Buses are the paths through which the digital signal flows internally of the PLC.
The four system buses are:
Data bus
It is used by the CPU to transfer data among different elements.
Control bus
It transfers signals related to the action that are controlled internally.
Address bus
It sends the location’s addresses to access the data.
System bus
It helps the I/O port and I/O unit to communicate with each other
Note: Depends on PLC manufacturer, the terminology may change.