Function Code; Data; Error Check; Function Codes - Thermo Scientific Omni FTIR Multi Gas CEMS Instruction Manual

Function Code

Error Check

Function Codes

(0x03) Read Holding
Thermo Fisher Scientific
Transaction Identifier
Protocol Identifier
Length
Unit Identifier
The function code is a single byte in length. The following function codes
are supported by the system:
Read Holding Registers
Write Single Register
If a function code is received that is not in this list, an invalid function
exception is returned.

Data

The data field varies depending on the function. For more description of
these data fields, see
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.
This section describes the various function codes that are supported by the
Omni FTIR Multi Gas CEMS.
Read Holding/Input Registers reads the measurement data from the
instrument. Issuing either of these function codes will generate the same
Registers
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
measurement 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.
2 Bytes 0x0000 to 0xFFFF (Passed back in response)
2 Bytes 0x00 (Modbus protocol)
2 Bytes 0x0000 to 0xFFFF (Number of following bytes)
1 Byte 0x00 to 0xFF (Passed back in response)
: 0x03
: 0x06
"Function Codes" below.
Omni FTIR Multi Gas CEMS Instruction Manual
Modbus Protocol
Function Codes
B-3