PLC controller output lines usually can be: -transistors in PNP connection -transistors in NPN connection -relays The following two pictures display a realistic way how a PLC manages external devices.
It ought to be noted that a main difference between these two pictures is a position of “output load device”. By “output load device” we mean some relay, signalization light or similar.
How something is connected with a PLC output depends on the element being connected. In short, it depends on whether this element of output load device is activated by a positive supply pole or a negative supply pole.