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1. DCS is a “distributed control system,” while PLC is merely a (programmable controller) “device”; the difference between the two is that one is a “system” and the other is a “device.” The system can enable the functionality and coordination of any devices, while the PLC devices only perform the functions specific to their own unit. 2. In terms of networking, the DCS network serves as the central nervous system of the entire system; the system network in Hilscher’s MACS system utilizes dual-redundant 100Mbps industrial Ethernet, based on the international standard protocol TCP/IP. It is a safe and reliable high-speed communication network with dual redundancy, offering better scalability and openness. In contrast, since PLCs generally operate independently, the networking structure they use when communicating with other PLCs or higher-level computers is usually single-network based, and their network protocols often do not conform to international standards. In terms of network security, PLCs do not have good protective measures. We adopt dual redundancy for power supply, CPU, and network. 3. In terms of the overall DCS design, each operator station has the functions of an engineer station, and once the operational programs are installed, the stations are closely interconnected. There is interlocking and coordinated control among any station, any function, and any controlled device. In contrast, in a system constructed using only PLCs for connectivity, the connection between stations (PLC to PLC) is loose, and coordinated control is not possible. 4. The DCS is designed with a large number of scalable interfaces, making it very convenient to connect external or additional systems. Once the entire system based on PLCs is established, it is difficult to add or remove operator stations as desired. 5. DCS Safety: To ensure the safety and reliability of the devices controlled by the DCS, it employs dual-redundant control units. When a critical control unit fails, the corresponding redundant unit will take over immediately and without disruption, thereby ensuring the safety and reliability of the entire system. The systems built with PLCs basically lack the concept of redundancy, let alone redundancy control strategies. Especially when one of its PLC units fails, the entire system has to be shut down in order to carry out replacement and maintenance as well as reprogramming. Therefore, the DCS system is one level higher in terms of safety and reliability. 6. System software: Updating various process control schemes is one of the most fundamental functions of a DCS. When a change is made to a scheme, engineers simply need to compile the modified scheme on the engineer station and then issue a download command; the download process is carried out automatically by the system, without affecting the operation of the original control scheme. Various control software and algorithms in the system can improve the control precision of the process control objects. For systems based on PLCs, the workload is extremely large. First, it is necessary to determine which PLC needs to be edited or updated; then, the program must be compiled using the corresponding compiler. Finally, a dedicated device (reader/writer) is used to transfer the program to that PLC on a one-to-one basis. During system debugging, this increases both the debugging time and costs significantly, and it also makes future maintenance very difficult. There is a huge difference in control accuracy. This explains why, in large and medium-sized control projects (with 500 points or more), systems entirely composed of PLCs are hardly used. 7. Modules: All I/O modules in the DCS system are equipped with CPUs, which enable assessment of the quality of acquired and output signals as well as scalar transformations; faulty modules can be removed and reinserted while the system is still operational, allowing for their replacement at any time. The PLC module is merely a simple electrical conversion unit; it lacks intelligent chips, and in the event of a failure, the entire corresponding unit becomes non-functional. 8. Nowadays, high-end PLCs and DCSs have similar functions. DCSs have stronger capabilities in terms of regularly scanning networks and distributed databases, and they are also good at handling calculations and analog values. PLCs are also classified into large, medium, small, and micro PLCs. Micro PLCs cost only a few hundred to 2,000 yuan, and they have a limited number of input/output points; large PLCs can handle thousands of such points. Their computing power is similar to that of DCS systems, but they lack strong capabilities for networking multiple units together. Currently, the two technical platforms are similar; the only difference lies in their focus. The distributed control system (DCS) has evolved from industrial automation instrument control systems to distributed systems centered around industrial control computers. As a result, it has certain advantages in terms of analog signal processing and loop regulation. In its early stages, it was primarily used for continuous process control, with an emphasis on loop regulation functions to meet the requirements for fast and large-scale data processing. In terms of hardware architecture, it features a high degree of bus standardization and strong compatibility, as well as abundant software resources; in particular, the support of real-time operating systems gives it an advantage in controlling industrial systems that require fast response times, real-time operation, complex modeling, and heavy computational loads. PLC evolved from relay logic systems and is primarily used in discrete manufacturing and process control. Initially, it was developed as a replacement for relay-based control systems, focusing on sequential control of digital signals. As it develops, the PLC also absorbs the advantages of DCS systems. The development of PLCs in process control will involve integration with fieldbus technology, the expansion of their functional capabilities, and the adoption of open buses. PLC was