我們都知道，利(li)用plc進行(xing)(xing)(xing)自動化生產加工，離不開在(zai)其運行(xing)(xing)(xing)之前(qian)的程(cheng)序的編寫(xie)與(yu)輸入，接著(zhu)在(zai)各種通信(xin)(xin)設備都與(yu)plc進行(xing)(xing)(xing)連接以后(hou)，plc接到相應的信(xin)(xin)息傳遞(di)就會開始進行(xing)(xing)(xing)程(cheng)序的執行(xing)(xing)(xing)。那么，plc在(zai)程(cheng)序執行(xing)(xing)(xing)時都有哪些階段性(xing)的步驟呢？

(1)輸(shu)(shu)(shu)入(ru)(ru)(ru)采(cai)樣階(jie)段(duan)。在這一階(jie)段(duan)中(zhong)(zhong)，PLC以掃(sao)描方式讀入(ru)(ru)(ru)所有(you)輸(shu)(shu)(shu)入(ru)(ru)(ru)端(duan)子上的(de)輸(shu)(shu)(shu)入(ru)(ru)(ru)信號(hao)，并將各輸(shu)(shu)(shu)入(ru)(ru)(ru)狀態存(cun)(cun)入(ru)(ru)(ru)對應(ying)的(de)輸(shu)(shu)(shu)入(ru)(ru)(ru)映像寄存(cun)(cun)器中(zhong)(zhong)。此時，輸(shu)(shu)(shu)入(ru)(ru)(ru)映像寄存(cun)(cun)器被(bei)刷斷。在程(cheng)序執行(xing)階(jie)段(duan)和輸(shu)(shu)(shu)出刷新(xin)階(jie)段(duan)中(zhong)(zhong)，輸(shu)(shu)(shu)入(ru)(ru)(ru)映像存(cun)(cun)儲器與外界隔離，其內容保持不變，直至下一個掃(sao)描周(zhou)斯的(de)輸(shu)(shu)(shu)入(ru)(ru)(ru)掃(sao)描階(jie)段(duan)，才(cai)被(bei)重(zhong)新(xin)讀入(ru)(ru)(ru)的(de)輸(shu)(shu)(shu)入(ru)(ru)(ru)信號(hao)刷新(xin)。可(ke)(ke)見(jian)，PLC在執行(xing)程(cheng)序和處理數(shu)據時，不直接使用現場當時的(de)輸(shu)(shu)(shu)入(ru)(ru)(ru)信號(hao)，而使用本(ben)次采(cai)樣時輸(shu)(shu)(shu)入(ru)(ru)(ru)到映像區(qu)中(zhong)(zhong)的(de)數(shu)據。一般來(lai)說(shuo)，輸(shu)(shu)(shu)入(ru)(ru)(ru)信號(hao)的(de)寬度要大(da)于一個掃(sao)描周(zhou)期，否(fou)則可(ke)(ke)能造(zao)成信號(hao)的(de)丟失。

(2）程(cheng)(cheng)序(xu)(xu)(xu)執(zhi)(zhi)行(xing)階段。在執(zhi)(zhi)行(xing)用戶程(cheng)(cheng)序(xu)(xu)(xu)過(guo)程(cheng)(cheng)中(zhong)，PLC按(an)照梯(ti)形圖程(cheng)(cheng)序(xu)(xu)(xu)掃描原則(ze)，一(yi)般來說(shuo)，PLC按(an)從(cong)左至右、從(cong)上到下的步驟逐(zhu)個執(zhi)(zhi)行(xing)程(cheng)(cheng)序(xu)(xu)(xu)。但(dan)遇到程(cheng)(cheng)序(xu)(xu)(xu)跳轉指(zhi)令，則(ze)根據跳轉條件(jian)是否滿(man)足來決定程(cheng)(cheng)序(xu)(xu)(xu)跳轉地址(zhi)。程(cheng)(cheng)序(xu)(xu)(xu)執(zhi)(zhi)行(xing)過(guo)程(cheng)(cheng)中(zhong)，當指(zhi)令中(zhong)涉及輸(shu)(shu)(shu)入(ru)、輸(shu)(shu)(shu)出(chu)狀(zhuang)態(tai)(tai)時，PLC就從(cong)輸(shu)(shu)(shu)入(ru)映(ying)像(xiang)寄存(cun)(cun)器(qi)(qi)中(zhong)"讀入(ru)"對(dui)(dui)應(ying)輸(shu)(shu)(shu)入(ru)端(duan)子狀(zhuang)態(tai)(tai)，從(cong)輸(shu)(shu)(shu)出(chu)映(ying)像(xiang)舂存(cun)(cun)器(qi)(qi)'讀入(ru)"對(dui)(dui)應(ying)元件(jian)（“"軟繼電(dian)(dian)器(qi)(qi)"')的當前狀(zhuang)態(tai)(tai)。然后進行(xing)相(xiang)應(ying)的運(yun)算，運(yun)算縷果再存(cun)(cun)入(ru)輸(shu)(shu)(shu)出(chu)映(ying)像(xiang)寄存(cun)(cun)器(qi)(qi)中(zhong)。對(dui)(dui)輸(shu)(shu)(shu)出(chu)映(ying)像(xiang)寄存(cun)(cun)器(qi)(qi)來說(shuo)，每(mei)一(yi)個元件(jian)“"軟繼電(dian)(dian)器(qi)(qi)"”)的狀(zhuang)態(tai)(tai)會隨(sui)著程(cheng)(cheng)序(xu)(xu)(xu)執(zhi)(zhi)行(xing)過(guo)程(cheng)(cheng)而變化。

(3）輸(shu)(shu)(shu)出(chu)(chu)(chu)刷新(xin)階(jie)段。程序執行階(jie)段的運算結(jie)果(guo)被存(cun)入輸(shu)(shu)(shu)出(chu)(chu)(chu)映像(xiang)區，而不送到輸(shu)(shu)(shu)出(chu)(chu)(chu)端(duan)口上。在輸(shu)(shu)(shu)出(chu)(chu)(chu)刷新(xin)階(jie)段，PLC將輸(shu)(shu)(shu)出(chu)(chu)(chu)映像(xiang)區中的輸(shu)(shu)(shu)出(chu)(chu)(chu)變量送入輸(shu)(shu)(shu)出(chu)(chu)(chu)鎖存(cun)器，然后由鎖存(cun)器通過輸(shu)(shu)(shu)出(chu)(chu)(chu)模塊產生(sheng)本周期(qi)的控(kong)制輸(shu)(shu)(shu)出(chu)(chu)(chu)。如果(guo)內部輸(shu)(shu)(shu)出(chu)(chu)(chu)繼(ji)電器的狀(zhuang)態為"1"，則輸(shu)(shu)(shu)出(chu)(chu)(chu)繼(ji)電器觸點閉合，經過輸(shu)(shu)(shu)出(chu)(chu)(chu)端(duan)子驅動外部負載。全部輸(shu)(shu)(shu)出(chu)(chu)(chu)設(she)備的狀(zhuang)態要保(bao)持一個掃描周期(qi)。