IC5002TBX0030，IC5002TBX0030，IC5002TBX0030
結(jié)合PLC的組成和結(jié)構(gòu)分析PLC的工作原理
結(jié)合PLC的組成和結(jié)構(gòu)分析PLC的工作原理更容易理解。PLC是采用周期循環(huán)掃描的工作方式，CPU連續(xù)執(zhí)行用戶程序和任務(wù)的循環(huán)序列稱為掃描。CPU對(duì)用戶程序的執(zhí)行過程是CPU的循環(huán)掃描，并用周期性地集中采樣、集中輸出的方式來完成的。一個(gè)掃描周期主要可分為：
（1）讀輸入階段。 每次掃描周期的開始，先讀取輸入點(diǎn)的當(dāng)前值，然后寫到輸入映像寄存器區(qū)域。在之后的用戶程序執(zhí)行的過程中，CPU訪問輸入映像寄存器區(qū)域，而并非讀取輸入端口的狀態(tài)，輸入信號(hào)的變化并不會(huì)影響到輸入映像寄存器的狀態(tài)，通常要求輸入信號(hào)有足夠的脈沖寬度，才能被響應(yīng)。
（2）執(zhí)行程序階段。 用戶程序執(zhí)行階段，PLC按照梯形圖的順序，自左而右，自上而下的逐行掃描，在這一階段CPU從用戶程序的第一條指令開始執(zhí)行直到最后一條指令結(jié)束，程序運(yùn)行結(jié)果放入輸出映像寄存器區(qū)域。在此階段，允許對(duì)數(shù)字量I/O指令和不設(shè)置數(shù)字濾波的模擬量I/O指令進(jìn)行處理，在掃描周期的各個(gè)部分，均可對(duì)中斷事件進(jìn)行響應(yīng)。
（3）處理通信請(qǐng)求階段。 是掃描周期的信息處理階段，CPU處理從通信端口接收到的信息。
（4）執(zhí)行CPU自診斷測(cè)試階段。在此階段CPU檢查其硬件，用戶程序存儲(chǔ)器和所有I/O模塊的狀態(tài)。
（5）寫輸出階段。每個(gè)掃描周期的結(jié)尾，CPU把存在輸出映像寄存器中的數(shù)據(jù)輸出給數(shù)字量輸出端點(diǎn)（寫入輸出鎖存器中），更新輸出狀態(tài)。然后PLC進(jìn)入下一個(gè)循環(huán)周期，重新執(zhí)行輸入采樣階段，周而復(fù)始。
如果程序中使用了中斷，中斷事件出現(xiàn)，立即執(zhí)行中斷程序，中斷程序可以在掃描周期的任意點(diǎn)被執(zhí)行。
如果程序中使用了立即I/O指令，可以直接存取I/O點(diǎn)。用立即I/O指令讀輸入點(diǎn)值時(shí)，相應(yīng)的輸入映像寄存器的值未被修改，用立即I/O指令寫輸出點(diǎn)值時(shí)，相應(yīng)的輸出映像寄存器的值被修改。
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