This Add-on uses the mbusd gateway. It provides a TCP-Slave (or server) which acts as a RTU-master to get data from Modbus RTU-slave devices.
I use this mainly for testing, but mbusd can also be deployed on a low-spec device (i.e. Raspberry Pi 1b) to convert an RS-485 connection to TCP.
Refer to the mbusd command line options for more detail.