developed by imitating the control principles of traditional relays. PLCs in the 1970s had only digital logic control, and they were first applied in the automotive manufacturing industry. It stores instructions for performing logical operations, sequential control, timing, counting, and arithmetic operations ； And various machines or production processes are controlled through digital input and output operations. The control programs developed by the user reflect the technological requirements of the production process, and are stored in the user program memory of the PLC in advance. During operation, it executes each instruction according to the content of the stored program in order to carry out the operations required by the process flow. The CPU of a PLC contains a program counter that indicates the storage address of the current program step. As the program runs, this counter increases by 1 with each step executed; the program proceeds sequentially from the starting step (with a step number of zero) to the final step (usually the END instruction), after which it returns to the starting step to repeat the cycle. The time required for a PLC to complete one cycle of operation is called a scan period. For different models of PLCs, the cycle scanning time ranges from 1 microsecond to several dozen microseconds. Loop operations such as the program counter are something that DCS does not have. DCS was developed based on operational amplifiers. All functions and the relationships between various process variables are implemented using function blocks (referred to as expansion blocks in some DCS systems). In the mid-1970s, DCS systems had only analog control. In a system like the TDC2000, one controller can perform calculations for 8 PID loops per second. The first industry to apply it is the chemical industry. The main difference between DCS and PLC lies in the logical processing of digital signals and the calculation of analog values; even though there has been some overlap between the two over time, differences still exist. After the 1980s, in addition to logical operations, PLCs also had algorithms for use in control circuits, but performing some complex calculations remained quite difficult. PLCs are programmed using ladder diagrams, and performing calculations on analog values is not very intuitive in programming, making it rather complicated. However, in terms of computation logic, it boasts fast performance; it takes less than 1 millisecond to process a 1K logic program, on the order of microseconds. It treats all inputs as digital signals, with 16 bits (or 32 bits) representing one analog value. DCS treats all inputs as analog values, with 1 bit representing a digital value. Solving a logic problem takes anywhere from a few hundred microseconds to several milliseconds. It takes the PLC a few dozen milliseconds to perform a PID calculation, which is on par with the processing time of a DCS. Large PLCs use another CPU to perform analog calculations. Send the calculation results to the PLC controller. Different models of DCS take varying amounts of time to calculate PID values, but all are in the range of several tens of milliseconds. Like the early TDC2000 system, it could complete control calculations for 8 circuits in 1 second. With the development of chip technology, the time required to compute an algorithm is decreasing. The time required to solve an algorithm depends on the arrangement and configuration of the functional blocks. Regarding ground resistance, the requirements may not be high for PLCs, but for DCS it must be below a few ohms (usually below 4 ohms). Analog isolation is also very important; in areas with explosion hazards, intrinsically safe barriers should be installed. For systems with the same number of I/O points, using a PLC is cheaper than using a DCS (saving approximately 40%). PLCs do not have dedicated operation stations; the software and hardware they use are standard, which means their maintenance costs are much lower than those of DCS. A PLC controller can handle thousands of I/O points (up to over 8,000 I/Os). The controllers of DCS can only handle a few hundred I/O points (no more than 500 I/Os). If the controlled objects mainly involve equipment interlocks with few circuits, a PLC is more suitable. If it is mainly analog control with many functional operations, a DCS is the best choice. In terms of redundancy in controllers, I/O cards, communication networks, etc., as well as advanced calculations and industry-specific requirements, DCS is far superior to PLC. Due to the use of general-purpose monitoring software, PLCs make it easier to design management information systems for enterprises. Originally, DCS systems used dedicated operation stations, while PLCs generally used general-purpose operation stations; however, nowadays DCS systems are also gradually adopting general-purpose operation stations. In fact, today’s PLC manufacturers in various countries are not only working to enhance the performance of PLCs as industrial field controllers, but they are also focusing on software development (special function software, programming software, configuration software, communication software) as well as on achieving greater hardware flexibility and openness in PLCs. As a result, the differences between PLCs and DCSs (Distributed Control Systems) as well as PCs are gradually narrowing. For example, Siemens introduced the concept of TIA (Totally Integrated Automation), and on this basis developed the PCS (Process Control System). Austrian company B&R also introduced the PCC (Programmable Computerized Controller, a programmable computer-based control system). Since 2003, the Programmable Automation Controller (PAC) has begun to become a popular term in the field of control. It is defined as: a centralized control engine that meets the various needs of PLC users, as well as the information requirements of manufacturing companies. PAC includes the main functions of PLC along with enhanced control capabilities, as well as object-based, open architecture formats, and network connectivity features found in PC-based control.