1.2 CPU212的結(jié)構(gòu)及工作原理
CPU212的工作原理是建立在下列單元的基上的：*，程序存儲(chǔ)器中存儲(chǔ)用戶程序；第二，處理器周期性掃描程序，即在一個(gè)周期開(kāi)始時(shí)，處理器將所有的輸入端的信號(hào)讀入，存儲(chǔ)于過(guò)程輸入映象寄存器(PII)，然后，在內(nèi)部計(jì)數(shù)器、位存儲(chǔ)器和定時(shí)器的控制下逐步掃描程序，接著處理器將新的信號(hào)狀態(tài)和過(guò)程輸出到映象寄存器(PIQ)中，并從這里傳送到輸出端。CPU212的結(jié)構(gòu)及工作原理示意圖如圖2所示。
1.3 CPU212的外部特征
CPU212外部包括輸入點(diǎn)、輸出點(diǎn)、編程口、面版狀態(tài)指示燈。CPU是PLC的大腦，它根據(jù)編制的應(yīng)用程序作出相應(yīng)的決定，而輸入/輸出是PLC的控制點(diǎn)。輸入用于監(jiān)視現(xiàn)場(chǎng)裝置，如開(kāi)關(guān)、傳感器等，輸出用于控制其它裝置，如電機(jī)、泵、開(kāi)關(guān)等；編程口用于連接編程裝置，如一臺(tái)計(jì)算機(jī)或手持編程器等。要裝載SETP7－MICRO/DOS軟件才能對(duì)CPU212進(jìn)行編程。由于CPU212的編程口傳輸信號(hào)采用的是RS－485標(biāo)準(zhǔn)，而計(jì)算機(jī)串行通訊口采用的是RS－232標(biāo)準(zhǔn)，因此連接時(shí)需要使用PC/PPI電纜將CPU212和計(jì)算機(jī)連接起來(lái)。由于PC/PPI電纜自帶轉(zhuǎn)換器，可完成兩種不同標(biāo)準(zhǔn)信號(hào)的轉(zhuǎn)換。如圖3所示。

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CPU212的面板狀態(tài)指示燈用于顯示CPU的運(yùn)行方式和I/O狀態(tài)。所有狀態(tài)指示燈每秒刷新多次，并用于指示CPU212內(nèi)部診斷測(cè)試結(jié)果。I/O指示燈用于指示傳感器輸入狀態(tài)和CPU控制的輸出狀態(tài)(導(dǎo)通則亮)。如圖4所示。
若I/O點(diǎn)未接，在通電時(shí)只有CPU狀態(tài)指示燈顯示CPU運(yùn)行方式。當(dāng)系統(tǒng)出現(xiàn)內(nèi)部錯(cuò)誤時(shí)，紅色的SF指示燈亮，此時(shí)應(yīng)切斷電源，清除故障。