QModMaster — The Complete Guide: Free Testing of Modbus RTU and TCP Devices

QModMaster is open-source (LGPL) graphical software for talking to Modbus devices, over serial lines (RTU) and over the network (TCP). It acts as the master: it sends requests to PLCs, energy meters, variable-frequency drives and sensors, and displays the replies.

QModMaster

QModMaster (open-source project)

Modbus RTU and TCP master with a bus monitor — for testing PLCs, energy meters, drives and sensors.

Get it from the vendor

What you get

Read and write — coils, discrete inputs, holding registers and input registers.
RTU and TCP — serial port, baud rate, parity and slave address, or IP address and port.
Bus monitor — the raw frames sent and received on the line.
Cyclic polling — repeated reads at a fixed interval to follow a changing value.
Data views — values in decimal, hexadecimal or binary.

When to use it

During commissioning of automation and building-control systems: check that an energy meter answers at the right address, that a drive accepts a command, or that RS-485 wiring is sound, before the SCADA system is connected. The key tip: check the manufacturer’s documentation for whether register numbering starts at 0 or 1 — a very common mistake. It is available for Windows and Linux.

Related tools

For electrical and control panel drawings — QElectroTech. To build an experimental Modbus device on a microcontroller — Arduino IDE. For automated test scripts — Python.