One day, Xiao U was engrossed in his work in the lab when Dr. Yi walked in. Dr. Yi: "Xiao U, what are you busy with?" Xiao U: "I'm wiring, Dr. Yi. I've encountered a problem, and I hope you can help me figure it out." Dr. Yi: "What problem?" Xiao U: "No matter how I wire the PLC's output points, it can only control one input point of the servo drive at a time." As Xiao U spoke, he showed Dr. Yi the products on the lab table. On the table was a CPU, model 224-1AD; next to it was a servo drive, which looked a bit old; and a 24V DC switching power supply. Several control and power cables were crisscrossed in between. "That's strange?" Xiao U's description piqued Dr. Yi's curiosity. "Yes, I want to use two output points of the 224-1AD, such as Q1.0 and Q1.1, to control two input points of the servo drive, such as DI1 and DI2, respectively, but I can't do it," Xiao U said. “Don’t rush, let’s start with the input point type of this servo drive,” Dr. Yi said. “Xiao U, go get the manual for this servo drive for me to look at.” A moment later, Dr. Yi asked, “Xiao U, where did you get this servo drive?” Xiao U ran out of the lab and returned with a manual. Xiao U handed the manual to Dr. Yi, who flipped to page 35 – Input Point Types – and began reading carefully. “I found this servo drive in the company’s storage room, and I thought I’d use it for experiments,” Xiao U replied. “This servo drive has been on the market for quite a while, hasn’t it? I estimate it’s 5-6 years old, and that’s exactly the reason for the problem you just encountered,” Dr. Yi said. “Dr. Yi, I don’t quite understand. Does my wiring have anything to do with the product’s history?” Xiao U asked, puzzled. “Of course. Some servo drive products, due to insufficient consideration of factors during early development and design, have wiring methods that differ significantly from current methods. If this isn’t taken into account in practical applications, it can cause the device to malfunction,” Dr. Yi explained. "Little U, look, the input point wiring diagram in this servo driver manual is shown in Figure 1. The area to the left of the red line represents the external wiring needed for corresponding function control; the area to the right shows the internal circuit structure of the driver. This part of the circuit uses an electronic device called an optocoupler, or simply an optocoupler. Figure 2 is a circuit diagram of the optocoupler. Little U, do you know how this optocoupler works?" Dr. Yi asked. "Dr. Yi, I think it works by using light to transmit signals. Why? Because as you can see in Figure 2, pins ① and ② of the optocoupler are the positive and negative terminals of the LED. When external conditions are met, the LED will conduct and emit light. Pins ③ and ④ of the optocoupler form a transistor that can receive light. When light is transmitted, this transistor will conduct, meaning pins ③ and ④ are shorted together. At this point, the internal controller can detect changes in the external signal," Xiao U replied. "Very good, Xiao U's answer is excellent!" Dr. Yi gave a thumbs up. “As everyone knows, LEDs have unidirectional conduction characteristics. For the LED in the optocoupler to conduct, the external conditions must be met: pin ① must be positive and pin ② must be negative, meaning the current flows from pin ① to pin ②. Only then can the optocoupler achieve its normal operating state and emit light. Otherwise, the optocoupler in Figure 2 will not work properly. Taking DI1 in Figure 1 as an example, when the SON switch is closed, the corresponding internal optocoupler conducts. The internal controller detects this conduction state and executes the command to enable the servo driver. Little U, can you draw the direction of the current flow at this time?” Dr. Yi asked. “Of course,” Little U said, picking up his pen and drawing, as shown by the red arrow in Figure 3. “When SON is closed, the current flows out from the positive terminal of the DC 12-24V power supply, through the 1K current-limiting resistor, into the positive terminal of the LED in the optocoupler, and finally out from the negative terminal of the LED, returning to the negative terminal of the power supply, forming a closed current loop,” Little U explained. “Excellent!” Dr. Yi nodded. “For any circuit to function properly, the current must form a closed loop. Similarly, when the other input points of DI2 and DI3 are closed, the current flows in the same direction as DI1. However, there is one thing to note: the internal circuit of the servo driver in Figure 1 short-circuits the pin 1 of all the optocouplers together, which limits its application.” Dr. Yi said. “What kind of limitation does it bring?” Little U asked. “Because the optocoupler in Figure 2 has unidirectional conductivity and all pin 1 are short-circuited together, in practical applications, COM+ in Figure 1 can only be connected to the positive terminal of the power supply to make the optocoupler conduct. If COM+ is connected to the negative terminal