Modbus TCP Server on port 502

You need to permit non root user to bind to port < 1024. Search this forum and google. Been covered a few times.

something like sudo setcap 'cap_net_bind_service=+ep' `which node` or sudo setcap 'cap_net_bind_service=+ep' $(eval readlink -f `which node`) should do the trick

Read this thread

and this