Troubleshooting¶

This page lists common startup, connectivity, build, and traffic issues.

Cannot Start On Port 502¶

Port 502 is the standard Modbus TCP port, but Windows may require administrator rights for low ports.

Use a high test port such as 1502, or run ModbusLab as administrator if you specifically need port 502.

Endpoint Already In Use¶

If startup fails because the endpoint is already in use:

• stop the other application using the same port;
• choose another port;
• check whether another ModbusLab server instance is already using the endpoint.

Bind Address Not Available¶

If the bind address is not assigned to this PC, use 0.0.0.0 or a local IP address shown by Windows network settings.

Client Cannot Connect¶

Check:

• server is running;
• client host and port match the selected endpoint;
• firewall allows inbound connections;
• Unit ID filter is empty or matches the client Unit ID;
• client and server are using Modbus TCP, not Modbus RTU.

Values Look Byte-Swapped¶

Check server byte order and word order settings. Multi-register values such as Float32, UInt32, and Double depend on both settings.

Logs Are Busy¶

High traffic is grouped in the Log view to keep it readable. Use Blocks and Stats when you need a higher-level traffic picture.

Clear logs or blocks from the context menu when you want a clean capture window.

Build From Source Fails¶

Confirm that you are on Windows and have a compatible .NET SDK:

dotnet --info
dotnet build .\ModbusLab.slnx

The app targets net11.0-windows and uses WPF, so non-Windows builds are not supported.