MODBUS Protocol
Function Codes
C-6
Model 43
i
Trace Level-Enhanced Instruction Manual
Thermo Fisher Scientific
response. These functions read the contents of one or more contiguous
registers.
These registers are 16 bits each and are organized as shown below. All of
the values are reported as 32-bit IEEE standard 754 floating point format.
This uses 2 sequential registers, least significant 16 bits first.
The request specifies the starting register address and the number of
registers. Registers are addressed starting at zero. Therefore, registers
numbered 1–16 are addressed as 0–15. The register data in the response
message are packed as two bytes per register, with the binary contents right
justified within each byte. For each register, the first byte contains the high
order bits and the second contains the low order bits.
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
