QModbusMaster is an excellent software tool for testing UCM/Modbus in Slave Mode
Download from https://sourceforge.net/projects/qmodmaster/
This can work in RTU (RS485) and TCP Mode to complement UCM/Modbbus in RS485 and TCP Mode
There is also a related tool for simulating Modbus Slave
https://sourceforge.net/projects/pymodslave/
This can be used for testing UCM/Modbus in Master Mode. It also works in RTU (RS485 ) and TCP mode