1. PLC, IPC, PC-Based PLC: With the rapid development of PC technology, IPC (Industrial Control Computer) and IPC-based application technologies have also experienced rapid development. Simultaneously, with the application of Internet technology and the integration and development of all production and control information processes, and the ability to browse manufacturing processes, operate and monitor intelligent field devices via the Internet/Intranet, IPCs are increasingly undertaking human-machine interaction control tasks for SCADA and coordinating control tasks with lower-level small controllers or intelligent field devices. Overall, IPCs are still the most suitable for automation control platforms. However, as a traditional mainstream controller, the PLC possesses advantages such as good stability, high reliability, and strong logical sequence control capabilities, giving it an irreplaceable advantage in the field of automation control. However, a major drawback is its closed architecture and closed system (requiring developers to have their own or OEM-developed CPU, chipset, BIOS, operating system, and ladder diagram programming software), and poor openness, which inevitably create barriers to its application and increase the difficulty of user maintenance and integration costs. Some predict that in the near future, PC-based controllers will gradually replace PLCs as the mainstream control devices. To address this, traditional PLC manufacturers are gradually PC-ifying the functions of their PLCs (such as Siemens' WinAC), while IPC manufacturers are gradually PLC-ifying the logic control functions of their IPCs, making PLCs and IPCs increasingly similar in function and specifications. This has led to the emergence of intermediate controllers based on PLC and IPC technologies: PC-Based PLCs. PC-Based PLCs, also known as embedded controllers, no longer rely on a chassis and motherboard as the main structure, combined with functional I/O boards such as A/D, D/A, and DI/DO. Instead, they are independent, dedicated systems based on embedded PC technology, suitable for small-scale SCADA systems. For example, ICP DARPA's I-8000 series features a 40MHz 80188 CPU in its main unit, running on the DOS-compatible MiniOS 7 operating system. Its programming environment is based on standard C language programming for PCs, and the program development process is extremely similar to that of a PLC: first, a resident task program is written on the PC, compiled, and then transferred to the main unit's Flash memory for offline execution. Furthermore, to leverage the advantages of PLCs, PC-Based PLCs can also use ladder logic programming, such as ICP DARPA's ISaGRAF (with the I-8417/8817 main unit). Compared to PLCs, PC-Based PLCs offer the powerful computing, data processing, and communication capabilities of IPCs. In terms of software, PC-Based PLCs support five international standard languages ​​and soft logic, including IEC-61131-3 (LD, SFC, FBD, IL, ST). Due to these characteristics, PC-Based PLCs will be more open and standardized, capable of adapting to more complex integrated control and management information needs. In general, IPC is an open architecture and open system, PLC is a closed architecture and closed system, and PC-Based PLC is in between, being an open architecture and a closed system. Strictly speaking, IPC generally undertakes management and control tasks and coordinates the control tasks of lower-level small controllers or intelligent field devices, while PLC is generally used as a local controller. Due to the alternating development of PC technology, information technology, and communication technology, the investment in the research and development of PC-Based PLC has been relatively reduced, and more manufacturers will jointly promote the development of PC-Based PLC. Therefore, PC-Based PLC has a very good development prospect, but this does not mean that PC-Based PLC will replace PLC in the short term. PLC and PC-Based PLC will gradually move towards integration in the process of competitive development [1, 2]. [b]2 Application Techniques of PC-Based PLC Architecture Systems[/b] 2.1 AI Module The number of AI (Analog Inputs) has a significant impact on the real-time performance and stability of the system, especially when there are many AI modules. The main reason is that the CPU of the I-8000 module is merely an 80188 controller with a clock speed of only 40MHz. Its data processing capabilities and storage space are limited, resulting in slower computation, logic processing, and event response compared to an IPC. Since the CPU must perform a series of processes—sampling, holding, synchronization, conversion, storage, processing, and computation—to complete an A/D conversion, it is relatively time-consuming. Therefore, when there are many AI channels to process, it will inevitably affect the real-time performance of sampling and the stability of the system. Generally speaking, it is best not to use more than two AI modules, such as the I-8017H series, in a single I-8000 module. 