產(chǎn)品詳情
過(guò)程映像輸入/輸出(I/Q)、變量存儲(chǔ)器V、內(nèi)部存儲(chǔ)器位M、定時(shí)器T、計(jì)數(shù)器C等屬于全局變量。S7-200的程序組織單元(ProgramOrganizationalUnit,簡(jiǎn)稱為POU)包括主程序、子程序和中斷程序。每個(gè)POU均有自己的64字節(jié)局部變量,局部變量只能在它所在的POU中使用。與此相反,全局變量可以在各POU中使用。
下面是子程序可以使用的局部變量:
1)TEMP(臨時(shí)變量)是暫時(shí)保存在局部數(shù)據(jù)區(qū)中的變量。只有在執(zhí)行該P(yáng)OU時(shí),定義的臨時(shí)變量才被使用,POU執(zhí)行完后,不再保存臨時(shí)變量的數(shù)值。
2)IN是由調(diào)用它的POU提供的輸入?yún)?shù)。
3)OUT是返回給調(diào)用它的POU的輸出參數(shù)(子程序的執(zhí)行結(jié)果)。
4)IN_OUT是輸入_輸出參數(shù),其初始值由調(diào)用它的POU傳送給子程序,并用同一變量將子程序的執(zhí)行結(jié)果返回給調(diào)用它的POU。
主程序和中斷程序的局部變量中只有臨時(shí)變量TEMP。
具有輸入、輸出參數(shù)和局部變量的子程序易于實(shí)現(xiàn)結(jié)構(gòu)化編程,對(duì)于長(zhǎng)期生產(chǎn)同類設(shè)備或生產(chǎn)線的廠家尤為有用。這些廠家的編程人員為設(shè)備的各組件或工藝功能編寫了大量的通用的子程序。即使不知道子程序的內(nèi)部代碼,只要知道子程序的功能和輸入、輸出參數(shù)的意義,就可以通過(guò)程序之間的調(diào)用快速“組裝”出滿足不同用戶要求的控制程序。就好像用數(shù)字集成電路芯片組成復(fù)雜的數(shù)字電路一樣。
子程序如果沒(méi)有輸入、輸出參數(shù),它和調(diào)用它的程序之間沒(méi)有清晰的接口,很難實(shí)現(xiàn)結(jié)構(gòu)化編程。
子程序如果沒(méi)有局部變量,它和調(diào)用它的程序之間只能通過(guò)全局變量來(lái)交換數(shù)據(jù),子程序內(nèi)部也只能使用全局變量。將子程序和中斷程序移植到別的項(xiàng)目時(shí),需要重新統(tǒng)一安排它們使用的全局變量,以保證不會(huì)出現(xiàn)地址沖突。當(dāng)程序很復(fù)雜,子程序和中斷程序很多時(shí),這種重新分配地址的工作量非常大。
如果子程序和中斷程序有局部變量,并且它們內(nèi)部只使用局部變量,不使用全局變量,因?yàn)榕c其他POU沒(méi)有地址沖突,不需作任何改動(dòng),就可以將子程序移植到別的項(xiàng)目中去。
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