傳統意義上，PLC主、備切換經常采用繼電器方式
，這種方式安全可靠
，但是，備用PLC無法獲得主用PLC中的某些狀態量，如模擬量、掃查量等
；如果主用PLC中的程序帶有順序標誌位，或者需要保持外圍設備的繼電器,一旦進行主、備切換，程序將無法繼續正常運行、外圍設備將有可能停運。本文針對GE Fanuc係列90-30 PLC利用網絡技術提出了一套解決方案，有效地解決了PLC主、備切換中容易產生的問題，並應用於實際工程中
，獲得了不錯的效果。

主、備切換；網絡技術
；繼電器
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、傳統主、備切換方式的原理及弊端 3 、新方法原理 4 、傳統方法與新方法的比較 5、實際項目應用 6 、結論 7、參考文獻 詳細情況，請下載本手冊