RELANCE 0-57407-4H 可編程控制器
模塊：DCS系統(tǒng)所有I/O模塊都帶有CPU，可以實現(xiàn)對采集及輸出信號品質(zhì)判斷與標量變換，故障帶電插拔，隨機更換。而PLC模塊只是簡單電氣轉(zhuǎn)換單元，沒有智能芯片，故障后相應單元全部癱瘓。
8、現(xiàn)在高端的西門子PLC與DCS的功能已經(jīng)差不多，DCS對網(wǎng)絡和分布式數(shù)據(jù)庫還要定時掃描有較強的功能，同時對運算和模擬量的處理比較拿手。
9、西門子PLC還分大、中、小、微PLC，其中微型的只賣幾百塊到2000塊，點數(shù)也好少，大型的可以帶數(shù)千點，運算能力與DCS差不多，但對多機聯(lián)網(wǎng)功能較弱?，F(xiàn)在兩個技術(shù)平臺都差不多，只是重點不一樣。
系統(tǒng)軟件：對各種工藝控制方案更新是DCS的一項最基本的功能，當某個方案發(fā)生變化后，工程師只需要在工程師站上將更改過的方案編譯后，執(zhí)行下裝命令就可以了，下裝過程是由系統(tǒng)自動完成的，不影響原控制方案運行。系統(tǒng)各種控制軟件與算法可以將工藝要求控制對象控制精度提高。而對于PLC構(gòu)成的系統(tǒng)來說，工作量極其龐大，首先需要確定所要編輯更新的是哪個PLC，然后要用與之對應的編譯器進行程序編譯，最后再用專用的機器（讀寫器）專門一對一的將程序傳送給這個西門子PLC，在系統(tǒng)調(diào)試期間，大量增加調(diào)試時間和調(diào)試成本，而且極其不利于日后的維護。在控制精度上相差甚遠。這就決定了為什么在大中型控制項目中（500點以上），基本不采用全部由西門子PLC所連接而成的系統(tǒng)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.