MODBUS Protocol
Function Codes
C-6
Model 42
i
Trace Level Instruction Manual
Thermo Fisher Scientific
Request
Function code
1 Byte
0x03 or 0x04
Starting Address
2 Bytes
0x0000 to maximum allowed by instrument
Quantity of Registers
2 Bytes
1 to maximum allowed by instrument
Response
Function code
1 Byte
0x03 or 0x04
Byte count
1 Byte
2 x N*
Register value
N* x 2 Bytes
N = N or N+1
*N = Quantity of Registers
Error Response
Function code
1 Byte
Function code + 0x80
Exception code
1 Byte
01=Illegal Function, 02=Illegal Address,
03=Illegal Data, 04=Slave Device Failure
Here is an example of a request and response to read registers 10–13:
Request
Field Name
(Hex)
Function 0x03
Starting Address Hi
0x00
Starting Address Lo
0x09
No. of Registers Hi
0x00
No. of Registers Lo
0x04
Response
Field Name
(Hex)
Function 0x03
Byte Count
0x06
Register value Hi (10)
0x02
Register value Lo (10)
0x2B
Register value Hi (11)
0x00