ModbusLab

Windows desktop software · Current version 1.0.0

Modbus RTU slave simulator

Create multiple simulated RTU units on one serial connection. Define each unit's data map, start the bus and inspect what a connected master sends and receives.

Read the setup guide · Check installer availability and Windows requirements

What it does

  1. Create a project and add a generic device or a bundled profile.
  2. Assign each device a unique unit address from 1 to 247.
  3. Edit coils, discrete inputs, input registers and holding registers in the table.
  4. Choose the serial port and matching RTU parameters, then start the bus.
  5. Connect a separate Modbus master and inspect requests, responses and errors in the traffic view.

Implemented simulator behavior

The simulator implements RTU server handling for function codes 01-06, 15-17 and 43/MEI 14. It checks frame CRCs, supports exception responses and address-zero broadcast writes, and can add a response delay or deliberately drop responses. Serial controls include baud rate, data bits, parity, stop bits, RTS direction and local-echo filtering.

The scanner is a separate read-only workflow. Depending on scan mode, it tries FC43/MEI 14 and FC17 Report Slave ID, with retries and an FC03 read in thorough mode. It does not send register writes.

Device profiles and projects

The bundled profile library includes selected Lumel instruments and the Fujitsu UTY-VMSX profile, as well as a generic RTU device. Profile maps provide starting register data and selected device-specific behavior. They are test models; check the target device's manual and verify the exchange in the traffic monitor before using a profile for an integration decision.

Project configuration can be saved to or opened from JSON. The current application also stores the active project locally on the computer.

Scope and requirements

  • Targets 64-bit Windows 10 and Windows 11.
  • Physical serial testing needs a compatible COM port or USB-to-RS-485 adapter. The adapter is not a Modbus master.
  • A PLC, HMI or master program must connect as a separate bus node and use the same serial parameters.
  • The simulator is a Modbus RTU slave. The app's gateway workspace polls RTU as a master; this does not add Modbus TCP or Modbus ASCII transport to the simulator.
  • RTU frame interpretation uses the PDU address in the request. Reference notation such as 40001 is not itself sent in the frame.
No hardware claim: ModbusLab currently sells software access. This page does not describe a physical RS-485 interface or gateway product.

Learn the protocol

For independent protocol explanations, see RTU framing, register addressing, function codes and troubleshooting.