 750
ETHERNET TCP/IP
6.3.3 Function Code FC3 (Read multiple registers)
This function reads the contents of holding registers from a slave device in
word format.
Request
The request specifies the reference number (start register) and the word count
(register quantity) of the registers to be read. The reference number of the
request is zero based, therefore, the first register starts at address 0.
Example: Read registers 0 and 1:
Byte Field name Example
Byte 0, 1 Transaction identifier 0x0000
Byte 2, 3 protocol identifier 0x0000
Byte 4, 5 length field 0x0006
Byte 6 unit identifier 0x01 not used
Byte 7 MODBUS function code 0x03
Byte 8, 9 reference number 0x0000
Byte 10, 11 Word count 0x0002
Response
The reply register data is packed as 2 bytes per register. The first byte contains
the higher value bits, the second the lower values.
Byte Field name Example
.....
Byte 7 MODBUS function code 0x03
Byte 8 Byte count 0x04
Byte 9, 10 Value Register 0 0x1234
Byte 11, 12 Value Register 1 0x2345
The contents of register 0 are displayed by the value 0x1234 and the contents
of register 1 is 0x2345.
Exception
Byte Field name Example
.....
Byte 7 MODBUS function code 0x83
Byte 8 Exception code 0x01 or 0x02