2.2 Relay Output Module The relay output module has the greatest impact on the entire system. Improper handling can lead to system crashes, frequent shutdowns, and motherboard burnout. Since the I-8000 module's power supply is typically 10-30VDC, with a total input power of 20W, unlike the 250W input power of an IPC, if too many relay output modules, especially high-power relay modules, are inserted, insufficient system power will cause abnormal outputs, frequent malfunctions in the control system, system crashes, shutdowns, and even burnout of the main control board. This creates numerous hidden dangers to the system's stability, safety, and reliability. Generally speaking, no more than two of modules such as I-8060, I-8058, I-8063, I-8064, I-8065, I-8066, I-8068, and I-8069 should be used, especially power modules like I-8060, I-8063, I-8064, I-8065, and I-8069, which are best used as a single unit. If the system needs to control a large number of power relays, ordinary opto-isolated switch input/output modules such as I-8042 can be used, connected using the principle of multi-stage amplification. 2.3 Communication Processing In a PC-based PLC architecture control system, one of the most crucial aspects is the real-time data communication process with the host computer. This process often limits the system's real-time performance and stability, and is prone to data bottlenecks. Because host computers typically run on Windows operating systems, and applications generally have human-computer interaction interfaces and real-time display interfaces, these interfaces are often designed as foreground windows, while data communication, analysis, and storage are designed to run in the background. However, Windows is not designed as a real-time operating system; it is a preemptive, multitasking operating system based on message passing mechanisms. Clearly, message scheduling alone cannot meet the requirements of a real-time system and cannot guarantee accurate and real-time completion of foreground and background control tasks. Therefore, in a Windows environment, employing multithreading technology can effectively utilize Windows' waiting time, accelerate program response, and improve execution efficiency. Using one thread to manage computer data communication and another thread to perform data processing, analysis, and storage enhances the real-time performance of system event response and communication control while ensuring continuous data acquisition. PC-based PLCs and host computers typically use RS-485, CAN, ModBus, or Ethernet. If RS-485, CAN, or ModBus is used, the communication ports must be allocated appropriately. Generally, RS-485, CAN, and ModBus communication adapters have two ports. Therefore, if the control system has two I-8000 modules, the host computer can use one communication port to communicate with both lower-level controllers. However, if there are four, six, or more, it's best to divide them into two groups, with the host computer using two communication ports to communicate with each group separately. The host computer uses two threads to write the communication program. See Figure 1 for the configuration diagram. Figure 1: Configuration Diagram 2.4 Power Supply Configuration If a control system has multiple I-8000 modules, considering system economy and safety, it's best to share a single switching or linear power supply between every two I-8000 modules. Considering the power consumption of the power supply itself, the power supply must be greater than 60W, and each power module should be connected to a ~220VAC or ~380VAC power supply separately. Never connect them in series. When selecting a switching power supply, it is important to choose one with a system power factor greater than 0.99, ripple voltage Vrms ≤ 1.0%, ripple coefficient ≤ 0.2%, high power density, good electromagnetic compatibility, and low ripple. Simultaneously, separating the power supply to the controller I/O channels and other devices using their own isolation transformers helps improve the anti-interference capability of the control system. 2.5 Signal Grounding: Proper and good grounding can introduce interference signals mixed into the power supply and I/O circuits to the ground, eliminating or reducing the impact of interference. This is an important means of safety protection and noise suppression, and is extremely important for improving the stability and reliability of the I-8000 system. To minimize the impact of electromagnetic noise, the power supply circuit and control circuit should have separate grounding electrodes. In control systems, power devices such as frequency converters are inevitable. Ensure that the frequency converter heat sink, power supply neutral line, frequency converter casing and neutral terminal, motor casing and Y-connection neutral terminal are reliably connected to the power supply circuit grounding electrode, and all grounding wires must not form a grounding loop. The lower the grounding resistance of the frequency converter, the better. The cross-sectional area of ​​the grounding conductor should not be less than 4mm², and the length should be controlled within 20m. The shielding layer and digital signal ground are connected to the grounding electrode of the control circuit. To prevent the formation of a loop, the shielding layer should be grounded at one end. The grounding wire of the controller should be separated from the power line and the power supply line. It is best to ground the I-8000 separately, or it can share a common ground with other equipment, but it is strictly forbidden to connect it in series with other equipment for grounding. 