Function Code Fc4 (Read Input Registers) - WAGO 750 Series Manual

166 • MODBUS Functions
Description of the MODBUS Functions

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
Byte 0, 1
Byte 2, 3
Byte 4, 5
Byte 6
Byte 7
Byte 8, 9
Byte 10, 11
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
.....
Byte 7
Byte 8
Byte 9, 10
Byte 11, 12
The contents of register 0 are shown by the value 0x1234 and the contents of
register 1 is 0x2345.
Exception
Byte
.....
Byte 7
Byte 8
Field name
Transaction identifier
protocol identifier
length field
unit identifier
MODBUS function code
reference number
Word count
Field name
MODBUS function code
Byte count
Value Register 0
Value Register 1
Field name
MODBUS function code
Exception code
Example
0x0000
0x0000
0x0006
0x01 not used
0x04
0x0000
0x0002
Example
0x04
0x04
0x1234
0x2345
Example
0x84
0x01 or 0x02
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