Common MODBUS functions • 285
Function code FC3 (Read multiple registers)
Modular I/O System
ETHERNET TCP/IP
6.2.3
Function code FC3 (Read multiple registers)
The binary contents of holding registers are read from the slave using this
function.
Request
The request determines the start word address (start register) and the number
the register to be read. The addressing starts with 0.
Example: An inquiry of the 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
