使用繼電器電路或PLC的梯形圖可以實現開關量的邏輯運算。圖1—4的上面是PLC的梯形圖，梯形圖中某些編程元件(如輸出繼電器和輔助繼電器)的線圈“通電”時，其常開觸點閉合，常閉觸點斷開，稱該編程元件為1狀態。當它們的線圈“斷電"時，其常開觸點斷開，常閉觸點閉合，稱該編程元件為0狀態。

    圖1—4中的A，B為輸入邏輯變量，M為輸出邏輯變量，它們之間的“與”、“或”、“非”邏輯運算關系如表1．1所示。用繼電器電路或梯形圖可以實現基本邏輯運算，觸點的串聯可實現“與”運算，觸點的并聯可實現“或”運算，用常閉觸點控制線圈可實現“非”運算(見圖1-4)。多個觸點的串、并聯電路可以實現復雜的邏輯運算，例如圖l-3中的繼電器電路實現的邏輯運算可用邏輯代數表達式表示為

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