The significance of PLC output and how to implement it

The input of PLC, the so-called input, is people commanding PLC to do things, and these commands are implemented through switches, buttons, proximity switches, etc. The output is that PLC drives machinery and equipment, which is achieved by relays, thyristors, and transistors. And these are the three types of PLC output. Today, I will tell you about the meaning of PLC output and how to achieve it.
The output of PLC is various circuits inside, and we as users see various wiring terminals.

As shown in Figure 1, the row of screws on the left side of the screen is where we connect the output. All PLCs are of this type, except that the arrangement is different and the screws are different. From the appearance alone, we cannot distinguish the type of PLC output. Generally, it is distinguished by 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 and can only connect to DC loads.

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

For example, the model of the PLC in Figure 1 is K7M-DRT20U, and the letter RT is the letter that represents output. It means that this PLC has a mixed output of relays and transistors. Most PLCs are represented by these letters. The most widely used Siemens PLC is represented by abbreviations, such as DC/DC/RLY, which represent the types of power input and output respectively. Obviously, RLY represents relay output. Transistor output can send high-speed pulses, which are generally used to control servos. There are two connection methods: PNP and NPN. Thyristor output can be directly connected to AC loads, which are generally 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 disadvantage is that the response time is slow (relative to transistors) and there is a mechanical life (transistors only age, but 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 output.

Figure 2 PLC Manual

As shown in Figure 2, 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.

Figure 3 Wiring diagram

As shown in Figure 3, 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 routines of PLCs. The relay output routines described in this article are common to all PLCs. I hope beginners can understand them carefully and draw inferences from them.