Programmable logical controllers (PLCs) are increasingly used in industrial control systems and play a crucial role in automatic control systems. Therefore, choosing the right PLC is essential. Faced with various types of PLCs from numerous manufacturers, each with its own advantages and disadvantages, capable of meeting diverse user needs, they are incompatible in terms of form, composition, function, network, and programming. There is no unified standard, making direct comparison impossible. Below are some perspectives on PLC selection in automatic control system design, which can serve as a reference when choosing a PLC. The following comparisons can help select a suitable product: I. Workload: This is particularly important. At the initial stage of automatic control system design, an accurate count of the number of control points (digital and analog) should be made. This is often the primary condition for selecting a PLC; generally, a PLC with 10% to 30% more control points than the required number should be chosen. This is due to several considerations: 1. It can compensate for points missed during the design process; 2. It ensures that there are replacement points in case of individual point failures during operation; 3. It accommodates future needs for increasing the number of control points. II. Working Environment: The working environment is a crucial indicator for PLC operation. Automatic control systems liberate people from busy work and harsh environments, requiring them to adapt to complex environments such as temperature, humidity, noise, signal shielding, and operating voltage. Different PLCs vary, so it's crucial to choose a product suited to the actual working environment. III. Communication Networks Modern PLCs are no longer simply for local control; remote communication has become a critical issue for control systems. However, communication protocols developed by different manufacturers vary significantly, resulting in poor compatibility. In this regard, the following aspects should be considered: 1. Communication between products from the same manufacturer. Each manufacturer has its own communication protocol, and often more than one. This is evident in large and medium-sized PLCs, but differs in small and micro PLCs. Some manufacturers, considering capacity, price, and functionality, often lack or use simpler communication protocols that differ from others. Therefore, this aspect primarily focuses on communication between different types of PLCs from the same manufacturer; 2. Communication between products from different manufacturers. If the automatic control system design involves partial modification of an existing automatic control system, and a different PLC is selected than the original system, or if the design requires two or more PLCs from different manufacturers, then the communication issues between products from different manufacturers need to be considered; 3. Whether it is beneficial for the future. Because different manufacturers use different communication protocols, and there is no unified international standard, PLC selection is greatly limited. Therefore, a communication protocol with a wide impact, development potential, complete functions, and near-universal compatibility should be considered. IV. The programming program is the "heart" of the entire automatic control system; the quality of the program directly affects the operation of the entire automatic control system. Some manufacturers require programmers and programming software to be purchased separately, and the price is considerable; this also needs to be considered. 1. Programming methods: One method is to use a dedicated programmer provided by the manufacturer. These come in various specifications and models. Large programmers have complete functions, are suitable for all PLC models, and are expensive; small programmers are compact, portable, and inexpensive, but have simple functions and poor applicability; another method is to use programming software based on a personal computer application platform, which is now adopted by most manufacturers. Due to the differences in their products, each manufacturer often develops programming software only suitable for its own products. The style, interface, application platform, flexibility, adaptability, and ease of programming of programming software can only be evaluated after the user has personally operated it. 2. Programming Languages ​​Programming languages ​​are the most complex and diverse, seemingly similar but not universal. The most commonly used ones can be divided into the following 5 categories of programming languages: (1) Ladder Diagram This is the programming language most used by PLC manufacturers. It was originally evolved from relay control diagrams. It is relatively simple and most useful for discrete control and interlocking logic; (2) Sequential Function Chart It provides the overall structure and coordinates with state positioning processing or machine control applications; (3) Function Block Diagram It provides an effective development environment and is particularly suitable for process control applications; (4) Structured Text This is a programming language similar to that used in computers. It is suitable for complex algorithms and data processing; (5) Instruction List It provides an environment for optimizing coding performance and is very similar