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MODBUS Protocol
Function Codes
A Slave address is not required in MODBUS over TCP/IP because the
higher-level protocols include device addressing. The unit identifier is not
used by the instrument.
Function Code
The function code is a single byte in length. The following function codes
are supported by the instrument:
Read Coils
:
0x01
Read Inputs
:
0x02
Read Holding Registers
:
0x03
Read Input Registers
:
0x04
Force (Write) Single Coil
:
0x05
Read Exception Status
:
0x07
If a function code is received that is not in this list, and invalid function
exception is returned.
Data
The data field varies depending on the function. For more description of
these data fields, see “Function Codes” below.
Error Check
In MODBUS over Serial an error check is included in the message. This is
not necessary in MODBUS over TCP/IP because the higher-level protocols
ensure error-free transmission. The error check is a two-byte (16-bit) CRC
value.
Function Codes
This section describes the various function codes that are supported by the
Model 49i Primary Standard.
(0x01/0x02) Read Coils /
Read Inputs
Read Coils/Inputs reads the status of the digital outputs (relays) in the
instrument. Issuing either of these function codes will generate the same
response.
These requests specify the starting address, i.e. the address of the first output
specified, and the number of outputs. The outputs are addressed starting at
zero. Therefore, outputs numbered 1–16 are addressed as 0–15.
Thermo Fisher Scientific
Model 49i Primary Standard Instruction Manual
C-3