A20B-2901-0721有了上述過程，用PLC實現(xiàn)控制顯然是可能的。因為：有了輸入刷新，可把輸入電路監(jiān)控得到的輸入信息存入PLC的輸入映射區(qū)；經(jīng)運行用戶程序，輸出映射區(qū)將得到變換后的信息；再經(jīng)輸出刷新，輸出鎖存器將反映輸出映射區(qū)的狀態(tài)，并通過輸出電路產(chǎn)生相應的輸出。又由于這個過程是永不停止地循環(huán)反復地進行著，所以，輸出總是反映輸入的變化的。只是響應的時間上，略有滯后。當然，這個滯后不宜太大，否則，所實現(xiàn)的控制不那么及時，也就失去控制的意義。
為此，PLC A20B-2901-0721 的工作速度要快。速度快、執(zhí)行指令時間短，是PLC實現(xiàn)控制的基礎。事實上，它的速度是很快的，執(zhí)行一條指令，多的幾微秒、幾十微秒，少的才零點幾，或零點零幾微秒。而且這個速度還在不斷提高中。
圖1－1－1所示的過程是簡化的過程，實際的PLC工作過程還要復雜些。除了I/O刷新及運行用戶程序，還要做些公共處理工作。
公共處理工作有：循環(huán)時間監(jiān)控、外設服務及通訊處理等。
監(jiān)控循環(huán)時間的目的是避免"※循環(huán)"，避免程序不能反復不斷地重復執(zhí)行。辦法是用"看門狗"（Watchingdog）。只要循環(huán)超時，它可報警，或作相應處理.
外設服務是讓PLC可接受編程器對它的操作，或通過接口向輸出設備如打印機輸出數(shù)據(jù).
通訊處理是實現(xiàn)PLC與PLC，或PLC與計算機，或PLC與其它工業(yè)控制裝置或智能部件間信息交換的。這也是增強PLC控制能力的需要。
也就是說，實際的PLC工作過程總是：公共處理--I/O刷新--運行用戶程序--再公共處理--……反復不停地重復著。
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