描述
PLC執(zhí)行程序的過程分為三個階段,即輸入采樣階段、程序執(zhí)行階段、輸出刷新階段。
在輸入采樣階段,PLC以掃描工作方式按順序?qū)λ休斎攵说妮斎霠顟B(tài)進行采樣,并存入輸入映象寄存器中,此時輸入映象寄存器被刷新。接著進入程序處理階段,在程序執(zhí)行階段或其它階段,即使輸入狀態(tài)發(fā)生變化,輸入映象寄存器的內(nèi)容也不會改變,輸入狀態(tài)的變化只有在下一個掃描周期的輸入處理階段才能被采樣到。
2.程序執(zhí)行階段
在程序執(zhí)行階段,PLC對程序按順序進行掃描執(zhí)行。若程序用梯形圖來表示,則總是按先上后下,先左后右的順序進行。當遇到程序跳轉(zhuǎn)指令時,則根據(jù)跳轉(zhuǎn)條件是否滿足來決定程序是否跳轉(zhuǎn)。當指令中涉及到輸入、輸出狀態(tài)時,PLC從輸入映像寄存器和元件映象寄存器中讀出,根據(jù)用戶程序進行運算,運算的結(jié)果再存入元件映象寄存器中。對于元件映象寄存器來說,其內(nèi)容會隨程序執(zhí)行的過程而變化。
3.輸出刷新階段
當所有程序執(zhí)行完畢后,進入輸出處理階段。在這一階段里,PLC將輸出映象寄存器中與輸出有關(guān)的狀態(tài)(輸出繼電器狀態(tài))轉(zhuǎn)存到輸出鎖存器中,并通過一定方式輸出,驅(qū)動外部負載。
因此,PLC在一個掃描周期內(nèi),對輸入狀態(tài)的采樣只在輸入采樣階段進行。當PLC進入程序執(zhí)行階段后輸入端將被封鎖,直到下一個掃描周期的輸入采樣階段才對輸入狀態(tài)進行重新采樣。這方式稱為集中采樣,即在一個掃描周期內(nèi),集中一段時間對輸入狀態(tài)進行采樣。
在用戶程序中如果對輸出結(jié)果多次賦值,則最后一次有效。在一個掃描周期內(nèi),只在輸出刷新階段才將輸出狀態(tài)從輸出映象寄存器中輸出,對輸出接口進行刷新。在其它階段里輸出狀態(tài)一直保存在輸出映象寄存器中。這種方式稱為集中輸出。
歐美品牌停產(chǎn)的,常用的“PLC”備品備件
我司產(chǎn)品應(yīng)用于以下領(lǐng)域:
1,《發(fā)電廠DCS監(jiān)控系統(tǒng)》
2,《智能平鋼化爐系統(tǒng)制造》
3,《PLC可編程輸送控制系統(tǒng)》
4,《DCS集散控制系統(tǒng)》
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5,《智能型消防供水控制系統(tǒng)》
6,《化工廠藥液恒流量計算機控制系統(tǒng)》
7,《電氣控制系統(tǒng)》造紙,印染生產(chǎn)線,變電站綜合自動化控制系列
本特利,英維思,伍德沃德,福克斯波羅、西屋、瑞恩、施耐德莫迪康、ABB、AB、西門子、摩托羅拉、GE發(fā)那科、安川、博世力士樂,ACSO,力士樂等各大品牌的DCS系統(tǒng)配件,機器人系統(tǒng)配件,大型伺服系統(tǒng)備件。
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