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Modbus/TCP & Modbus RTU Function Codes
Definition
Revision 4 HC900 Hybrid Controller Communications User Guide 17
9/03
4. Modbus/TCP & Modbus RTU Function Codes
4.1 Definition
The HC900 Modbus protocol uses a subset of the standard Modbus function codes to provide access to process-
related information. These standard function codes provide basic support for IEEE 32-bit floating point numbers,
32-bit unsigned/signed integer and 16-bit integer register representation of instrument’s process data.
Table 4-1, Table 4-2, and Table 4-3 list the Function Code definitions, the maximum number of Object Addresses
and maximum number of registers allowed per request.
Repolling of data is not supported by this instrument.
Table 4-1 Modbus/TCP and Modbus RTU Function Codes Definitions
Function Code Name Usage
01 Read Coil Status Read the state of a digital output
02 Read Input Status Read the state of a digital input
03
Read Holding Registers
Read data in 16-bit Register Format (high/low). Used to read
integer or floating point process data. Registers are
consecutive and are imaged from the instrument to the host.
04 Read Input Registers Provides Read access to any Analog Input Channel
positioned in any Rack or Slot.
05 Force Single Coil Write data to force a digital output ON/OFF
Values of FF 00 forces digital output ON
Values of 00 00 forces digital output OFF
Values of FF FF releases the force of the digital output
All other values are illegal and will not effect the digital output.
06 Preset Single Register Write Data in 16-bit Integer Format (high/low) ONLY.
08 Loopback Test Used for diagnostic testing of the communications port.
16 (10h) Preset Multiple Registers Write Data in 16-bit Format (high/low). Used to write integer
and floating point override data. Registers are consecutive
and are imaged from the host to the instrument.
17 (11h) Report Device ID Read instrument ID and connection information, ROM version,
etc.