Technical term for this kind of PLC I/O?
In Figures 1 and 2, below, an external sensor is connected to a PLC input. Let's assume that the PLC input has an opto-coupler to the internal logic to isolate the sensitive logic circuitry from the outside world. The circuit, as far as external switches are concerned, is an LED with series resistor. If we short the input terminal to the common terminal we should expect about 5 to 15 mA to flow (determined by the internal series resistor).

In Figure 1. The opto-LED anode is connected to +24 V supply and the cathode is connected through the resistor to the input pin. When the input pin is connected to COMM- the LED will light giving a logic '1' to the PLC CPU. We can, of course, use a switch contact to do this but many interfaces will use a transistor. Either way, the PLC input provides or sources the current through the LED (red arrow) and is known as a "sourcing" input. Since NPN transistors can be very easily switched in this configuration they are generally used - hence "NPN" inputs.



One major advantage of this arrangement is that the transistor can be powered from a supply of different voltage to the PLC - e.g., a 5 V micro-controller and once it shares the common negative it effectively becomes a level shifter between the two systems.

The main disadvantage is that the logic is somewhat inverted. A high voltage on the input is logic 0 and a low voltage is logic 1. This can be confusing.



Figure 2 shows the PNP / sinking circuit. Here current flows from the + supply, through the transistor and the PLC "sinks" it.

The logic is the right way up now and this style of input is preferred on most industrial equipment at present.

For outputs the situation is similar. A current sourcing output will supply the current from the + supply, through the load to the COMM-. For a current sinking output the current will flow from the + supply, through the load, into the PLC input where an NPN transistor will "sink" it to COMM-.

Note some PLCs use bi-directional opto-isolators - two LEDs connected in opposite directions. By connecting the input common terminal to + or - supply the inputs can be made sourcing or sinking.