of the power supply, the optocoupler cannot conduct regardless of the operation of the external switch. See Figure 4,” Dr. Yi said. “I see. But in actual wiring, we always connect COM+ to the positive power supply because there’s a '+' symbol in COM+, which I believe many engineers can recognize,” Xiao U said. “Connecting COM+ to the positive power supply is the correct connection, nothing wrong with that. But if you want to use the output points of the 214-1AD to control multiple input points of the servo driver in Figure 1, as you just described, problems will arise,” Dr. Yi said. “Hmm. Let me think about it,” Xiao U said. “The output point circuit of the 214-1AD is shown in Figure 5. As you can see, when you write 1 in the programming software, the internal switching transistor of the PLC will conduct, and Q1.0 will then output 24V to the external load.” “Your analysis is correct, Little U. Based on Figure 5, draw the current flow when Q1.0 has an output,” Dr. Yi said. At this moment, Little U picked up his pen again and started drawing, as shown in Figure 6. “When Q1.0 has an output, the current flows out from +24V, passes through the switching transistor, flows out from the Q1.0 terminal to the load, and then flows into the ground of the 24V power supply, forming a closed loop,” Little U explained as he drew the diagram. “Yes, as we can see from Figure 6, when the 214-1AD output point has an output, the current flows out from the output point. It is this characteristic that causes the problem you just encountered. Little U, now you can combine Figure 6 and Figure 3 to see where the problem is. You can try controlling one point individually first, and then control multiple points simultaneously,” Dr. Yi said. “Okay,” Little U replied. “As Dr. Yi just said, the current can only flow out from the 214-1AD output point; due to the unidirectional conduction characteristic of the optocoupler in the servo driver, COM+ is the common terminal of the servo driver input point, and the current can only flow into the internal circuit of the servo driver from COM+. Therefore, in the case of controlling one point individually, the output point Q1.0 of the 214-1AD must be connected to COM+, and the input point DI1 of the servo driver must be connected to the M terminal of the 214-1AD to ensure that the current forms a closed loop and to control DI1 normally. Just like the blue and red wiring in Figure 7.” “Yes! Figure 7 shows the control of a single input point DI1. What if we want to use Q1.0 and Q1.1 to control both DI1 and DI2 simultaneously? What will happen?” Dr. Yi asked. “Ah? Dr. Yi, I know!” Little U seemed to understand something and became extremely excited. “Dr. Yi, the principle and wiring method of Q1.1 controlling DI2 and Q1.0 controlling DI1 are the same, as shown in Figure 8. As you can see, Q1.0 and Q1.1 are actually connected together, which means we can't achieve the goal of controlling DI1 and DI2 separately. As long as either Q1.0 or Q1.1 is conducting, it will cause both DI1 and DI2 to have signal input simultaneously.” "Haha, Little U is really amazing." Dr. Yi gave another thumbs up. "There are two main reasons for this phenomenon. One is that the internal circuit of this servo driver uses an optocoupler with unidirectional conduction characteristics. The other is that the internal circuit of this servo driver connects all the pins 1 of the optocouplers together as a common terminal COM+." " Why is this servo driver designed this way? This design will cause very troublesome applications. Didn't the engineers who developed this servo driver know that this would cause such a situation?" Little U asked doubtfully. "Of course, the engineers had their considerations for this design. First, in terms of the number of terminals, connecting all the pins 1 of the optocouplers together as a common terminal COM+ can save the number of terminals and avoid wiring trouble. Little U, there are currently nine input points, DI1-DI9. Through the common terminal, only 10 terminals are needed. If all the pins of the optocouplers are brought out individually, how many terminals would be needed in total?" Dr. Yi asked. "A total of 18 terminals," Little U replied. "Yes, the number of terminals increases, and it also brings trouble to the application wiring," Dr. Yi said. "Dr. Yi, if that's the case, why didn't the design and development engineers connect all the pins 2 of the optocouplers, which are the negative terminals of the LEDs, together as the common terminal COM-, and define the positive terminals of the LEDs as the input points? This way, the problem we just encountered wouldn't have happened," Xiao U asked. "Xiao U, that's a good question. If we followed what you just said, we could indeed avoid the problem we just encountered." What did you learn from Xiao U's conversation with Dr. Yi? Share it with Xiao U! You can ask Xiao U any questions in the UniMAT subscription account (unimat2004)! To learn more about Xiao U's story, please follow UniMAT!