If you have PC and PLC how do they communicate by means of hardware and software if e.x: I have Siemens PLC s7 300, 400 and Allen Bradley RX LOGIX 5000?

in case of siemens plc s7300 and s7400 commnication is done by ethernet or mpi

in general

you can connect between two control system by hard wire such as relay in on plc and concoct input in other

also you can use modbus or opc to interchange the data by software