SST-DN3-PCU-1

SST-DN3-PCU-1

SST-DN3-PCU-1在RS485通信網(wǎng)絡(luò)中般采用的是主從通信方式,即個主機(jī)帶多個從機(jī)。很多情況下,連接RS-485通信鏈路時只是簡單地用對雙絞線將各個接口的“A”、“B”端連接起來。而忽略了信號地的連接,這種連接方法在許多場合是能正常作的,但卻埋下了很大的隱患,
    這有二個原因：
    (1)共模干擾問題： RS-485接口采用差分方式傳輸信號方式,并不需要相對于某個參照點(diǎn)來檢測信號,系統(tǒng)只需檢測兩線之間的電位差就可以了。但人們往往忽視了收發(fā)器有定的共模電壓范圍,RS-485收發(fā)器共模電壓范圍為-7～+12V,只有滿足上述條件,整個網(wǎng)絡(luò)才能正常作。當(dāng)網(wǎng)絡(luò)線路中共模電壓超出此范圍時就會影響通信的穩(wěn)定可靠,甚至損壞接口。
    (2)EMI問題：發(fā)送驅(qū)動器輸出信號中的共模部分需要個返回通路,如沒有個低阻的返回通道（信號地）,就會以輻射的形式返回源端,整個總線就會像個巨大的天線向外輻射電磁波 
    現(xiàn)有的網(wǎng)絡(luò)拓?fù)浣Y(jié)構(gòu)般采用終端匹配120歐姆電阻的手牽手總線型結(jié)構(gòu),不支持環(huán)形或星形網(wǎng)絡(luò)。在構(gòu)建網(wǎng)絡(luò)時,請注意如下幾點(diǎn)： 
    （1）采用條雙絞線電纜作總線,將各個節(jié)點(diǎn)串接起來,從總線到每個節(jié)點(diǎn)的引出線長度應(yīng)盡量短,以便使引出線中的反射信號對總線信號的影響低。有些網(wǎng)絡(luò)連接盡管不正確,在短距離、低速率仍可能正常作,但隨著通信距離的延長或通信速率的提高,其不良影響會越來越嚴(yán)重,主要原因是信號在各支路末端反射后與原信號疊加,會造成信號質(zhì)量下降。 

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