to assembly language. The programming software provided by the manufacturer generally includes one or more programming languages. For example, TE's XTEL programming software can use three programming languages: Ladder, Sequential Function Chart, and Literal. Siemens' Step7 programming software can use two programming languages: Ladder and STL. Modicon's Modsoft programming software only uses Ladder (984 Ladder), while another software, Concept, can use five programming languages: Ladder (LD), Sequential Function Chart (SFC), Function Block Diagram (FBD), Structured Text (ST), and Instruction List (IL). Most programming languages ​​within the same programming software are interchangeable; generally, you should choose the language you are most familiar with. 3. Memory: The PLC memory is where programs and data are stored. There are two types: internal and external. Memory capacity ranges from 512 to 128 MB. It is essential to select a sufficiently large memory based on the actual situation, and some spare space should be reserved for buffering. PLC memory can be categorized into Random Access Memory (RAM), Read-Only Memory (ROM), and Erasable Read-Only Memory (EPROM). RAM allows for arbitrary reading and writing, but the program is only retained for a short period after power failure, making it ideal for debugging automatic control systems. ROM is read-only; the program is pre-programmed by the manufacturer or developer and cannot be changed, even in the event of power failure. EPROM differs from ROM in that it can be erased and rewritten using special methods (such as ultraviolet light), making it suitable for systems that operate for extended periods with minimal modifications. 4. Ease of Modification: Another advantage of PLCs over relay control is their ability to modify the control structure (or process) according to actual needs, requiring convenient and quick program changes. 5. Dedicated Modules: Some PLC manufacturers offer dedicated modules such as communication modules, PID control modules, counter modules, and analog input/output modules. Corresponding program blocks are also provided in the software, often requiring only simple parameter input for easy programming. V. Communication with Monitoring Systems 1. Human-Machine Interface (HMI): This is an early type of monitoring system, designed by manufacturers specifically for their PLC products, and best suited for point-to-point control. With its simple structure, limited functions, panel control, and ease of operation, it is still widely used in field control systems. Its advantage lies in its ability to maintain good field control even when remote control fails. 2. With the continuous development of computers, PC-based (including industrial PCs) monitoring systems are increasingly used in automation systems. These systems fall into two categories: one is custom-designed by PLC developers for their own (or specific) products; the other is developed by software companies to suit most PLC products. The former has strong compatibility with PLC products and can formulate corresponding control schemes based on the characteristics of the PLC, essentially remaining PLC-centric. The latter, however, disregards the PLC product, focusing on the advantages of computers in image, animation, sound, network, and data processing, providing more flexible development conditions for secondary developers. It often produces excellent monitoring systems, and as long as there is a corresponding communication protocol (currently, most manufacturers' communication protocols are available), it can connect to various types of PLCs, making it the preferred choice for today's automation systems. Therefore, in this regard, the feasibility of the communication method between the selected PLC and the monitoring system should be considered. VI. Sustainability This includes three aspects: 1. Product lifespan. 1. To ensure the lifespan of the selected PLC, try to purchase products with a more recent production date. 2. Product continuity: Whether the manufacturer's continuous development and upgrades of the PLC products are backward compatible determines whether the current system can accommodate future new functions. 3. Product update cycle: When a certain model of PLC (or PLC module) becomes obsolete, can the manufacturer guarantee sufficient spare parts? In this case, consider choosing a newer PLC at that time. VII. After-sales service and technical support: 1. Choose products from reputable companies; 2. Choose reputable distributors; 3. Have strong after-sales service and technical support. VIII. Cost-effectiveness: Prioritize the performance of the automation system over price. Only consider price when the previous factors are similar and difficult to choose between, selecting a product with a higher cost-effectiveness. In actual selection, there are often many constraints, and it is not necessary to consider all of the above aspects, but some are essential, and any problems must be solved through alternative methods. Generally, comparing the first five factors will allow you to narrow down your choices to two or three products. Considering the remaining factors will then help you select a more satisfactory PLC. With the continuous development of science and technology, PLC products will inevitably have a unified standard. At that time, selecting a PLC will no longer be a difficult task.