RELANCE 0-57407-4H 可编程控制器 

模块：DCS系统所有I/O模块都带有CPU，可以实现对采集及输出信号品质判断与标量变换，故障带电插拔，随机更换。而PLC模块只是简单电气转换单元，没有智能芯片，故障后相应单元全部瘫痪。

8、现在高端的西门子PLC与DCS的功能已经差不多，DCS对网络和分布式数据库还要定时扫描有较强的功能，同时对运算和模拟量的处理比较拿手。

9、西门子PLC还分大、中、小、微PLC，其中微型的只卖几百块到2000块，点数也好少，大型的可以带数千点，运算能力与DCS差不多，但对多机联网功能较弱。现在两个技术平台都差不多，只是重点不一样。

系统软件：对各种工艺控制方案更新是DCS的一项最基本的功能，当某个方案发生变化后，工程师只需要在工程师站上将更改过的方案编译后，执行下装命令就可以了，下装过程是由系统自动完成的，不影响原控制方案运行。系统各种控制软件与算法可以将工艺要求控制对象控制精度提高。而对于PLC构成的系统来说，工作量极其庞大，首先需要确定所要编辑更新的是哪个PLC，然后要用与之对应的编译器进行程序编译，最后再用专用的机器（读写器）专门一对一的将程序传送给这个西门子PLC，在系统调试期间，大量增加调试时间和调试成本，而且极其不利于日后的维护。在控制精度上相差甚远。这就决定了为什么在大中型控制项目中（500点以上），基本不采用全部由西门子PLC所连接而成的系统的原因。

DCS security: To ensure the safety and reliability of equipment controlled by DCS, DCS adopts dual redundant control units. When important control units fail, relevant redundant units will switch to working units in real-time without disturbance, ensuring the safety and reliability of the entire system. The systems connected by Siemens PLC have almost no concept of redundancy, let alone redundant control strategies. Especially when one of its Siemens PLC units fails, the entire system has to be shut down before replacement and maintenance can be carried out, and reprogramming is required. So the DCS system needs to be one level higher in terms of safety and reliability.

6. System software: Updating various process control schemes is the most basic function of DCS. When a certain scheme changes, engineers only need to compile the modified scheme on the engineer station and execute the download command. 

Siemens PLC does not have good protection measures in terms of network security. We adopt dual redundancy of power supply, CPU, and network.

3. DCS considers the overall plan, and the operator station has the function of an engineer station. After the program is installed in the operation plan, there is a close joint relationship between stations. Any station, any function, or any controlled device is interlocked and controlled in a coordinated manner; The system composed solely of PLCs connected to each other has a loose connection between stations (Siemens PLC and Siemens PLC), which cannot achieve coordinated control functions.

4. DCS has a large number of expandable interfaces in the entire design, making it very convenient for external or extended systems. After the completion of the entire system connected by Siemens PLC, it is difficult to arbitrarily add or reduce operator stations.

The key difference between DCS and Siemens PLC is that DCS is a "decentralized control system", while Siemens PLC (programmable controller) is just a control "device", and the two are the difference between "system" and "device". The system can achieve the functions and coordination of any device, while Siemens PLC devices only realize the functions that this unit has.

2. In terms of network, DCS network is the central nervous system of the entire system. It is a safe and reliable dual redundant high-speed communication network, with better scalability and openness of the system. However, Siemens PLCs, as they mostly work as individuals, use a single network structure when communicating with other Siemens PLCs or upper computers, and the network protocol often does not comply with international standards.