Technical data

Fieldbus Communication • 221
MODBUS Functions
WAGO-I/O-SYSTEM 750
BACnet/IP Controller
4.3.3.4 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 re-
quest 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