MPL-SSN-A5B5

MPL-SSN-A5B5

现在一些带现场总线的现场仪表本身装了许多功能块，虽然不同产品同种功能块在性能上会稍有差别，但一个网络支路上有许多功能雷同功能块的情况是客观存在的。选用哪一个现场仪表上的功能块，是系统组态要解决的问题。

　　考虑这个问题的原则是：尽量减少总线上的信息往返。一般可以选择与该功能有关的信息输出多的那台仪表上的功能块。

　　典型系统比较

　　通过使用现场总线，用户可以大量减少现场接线，用单个现场仪表可实现多变量通信，不同制造厂生产的装置间可以完全互操作，增加现场一级的控制功能，系统集成大大简化，并且维护十分简便。传统的过程控制仪表系统每个现场装置到控制室都需使用一对的双绞线，以传送4~20mA信号；现场总线系统中，每个现场装置到接线盒的双绞线仍然可以使用，但是从现场接线盒到中央控制室仅用一根双绞线完成数字通信。

　　通过采用现场总线控制系统，到底能节省多少电缆，编者尚未做此计算。但是，我们从采用DCS系统的电厂中与自动控制系统有关的所用电缆公里数看出，电缆在基建投资中所占份额。

　　某电厂，2×300MW燃煤机组。热力系统为单元制。每台机组设置一座集中控制楼，采用机、炉、电单元集中控制方式。单元控制室的标高为12.6米，与运行层标高一致。DCS采用WDPF—Ⅱ，每台机组设计的I/O点为4500点。

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