传统意义上，PLC主、备切换经常采用继电器方式，这种方式安全可靠，但是，备用PLC无法获得主用PLC中的某些状态量，如模拟量、扫查量等；如果主用PLC中的程序带有顺序标志位，或者需要保持外围设备的继电器,一旦进行主、备切换，程序将无法继续正常运行、外围设备将有可能停运。本文针对GE Fanuc系列90-30 PLC利用网络技术提出了一套解决方案，有效地解决了PLC主、备切换中容易产生的问题，并应用于实际工程中，获得了不错的效果。

主、备切换；网络技术；继电器
Abstract: Traditionally, the switch of PLC between master and slave always uses relay, it is reliable and secure, however, slave PLC can not acquire status variables with this method, such as analogue values, digital input status, and so on; if there were some sequence flags or must keep relays of peripheral equipment, program can not run as usual, and peripheral equipment may be stopped when the switch completed. This paper provides a set of method, which is used in GE Fanuc Series 90-30 PLC with network technology; it can solve problems produced by the switch, and acquires good result in real project.
Keyword: switch between master and slave; network technology; relay

1、前言 2、传统主、备切换方式的原理及弊端 3、新方法原理 4、传统方法与新方法的比较 5、实际项目应用 6、结论 7、参考文献 详细情况，请下载本手册