IC647ABK001，IC647ABK001，IC647ABK001
可編程控制器的掃描周期及工作過程
可編程控制器的工作過程包括兩部分：自診斷及通信響應(yīng)的固定過程和用戶程序執(zhí)行過程，如圖1所示。PLC在每次執(zhí)行用戶程序之前，都先執(zhí)行故障自診斷程序、復(fù)位、監(jiān)視、定時(shí)等內(nèi)部固定程序，若自診斷正常，繼續(xù)向下掃描，然后PLC檢查是否有與編程器、計(jì)算機(jī)等的通信請(qǐng)求。如果有與計(jì)算機(jī)等的通信請(qǐng)求，則進(jìn)行相應(yīng)處理。當(dāng)PLC處于停止（STOP）狀態(tài)時(shí)，只循環(huán)進(jìn)行前兩個(gè)過程。而在PLC處于運(yùn)行（RUN）狀態(tài)時(shí)，PLC從內(nèi)部處理、通信操作、輸入掃描、執(zhí)行用戶程序、輸出刷新五個(gè)工作階段循環(huán)工作。每完成一次以上五個(gè)階段所需要的時(shí)間稱為一個(gè)掃描周期。
掃描周期是PLC的一個(gè)重要指標(biāo)，小型PLC的掃描周期一般為十幾毫秒到幾十毫秒。PLC的掃描周期長短取決于掃描速度和用戶程序的長短。毫秒級(jí)的掃描時(shí)間對(duì)于一般工業(yè)設(shè)備通常是允許的，PLC對(duì)輸入的短暫滯后也是允許的。但對(duì)某些I/O快速響應(yīng)的設(shè)備，則應(yīng)采取相應(yīng)的處理措施。如選擇高速CPU，提高掃描速度；選擇快速響應(yīng)模塊、高速計(jì)數(shù)模塊以及不同的中斷處理等措施減少滯后時(shí)間。對(duì)于用戶來說，要提高編程能力，盡可能優(yōu)化程序；而在編寫大型設(shè)備的控制程序時(shí)，盡量減少程序長度，選擇分支或跳步程序等，都可以減少用戶程序執(zhí)行時(shí)間。
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Allen-Bradley Overload Relay L.H. X-36182 ! WOW !
Allen-Bradley Overload Relay 129-BSC 32 B ! WOW !
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Allen-Bradley Industrial Circuit Breaker -- 1489-A1C050 /A --
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ALLEN-BRADLEY AC DRIVE 1336F-C025-AN-EN
Allen-Bradley AB flex I/O 24vcd power supply 1794-ps13
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ALLEN-BRADLEY 872C-A5N18-N3 PROXIMITY SENSOR
