PLC controller output lines usually can be:-transistors in PNP connection-transistors in NPN connection-relaysThe 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.