WAGO 750-830 Manual page 220

220 • Fieldbus Communication
MODBUS Functions
4.3.3.3 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 re-
quest 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 Word count
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
.....
Byte 7
Byte 8
Byte 9, 10
Byte 11, 12 Value Register 1
The contents of register 0 are displayed 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
Field name
MODBUS function code
Byte count
Value Register 0
Field name
MODBUS function code
Exception code
Example
0x0000
0x0000
0x0006
0x01 not used
0x03
0x0000
0x0002
Example
0x03
0x04
0x1234
0x2345
Example
0x83
0x01 or 0x02
WAGO-I/O-SYSTEM 750
BACnet/IP Controller