1. Application field: The initial design goal of PLC is only to replace complex relay circuits, and DDC is originally only used in industrial automation instruments: PLC emphasizes versatility, DDC emphasizes specificity: PLC is used in the field of industrial control, and DDC is used in professional building systems.
2. Structural differences: DDC is a kind of "decentralized control system", the composition of the system is a hierarchical structure, which can achieve point-to-point communication, while PLC is just a control "device", often used for the control of a certain part of the production line, the composition of the system is connected by a unique protocol of the fieldbus, and the PLC communicates with other PLCs through the host computer: the two are the difference between "system" and "device".
3. Protocol differences: The DDC system generally supports a variety of protocol standards, has rich integration interfaces, has a strong ability to integrate third-party equipment, and has better scalability and openness of the system itself; However, because PLC basically works for individuals, when communicating with other PLCs or host computers, the network form used is basically a single network structure, and the network protocol is generally a proprietary fieldbus standard, and the integration ability with third-party equipment is relatively poor.
4. Software features: The upper computer software of the DDC system is mostly special software: the upper computer software of the PLC system is mostly general configuration software. Specific to the field of building automation, it is more convenient to use a dedicated DDC, especially the workload of the host computer is small; The use of PLC is more troublesome for both the lower computer programming and the upper computer unit, and it is necessary to start from the basics, and the technical level and English requirements of the design programmers and users are high.
5. Professionalism: PLC (such as the commonly used Siemens S7200 and S7300) is a general industrial control product, and there is no built-in energy management and energy-saving procedures that have been rigorously tested; It requires very professional designers to do a lot of on-site debugging work, and the debugging cycle is long. DDC solidifies the professional version of the software, with standard applications and special functions such as PID algorithms and energy management programs that have been rigorously experimented, DDC usually has: peak load control, optimized start and stop control, optimized equipment scheduling, energy saving cycle control, a variety of air conditioning operation modes, temporary plan replacement, holiday schedule, basic calendar schedule, event schedule, trend record and report and other functions.
6. Scalability: DDCS has a large number of expandable interfaces in the whole design, the external system or expansion system is very convenient, and it is difficult to increase or decrease the operator station at will after the completion of the whole system connected by the PLC.
7. Safety: When the DDC fails, it can be replaced online, which does not affect the network communication of other DDCs on the network, the DDC itself can work independently, and the central operation station can be shut down when not needed, so as to ensure the safety and reliability of the whole system. When the PLC unit module fails, the entire system has to be stopped for replacement maintenance and reprogramming, and the PLC relies on the host computer to work. Therefore, the DDC system is one level higher than the PLC system in terms of safety and reliability.
8. Modularization: PLC is also divided into large, medium, small and micro PLC, such as S7-200 belongs to Siemens micro PLC, S7300 belongs to Siemens lower performance series, the number of points that can be brought is very limited, the scale of the network is limited, and it is not easy to expand. DDC has a variety of modular series to choose from, suitable for different air conditioning processes, with a reasonable ratio of IO points and appropriate redundancy.
9. Debugging complexity: Due to the different control fields of the application, the PLC system is more troublesome to debug, and the DDC system is easy to debug. And for the system that PLC is composed of, the workload is extremely huge, first need to determine which PLC is to be edited and updated, then use the corresponding compiler to compile the program, and finally use the special machine (reader) to send the program to this PLC on a special one-to-one basis, during the system debugging, greatly increase the debugging time and debugging cost, and are extremely unfavorable to the future maintenance. There is a big difference in the accuracy of control. This is why in medium- and large-sized control projects (more than 500 points), systems that are all connected to PLCs are not used. High requirements for the technical level of future maintenance personnel
In short, due to the different fields of application, DDC and PLC are very different in terms of working mode, network communication, system function, professionalism, scalability, and security.
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