What is the difference between a two-wire interface and a PNP/NPN interface?

Two-wire interface

The two-wire interface is a type of interface used in automation and control technology to connect sensors and actuators.

With a two-wire interface, the sensor or actuator is connected between two wires: the positive power supply wire (+) and the negative signal return wire (-). The power supply wire is used to supply power to the device, while the signal return wire is used to transmit the signal back to the control system.

When the sensor or actuator in a two-wire interface is activated, it interrupts or connects the connection between the two wires and generates a signal.

The two-wire interface is typically used for simple sensors and switches that only require a single binary output. This type of interface is relatively easy to implement and is used in various applications such as presence detection.

It is important to note that the two-wire interface does not provide the ability to transmit additional information or communicate with the control system. If more information is to be transmitted, e.g. an analog signal, a different type of interface is required.

The two-wire interface can be polarized/non-polarized with the switching function normally open/normally closed.

Example of a non-polarized, normally open two-wire interface:

Example of a polarized two-wire interface with normally closed switching function:

PNP/NPN interface

Most industrial proximity sensors (inductive, capacitive, ultrasonic, photoelectric) are semiconductor-based.

Electronic semiconductor components, such as transistors, are used to switch the sensor output when an object is detected.

There are two specific types of three-wire sensors: PNP and NPN. The difference lies in the internal circuitry and the type of transistors used.

With the PNP interface, the output pin of the sensor, when activated (when it detects the presence of an object), provides a voltage close to the supply voltage (usually around 24 V DC in industrial applications). When the sensor is not activated, a low or no voltage is present at the output pin.

PNP devices usually have three connections: Power (+), ground (-) and output. The output is connected to the load, which in turn is connected to ground. This means that the load is activated when a voltage is applied to the output.

Example of a normally open PNP interface:

An NPN sensor works in such a way that when it detects a target object or a target state, it closes the circuit between the output of the sensor and ground. This means that the output of the sensor is connected to ground (negative voltage of the source).

When the sensor is activated (when it detects an object), the output of the sensor is connected to ground, causing the output voltage to drop to a low value or 0V.

Example of a normally closed NPN interface:

Essentially, a PNP sensor connects the load to power, while an NPN sensor connects the load to ground.

Note: When connecting to a PLC, it should be noted that the sensor must have the output type that the PLC expects. The input stage of the PLC normally recognizes the state of the sensor (on or off). However, it expects either a PNP or an NPN sensor.

Transition from a two-wire interface to PNP/NPN

When switching from a two-wire interface to a PNP/NPN interface, it is important to check the compatibility of the device with the new interface. Some sensors and actuators may only be two-wire and cannot simply be converted to a PNP/NPN interface.

If the device is compatible with a PNP/NPN interface, the power supply requirements must be determined. The positive (+) power supply line must be connected to the power source and the negative (-) signal line to the device. It must be ensured that they are correctly connected and insulated.

When converting to an NPN interface, it is important to connect the ground (GND) to the device. The ground serves as a reference point for signals in an NPN interface.

The device settings must be checked for this. Some devices have the option of switching between different interfaces via a switch or a configuration interface. It must be checked that the device is correctly configured for the PNP/NPN interface.

Once the changes have been made, the device must be tested and checked to ensure that it functions correctly with the new interface. It must be remembered that additional changes to the system may be necessary, such as changing the inputs/outputs of the control system or adapting the program.