3 Practical Application Case In small oil companies, a large amount of oil metering work is required, such as light oil, 0# gasoline, 90# gasoline, etc. The metering process often involves the truck fleet weighing the gross weight at the company's weighbridge after transporting the goods from the freight station, unloading, and weighing the vehicle again when leaving the factory. The weighing process, procedures, and registration are extremely cumbersome, and sometimes errors and omissions in weighing occur. It is very difficult to manage and brings great difficulties to the statistical and metering work. The labor intensity of the weighing workers is high, and there are often queues of truck fleets at the weighbridge. The efficiency is extremely low. To change this situation, a PC-Based PLC is adopted. The I-8411 embedded controller, equipped with an I-8017H analog signal input module, an I-8024 analog signal output module, an I-8042 opto-isolated digital input/output module, an I-8060 relay output module, and an I-7520 RS232/RS485 converter, utilizes computer control technology to develop a distributed oil metering and statistical management system for various oil types, including inbound metering, outbound metering, and statistics. This system is time-saving and labor-saving, and has been well-received by users. The system architecture is shown in Figure 2. Figure 2: Distributed metering architecture based on I-8411 3.1 Functional Modules 1) Utilize the 6 differential input channels of I-8017H to collect the liquid level, liquid temperature, flow rate values ​​of two LUGB series vortex flow transmitters (for calculation purposes, the average value of the two flow meters is taken as the actual flow rate value), and store the liquid level value of the tank to prevent liquid overflow, temperature, etc.; 2) Utilize the D/A function of I-8024 to output a 0-10V DC signal as the frequency conversion control input signal for Siemens' Micro Master general-purpose frequency converter, enabling the frequency converter to perform V/F conversion, converting it into a 0-50Hz alternating signal to control the three-phase asynchronous motor in real time, achieving the purpose of frequency conversion operation of the motor and promoting constant speed flow of the liquid. 3) Use the output signal of the I-8060 power relay to control the opening of various flow relays, flow control solenoid valves, and electrical contactors in real time; 4) Use the digital I/O of the I-8042 to detect and control various switches, and simultaneously detect the closing status of flow relays, flow control solenoid valves, and electrical contactors in real time; 5) Use the I-7520 as an RS-232/RS-485 converter to enable data communication between the I-8411 and the serial port of the host computer server. 3.2 Safety and Reliability Measures 1) Spike Pulse Handling: Because large thyristors are used in this system, their closing and opening will generate huge energy spike pulses. Once this pulse enters the signal system, it will not only cause malfunctions in the control system, but more seriously, it will burn out the control equipment and lock the control signal input channel. In particular, it has a greater impact on modules such as I-8017H, I-8024, and I-8042. In order to reduce its impact, an RC protection circuit is added to the input or output terminal of each control module to absorb its spike pulses. Meanwhile, the signal ground and power ground must be separated. 2) Handling of inverter overvoltage: In this system, the inverter drives a traction motor with large inertia. Because the inverter output speed is relatively fast, while the load decelerates relatively slowly due to its own resistance, the speed of the motor driven by the load is higher than the speed corresponding to the frequency output by the inverter. The motor is in a generating state, and the inverter does not have an energy feedback unit. Therefore, the voltage of the inverter's DC circuit rises and exceeds the protection value, resulting in an overvoltage fault. Therefore, a regenerative braking unit must be added, otherwise it will interfere with the SCADA system. 3.3 System functions 1) Data display: Displays real-time collected parameters such as flow rate, temperature, switch status, and motor speed for each type of oil in digital, bar, and curve formats; 2) Can calculate flow rate and total volume, generate daily, monthly, and annual reports, and store historical records for many years; 3) Data repair and maintenance: Has parameter setting and data loss repair functions. 4) Real-time data exchange with the company's MIS system. 4. Conclusion: The development of PC-Based PLCs has benefited from the development of embedded CPUs, embedded operating systems, and IEC-61131-3 (LD, SFC, FBD, IL, ST) standardized programming languages. PC-Based PLCs possess the dual characteristics of IPCs and PLCs, having the system structure of a PLC and the open architecture of an IPC. Currently, the industrial control field is in an era of coexistence of IPCs, PLCs, and PC-Based PLCs, and also an era of gradual integration among the three. With the development of embedded CPUs, embedded operating systems, and development tools conforming to the IEC-61131-3 international standard languages, PC-Based PLCs or embedded controllers will become more open and standardized, with more powerful functions, stronger data communication capabilities, and faster data processing capabilities. They will be better able to adapt to more complex industrial control needs. Editor: He Shiping