Technical data

MODBUS Functions • 151
Description of the MODBUS Functions
WAGO-I/O-SYSTEM 750
ETHERNET TCP/IP
6.3.4 Function code FC4 (Read input registers)
This function reads contents of input registers from the slave device in word
format.
Request
The request specifies a 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 0x04
Byte 8, 9 reference number 0x0000
Byte 10, 11 Word count 0x0002
Response
The register data of the response is packed as 2 bytes per register. The first
byte has the higher value bits, the second the lower values.
Byte Field name Example
.....
Byte 7 MODBUS function code 0x04
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 shown by the value 0x1234 and the contents of
register 1 is 0x2345.
Exception
Byte Field name Example
.....
Byte 7 MODBUS function code 0x84
Byte 8 Exception code 0x01 or 0x02