I. PLC Input Internal Circuitry
The internal circuitry of a PLC input typically employs optocoupler circuits, as shown in the diagram below (common cathode). This is done to isolate the external circuitry from the PLC's internal circuitry, thereby preventing interference from external circuits. This diagram only shows one PLC input; the other inputs are similar, and the common terminal (COM) of all inputs can be connected together or divided into several groups for shared use.
It's worth noting that the common terminal can be either the anode or cathode of the LED connected together. Depending on the connection of the LED's COM terminal, it can be classified as "common anode" or "common cathode". For example, the input circuit of Mitsubishi FX series PLCs uses a "common anode" connection, while the COM terminal of Siemens or Delta PLCs is left floating. Users can choose between "common anode" or "common cathode" based on their actual needs or habits.
As can be seen from the diagram, in order for a certain input terminal of the PLC to have an input, the two ends of the optocoupler LED must form a circuit. That is, when the COM terminal is connected to "+", the input must be introduced with a "-" level (common anode); when the COM terminal is connected to "-", the input terminal must be introduced with a "+" level (common cathode).
II. Forms of PLC Input External Circuits
There are three types of external node forms for PLC input external circuits:
1. Passive node input, i.e., switch node input.
2. Input of NPN and PNP nodes
3. Diode input
Below is a brief explanation of the three types of node inputs and their wiring methods.
1. Passive node input (digital input)
This type of node configuration is the most commonly used input configuration for PLCs. When using this configuration, simply pay attention to whether the PLC's input common terminal is common anode or common cathode. If it's common anode, the terminal introduced through the switching node should be negative; if it's common cathode, the terminal introduced through the switching node should be positive. See the diagram below (the part in parentheses indicates common anode):
2. Input of NPN and PNP nodes
Some sensors or proximity switches have NPN or PNP output nodes. In this case, whether to select an NPN or PNP node as the PLC input depends on both the PLC's wiring configuration and the sensor's or proximity switch's wiring configuration. The following example illustrates this: As shown in the diagram, the sensor's output is NPN. From the load wiring diagram, we can see that when the sensor activates, it outputs 0V (black line ④). This requires the PLC's common terminal (COM) to be positive. Therefore, for this circuit, when the PLC's common terminal (CON) is connected to the positive terminal, the PLC input can only be in NPN form.
The diagram below shows the opposite scenario. When the sensor activates, its output is positive (black line ④). In this case, the PLC's common terminal (COM) must be connected to the negative terminal. Therefore, for this circuit, when the PLC's common terminal is connected to the negative terminal, the PLC input can only be in PNP form.
Whether a PLC's input nodes use PNP or NPN is not something you need to memorize. You only need to understand the internal circuit principles of the PLC input: whether PNP or NPN nodes are used, the LEDs in the optocoupler section of the PLC input circuit must be energized.
The above two examples use Siemens PLCs. The common terminal of the optocoupler in the internal circuit of a Siemens PLC can be either common cathode or common anode. Therefore, when considering using NPN or PNP inputs, the positive or negative terminal of the common terminal (COM) can be changed to use them respectively. However, for Mitsubishi FX series PLCs, since the common terminal of the optocoupler is fixed to be common anode, the common terminal can only be connected to the positive terminal, and the input can only use the NPN node input method.
3. Series diode input
Sometimes, it's necessary to insert an LED in series with the input node of a PLC for indication. As shown in the diagram below: In this case, PLCs typically specify the allowable voltage drop of the LED and the allowed number of LEDs. For example, the FX series PLC shown in the diagram specifies a allowable voltage drop of 4V for the LED, and a maximum of two LEDs can be inserted in series at a time.
