How to understand the input signal of PLC? Wiring and implementation of NPN and PNP logic

PLC (Programmable Logic Controller) is a technology that combines software and hardware, because the programs we write require external wiring to be implemented, and a PLC with only wiring but no program has no engineering significance. Therefore, when starting to learn PLC, you must correct a concept, that is, PLC is not just programming, but also includes the peripheral circuits of PLC.

PLC wiring can be divided into two parts, one is power wiring, the other is IO wiring. Power wiring is very simple. Confirm the power supply of PLC. PLCs on the market generally have two specifications of power supply, DC24V and AC220V. Before wiring, be sure to confirm the power supply of PLC, otherwise it will be a puff of smoke. Generally, the power supply part of PLC will be marked. You can refer to the hardware manual for details. Generally, there will be a simple instruction in the PLC packaging box. It should be noted that the ground wire must be connected and cannot be omitted.

IO wiring is the focus of PLC wiring, that is, the input and output wiring of PLC, which includes digital and analog quantities. From the application point of view, the so-called PLC input is the command signal sent by people to PLC. These commands are implemented through various switches, buttons, limit switches, temperature sensors, potentiometers, etc.

Let's first explain the wiring of digital quantities. It includes two forms, PNP and NPN. In fact, it is very simple. You can connect it according to the manual. However, many beginners do not have the patience to read the manual, which is wrong. The difference between these two wiring methods is that the connection method of the common terminal is different. PNP is also called positive logic, source connection, high level is valid. NPN is also called negative logic, drain connection, low level is valid.

1. How do beginners understand the PLC input signals, as well as the wiring and implementation of NPN and PNP logic?

As shown in the figure above, this is the connection method in the hardware manual of Schneider PLC. From the figure, we can see that 0V and COM0 are short-circuited. When the switch on I0 is closed, input I0 will have 24V input. Obviously, this is a PNP connection. So, how do we apply it in practice? You can refer to a drawing:

You can take a look at what is the difference between this and the manual? By the way, the drawing does not use the PLC's DC24V power supply, but an external power supply. Generally, this solution is adopted in practical applications, because when the number of inputs is too large or the external wiring is too long, the power provided by the PLC is relatively low and overload may occur. Therefore, it is recommended that everyone is equipped with a DC24 V switching power supply.

The next step is on-site wiring. Once the wires are connected according to the drawing, the input part of the PLC is completed.

The output of PLC is the PLC driving the machine equipment, which is realized by relays, thyristors and transistors. These are the three types of PLC output. Today, I will tell you the meaning of PLC output and how to achieve it.

The output of the PLC is various circuits internally, and what we as users see are various wiring terminals.

As shown in the picture above, the row of screws on the left side of the screen is where we connect the output. All PLCs are in this form, except that the arrangement is different and the screws are different. We cannot distinguish the type of PLC output from the appearance alone. Generally, we distinguish them from the model. The PLC model usually has letters indicating the output characteristics.

R is the abbreviation of Relay, which means relay output, which can connect AC and DC loads;

T is the abbreviation of Transistor, which means transistor output, which can only connect to DC load;

S is the abbreviation of Silicon, which means thyristor output can only be connected to AC loads;

For example, the PLC model in the figure is K7M-DRT20U, and the letters R and T represent output. It means that this PLC has a mixed output of relays and transistors. Most PLCs are represented by these letters, and the most widely used Siemens PLC is represented by abbreviations, such as DC/DC/RLY, which represent the types of power supply/input/output respectively. Obviously, RLY means relay output.

Transistor output can send high-speed pulses, usually used to control servos, and has two connection methods: PNP and NPN. Thyristor output can be directly connected to AC loads, but is rarely used. The most commonly used is relay output, which is exactly the same as the relay we usually use. It has a high cost-effectiveness and can be connected to AC and DC loads. It is just a contact, so it is not divided into NPN and PNP.

The disadvantages are: slow response time (compared to transistors), and mechanical life (transistors only age, not life). So, how do we connect according to the manual? Let me take Schneider TM218LDA40DRPHN as an example to explain to you. From the model, it is a relay type output.

As shown in the picture, this is the hardware manual of the PLC. Looking at the yellow highlighter part, I have drawn an output circuit. From this, we can see that the essence of the PLC relay output is a switch, and the COM terminal and the output terminal form a switch. When Q8 has an output, COM and Q8 are connected, which drives external components, such as relays, indicator lights, etc. The relay can drive components such as motors, inverters, cylinders, etc. to realize machine action.

As shown in the figure, this is the actual wiring. The red circle part is the external power supply, and the yellow highlighter part is the circuit of the output point Q8. The current flows from the 24+ terminal on the right into COM3. When Q8 has an output, COM3 and Q8 are connected, driving the relay, and the host 3 will start. Therefore, the output of the PLC is actually the connection between the output point and the COM terminal. Similarly, when Q9 has an output, it is the connection between the COM terminal and Q9, and when Q12 has an output, it is the connection between COM4 and Q12. It should be noted that the output terminal of the PLC has more than one common terminal COM, and it must be used separately.

Summary: Regardless of the brand, PLCs are essentially the same. When learning PLCs, don’t learn for the sake of learning, or for the sake of learning a certain brand. Instead, learn the principles and implementation methods. This way, even if you change to another PLC, you can easily master it. Therefore, you still need to master the general routines of PLCs from manuals to drawings. I hope that beginners can understand them carefully and draw inferences from one example.