IC693PRG300輸出刷新階段

當(dāng)用戶程序掃描結(jié)束后，PLC就進(jìn)入輸出刷新階段。在此期間，CPU按照I/O映象區(qū)內(nèi)對應(yīng)的狀態(tài)和數(shù)據(jù)刷新所有的輸出鎖存電路，再經(jīng)輸出電路驅(qū)動相應(yīng)的外設(shè)。這時，才是PLC的真正輸出。

輸入/輸出滯后現(xiàn)象

從PLC的工作過程，可以總結(jié)如下幾個結(jié)論。

·以掃描的方式執(zhí)行程序，其輸入/輸出信號間的邏輯關(guān)系存在著原理上的滯后。掃描周期越長，滯后就越嚴(yán)重。

·掃描周期除了包括輸入采樣階段、用戶程序執(zhí)行階段、輸出刷新階段三個主要工作階段所占的時間外，還包括系統(tǒng)管理操作占用的時間。其中，程序執(zhí)行的時間與程序的長短及指令操作的復(fù)雜程度有關(guān)，其他基本不變。掃描周期一般為毫微秒級。

·第n次掃描執(zhí)行程序時，所依據(jù)的輸入數(shù)據(jù)是該次掃描周期中采樣階段的掃描值X依據(jù)的輸出數(shù)據(jù)有上一次掃描的輸出值Y（n-1），也有本次的輸出值Yn；所n送往輸出端子的信號，即是本次執(zhí)行全部運算后的結(jié)果Yn。

·輸入/輸出響應(yīng)滯后不僅與掃描方式有關(guān)，還與程序設(shè)計安排有關(guān)。

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