描述
? 一個嵌入式系統(tǒng)如果有電源、晶振和CPU,那么這就是我們熟悉的最小系統(tǒng)。如果該嵌入式系統(tǒng)需要運行大點的操作系統(tǒng),那么不但需要CPU具有MMU,CPU還需要外接SDRAM和NANDFLASH。如果該cpu具有SDRAM和NANDFLASH控制器,那么在硬件設(shè)計上不用過多的考慮地址線的使用。如果沒有相關(guān)的控制器,那么需要注意地址線的使用。 這部分在LAYOUT的時候是一個重點,究其原因就是要使相關(guān)信號線等長以確保信號的延時相等,時鐘和DQS的差分信號線走線。在布線的時候各種布線技巧需要綜合使用,例如與cpu對稱分布,菊花鏈布線、T型布線,這都需要依據(jù)內(nèi)存的個數(shù)多少來進行選擇,一般來說個數(shù)越多,布線越復(fù)雜,但是知道其關(guān)鍵點,一切迎刃而解。同樣道理,只有不斷地在PLC上運行這些指令,觀察運行的結(jié)果,才能弄清PLC指令的作用。很多初學(xué)者對PLC一臉的迷茫,往往是出于一種畏懼,擔(dān)心損壞設(shè)備。而這些畏懼是沒有任何道理的。仔細(xì)的閱讀手冊是非常重要的,但是僅靠讀書是成不了一個工程師的。更何況手冊上的內(nèi)容并非面面俱到。我在接觸到那些不熟悉的指令時,喜歡單獨編一個小程序,讓PLC運行。然后逐個修改條件,觀察運行的結(jié)果(MicroWin為用戶提供了非常好的監(jiān)控手段),反過來再重新理解手冊的描述,這樣就可以非常直觀的理解這些指令的作用和使用方法。不必?fù)?dān)心自己寫的程序會有什么問題,會影響PLC的正常工作。程序有沒有問題,只有讓PLC運行了才能發(fā)現(xiàn)。而發(fā)現(xiàn)問題并解決問題就是對自己能力的提高。撇開硬件操作不談,單就軟件來說,我還真沒有遇到過由于軟件問題而損壞PLC的事。在這里不必?fù)?dān)心繼電器電路接錯線可能造成的后果。所以,大膽的實踐是PLC編程的必由之路。當(dāng)然,大膽實踐并不是野蠻操作,而是必須遵循必要的規(guī)范。還有一個要注意的,在程序未經(jīng)可靠性證實之前,千萬不要掛接負(fù)載,以免造成不必要的損失。數(shù)字量的輸出有LED顯示;而模擬量處理可以采用一些硬件或軟件模擬手段來解決。
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主營DCS系統(tǒng)配件
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