Modbus Protocol
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Overview
The Modbus communication protocol is a serial communication protocol. The Modbus protocol does not define the physical layer. Instead it uses RS-232C compatible serial interfaces, which are very common on microcontrollers.
Check out the Mobicon Modbus Protocol Reference Guide for detailed info. Another good tutorial is Lammert Bies’ Modbus interface.
Terminology
- Coil: Discrete output (e.g. digital GPIO output pin on microcontroller)
- Holding register: Internal, two-byte wide digital register that are used for storing values
ASCII Framing
Messages start with a colon (:) and end with a carriage return-line feed pair (CR-LF). Each byte of data between is sent as ASCII-encoded HEX (0-9, A-F).
Function Codes
The common function codes are shown in the below table.
| Code | Description |
|---|---|
| 01 | Read coil status |
| 02 | Read input status |
| 03 | Read holding registers |
| 04 | Read input registers |
| 05 | Force single coil |
| 06 | Preset single register |
| 07 | Read exception status |
| 15 | Force multiple coils |
| 16 | Preset multiple registers |
| 17 | Report slave ID |
Software
FreeMODBUS - A Modbus ASCII/RTU and TCP implementation: Software targeted at embedded platforms.
free-dotnet-modbus: A free .NET implementation of the Modbus protocol.
ModNet: Free Windows-based Modbus protocol client, which checks Modbus RTU compliance.
Authors
This work is licensed under a Creative Commons Attribution 4.0 International License .
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