Disassembly of domestic PLC: Is it truly domestic or a foreign product disguised as a domestic PLC?

PLC is what we usually call programmable logic controller, which is widely used in the field of industrial automation control. Its components mainly include power supply, processing control unit, memory, input and output unit.

The well-known PLC products on the market are basically designed and manufactured by foreign companies, such as Siemens, Schneider, Mitsubishi, Rockwell, Omron, etc. Although the quality of these products is relatively reliable, they are relatively expensive. In addition, in today's world where domestic products should be self-reliant, do we have to rely on foreign PLC products? With such a doubt, this issue of Hard Core Review will dismantle a domestic PLC, a product with good sales and reputation, and the price of the product is only half of that of foreign PLC products with similar functions - Xinjie's XC3 series PLC.

Exterior

The specific model of the XC3 series PLC disassembled this time is XC3-24R-E.

Before disassembling, let's take a look at the design structure and functions of this PLC.

• Red: Input terminal

• Green: Output terminal

• Yellow: Input/Output/Status indicator

• Gray: BD expansion interface

• Cyan: module expansion interface

• Black: RS232, RS485 communication interface

Disassembly

Because it is fixed directly with a snap-on structure, it is very convenient to disassemble. Below is a family photo of the entire product after disassembly. Except for a screw on the power board that fixes it to the bottom case, no additional tools are required.

Xinje XC3 PLC is divided into three boards in PCB design, from left to right they are power board, IO port board and main control board.

Power Board

Let’s start with the power board. First of all, this product is powered by 220V AC, and then needs to output a 24V DC voltage, so the entire power board is responsible for this part of the work.

Because there is a lot of space left for the power board, the design is relatively easy. This abundant PCB space also makes the entire power supply very simple and reliable, and the heat dissipation is also guaranteed. The board is a two-layer board, and the power structure is also very clear. The red line in the above picture distinguishes the primary and secondary circuits of the power supply.

The primary circuit first converts AC power into DC power through a filter network composed of fuses, capacitors and inductors, and a bridge rectifier network (here you can see the KPB2010 bridge rectifier), and then achieves step-down output through the ON Semiconductor power switch device (model KA5H0380R , with an internal integrated PWM controller) and transformer on the board;

The secondary circuit includes the rectifier circuit, filter circuit and feedback circuit of the power output part. However, the more efficient synchronous rectifier circuit design is not used in the rectifier circuit. It is probably because the output voltage is relatively high, and the improvement effect of synchronous rectification is not too great. Of course, it may also be a compromise made by taking into account the cost of the entire power module. As for the feedback network part, the voltage divider resistor sends the divided voltage at the output end to the precision voltage regulator 431 for comparison with the reference voltage, and then converts it into a current signal through the optocoupler and feeds it back to the ON Semiconductor power switch device on the primary side. This constitutes a "circulation system" of the entire power board, which continuously adjusts the output 24V voltage regulator.

As for the back side of the power PCB, there is no device layout.

IO interface board

The IO interface board is mainly the input and output unit of the PLC. The optical coupler isolation is used at the input end (different types of optical couplers are used, one of which is 6N137 ). Through the PCB wiring on the board, it can be known that the signal output from the optical coupler is passed to the main control board through the interface row socket (left side) on the IO interface board;

Secondly, we can also find on this IO interface board that this PLC is based on relay output. Here we use Omron relays (model G5NB-1A-E-DC24 ). In addition, if the output is to directly drive the relay, the general microcontroller or IO port does not have such a strong driving capability, so STMicroelectronics' compound transistor driver (model ULN2003 ) is used here. This is also called a Darlington tube, which integrates 7 groups of NPN transistors. The driving capacity of each group of transistors is 50V/500mA, so it can be used directly to drive the relay.

In addition, a TI RS485 transceiver (model  DS3696A , ) can be found on the IO interface board, which is used to build the RS485 communication circuit of the PLC.

On the back of the IO interface board, you can see the PCB design of its circuit. What is more surprising is that we found that there are 3 power signals from the interface coming from the power board. We can probably guess that one is the natural ground on the IO port board itself, which is used to improve the EMI effect, one is the 24V wet-connected power supply for the PLC output end, and it is not difficult to guess that the other 24V will be used for the step-down output of the main control board to provide different voltage power supplies to various chips on the main control board.

main control board

• Black: Model DS1308Z, Maxim RTC clock chip

• Yellow: Model ADM3232E, ADI RS232 transceiver

• Gray: Model DS28E11, Maxim's dedicated encryption chip EPROM

• Red: Model R5F36506CDFA, Renesas M16C/60 (16-bit) MCU

• Orange: Model R1LP0108ESN, Renesas SRAM, 1Mbit

• Purple: Model SST39SF020, Microchip NOR Flash, 2Mbit

• White: Model M4A5-128/64-10VNC, Lattice programmable interface device

• Cyan: Model 74HC14, Nexperia Schmitt trigger

Finally, let's take a look at the main control board. The hardware circuit design of the main control board is relatively complex, but we can easily understand the hardware circuit layout and design logic of the entire XC3 PLC by combining the front and back of the main control board and the IO interface board.

First of all, the components on the front of the main control board are basically painted with three-conformal paint, which can improve the product's dust, moisture and humidity resistance, and improve product reliability and stability. However, from what we have disassembled, only the main control board has been painted. This is like you did a good thing but only did half of it and didn't continue, which makes people feel like a thorn in the throat.

The core of the main control board is Renesas' M16C/60 (16-bit) microcontroller, which has very good EMI. The memory part uses Renesas' SRAM, Microchip's NOR FLASH and Maxim's dedicated encryption chip EPROM. The Lattice IO interface device is responsible for the input and output units. On the IO interface board above, we mentioned that it is not appropriate to directly connect the signal from the input terminal through the optocoupler to the digital IO port, because the output signal has a long rise time if the optocoupler is isolated. If you want to connect a digital I/O port, such as FPGA, it is best to connect a Schmitt trigger for shaping or waveform flipping, and increase the driving capability. So here we can see from the layout of the main control board circuit that before connecting the IO interface device, it will pass through a Schmitt trigger (model 74HC14, cyan).

In addition, you can also see the hardware design of other circuit modules on the board. The RS232 communication circuit is built through the ADI RS232 transceiver, the RTC circuit of the board is designed through the Maxim RTC clock chip, and the IO interface input and output circuit also includes the input and output LED circuit design. In addition, the external function expansion module of the board is expanded through the Lattice programmable interface device, etc.

By disassembling the XC3 PLC, we can roughly infer that its hardware circuit design block diagram is as follows:

The BOM table of the main components of the entire PLC is as follows:

summary

Although the author has not further tested the function and reliability of this product, judging from the feedback from engineers who have used it and the sales volume of this product, it is a relatively successful product. From the actual disassembly, the product's design circuit structure and hardware circuit design logic are quite representative, the workmanship design of the main control board and IO board is relatively solid, and the workmanship of the power board is a bit rough, but the flaws do not outweigh the merits, and it can be said to be a good product.

At the same time, it was also found from the actual disassembly that although it is a domestic PLC product, the important components used inside it are basically devices from internationally renowned semiconductor manufacturers, including Lattice, ADI, ST, Omron, Renesas, ON Semiconductor, Ansem, Microchip, TI, Maxim, etc., and there are basically very few domestic devices, or some not very important domestic resistors and capacitors may be used. While everyone is talking about the independence and self-reliance of domestic chips, we must also recognize our own shortcomings. The road to the rise of domestic chips is still long.