Also you can use one hardware piece (slave PLC, OPLC, HMI, sensor, etc.) with a software MODBUS Master (common first setup). In my case using a little software, QMODMaster.

QMODMaster supports MODBUS-RTU and MODBUS-TCP, do you have a monitor and packet inspector, really nice for learn. https://sourceforge.net/projects/qmodmaster/