mbusd

mbusd is an open-source Modbus TCP to Modbus RTU (RS-232/485) gateway. It presents a network of RTU servers as single TCP server.

That is a TCP server, which acts as an RTU client to get data from Modbus RTU servers. See Modbus for client / server.

When you add the Sunsynk HASS repository, it includes a mbusd addon called "Modbus TCP to Modbus RTU Gateway Add-on".

https://github.com/kellerza/sunsynk/tree/main/hass-addon-mbusd

Point the Sunsynk add-on at tcp://homeassistant.local:502 (or the host that runs mbusd). mbusd has its own wait/retry on the RS485 leg — in add-on language that is 3 READ_ATTEMPTS, 500 msTIMEOUT (-W), and 100 ms READ_MESSAGE_SPACING (-R). Keep that RTU wait shorter than the Sunsynk add-on's TIMEOUT. Comparison table: Wait times by component.