modsim

Modsim is a Modbus simulator for local development and integration with modpoll. Its default datastore contains coils, discrete inputs, holding registers and input registers. All Unit IDs share that datastore. A public Modbus TCP simulator is available at modsim.topmaker.net:502 for quick testing.

Run the default simulator

pip install modsim
modsim --comm tcp --port 5020

Alternatively, start the published container:

docker run --rm -p 5020:5020 topmaker/modsim:latest

The default TCP listener uses port 5020. Coils and holding registers are writable; discrete inputs and input registers are read-only. Unsupported addresses return a Modbus exception.

Transports and options

TCP and UDP use socket framing; UDP listens on loopback. TLS uses TLS framing and expects examples/certificates/pymodbus.crt and pymodbus.key relative to the working directory. The TLS server does not authenticate clients. Serial supports RTU and ASCII framing; physical serial devices have not been validated in the local regression suite.

Only the sequential datastore is implemented. The CLI rejects non-default --store values and the unsupported --slaves and --context options. Run modsim --help for the full option list.

The local TCP, UDP and temporary-certificate TLS tests cover startup, reads and shutdown. SIGINT and SIGTERM handling on Windows has not been verified.