QModbus

QModbus is a Qt/C++ wrapper around libmodbus that provides an object-oriented, Qt-style API for Modbus communication. It lets a Qt application act as a Modbus master over TCP, RTU, or TCP-PI and exchange coils, discrete inputs, and holding/input registers.

Features

• Modbus master over:
  • Modbus/TCP (QModbusMaster("127.0.0.1", 502))
  • Modbus/RTU (serial, with baud/parity/data/stop bits)
  • Modbus/TCP-PI (IPv4/IPv6 node + service)
• Typed access to registers and bits:
  • QModbusRegisters — signed/unsigned 16/32/64-bit integers and float/double, encoded in standard Modbus big-endian (abcd) order.
  • QModbusBits — coils and discrete inputs.
  • QModbusError — isValid() / message() error reporting.
• Qt-style error handling via a per-object last-error state.

Requirements

• Qt 5.12 or later (Core; Widgets/Gui only for the GUI app, Test only for tests)
• libmodbus 3.1.x or later (LGPL-2.1+)
• A C++11 toolchain (tested with MinGW-w64 7.3)

Building

The project uses qmake. The dependency paths are configured through qmake feature files (.prf), which contain absolute paths that you must adjust to your machine.

1. Copy the feature file into your Qt mkspecs/features directory and edit the library path inside it, for example:

   cp libmodbus-3.1.10.prf $QTDIR/5.12.12/mingw73_32/mkspecs/features/

   and set LIB_DIR in that file to your libmodbus installation.

2. Build the library:

   qmake QModbus.pro
   make        # or mingw32-make / nmake on Windows

3. Build the GUI app (optional):

   qmake QModbusMasterGUI.pro
   make

4. Build and run the tests (optional):

   qmake QModbusTest.pro
   make

Usage

#include <QDebug>
#include "qmodbusmaster.h"
#include "qmodbusregisters.h"

using namespace Modbus;

QModbusMaster master("127.0.0.1", 502);
master.connect();

QModbusRegisters regs(0, 4);   // start address 0, 4 registers
if (master.readRegisters(regs)) {
    qDebug() << regs.getInteger32(0)
             << regs.getFloat32(0);
} else {
    qDebug() << master.lastError().message();
}

master.close();

API overview

• QModbusMaster — connect/close/flush, setSlave(), setBroadcast(), readBits()/readInputBits()/readRegisters()/readInputRegisters(), writeBit()/writeBits()/writeRegister()/writeRegisters(), and writeAndReadRegisters(), plus timeout/debug/error-recovery settings.
• QModbusRegisters — getValue()/setValue() and typed accessors: getInteger16/32/64, getUInteger16/32/64, getFloat32/64 (with matching set* methods).
• QModbusBits — getValue()/setValue() for individual bits.
• QModbusError — isValid() and message().

Byte order

32- and 64-bit values are written to the Modbus wire in big-endian (abcd) word order, as required by the Modbus specification, independently of the host CPU endianness. If a device uses a non-standard word order, swap the data before/after calling the accessors.

Testing

Unit tests (TestQModbusBits, TestQModbusRegisters, TestQModbusError) run standalone. The integration tests (TestQModbusMaster) need a Modbus server at 127.0.0.1:502; you can use diagslave:

diagslave -m tcp -p 502 &
QModbusTest.exe

License

QModbus is licensed under the GNU Lesser General Public License, version 3 or later (LGPL-3.0-or-later). See LICENSE.txt.

Third-party components and their licenses are listed in THIRD-PARTY-NOTICES.txt.