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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(gè)主機(jī)帶多個(gè)從機(jī)。很多情況下,連接RS-485通信鏈路時(shí)只是簡(jiǎn)單地用對(duì)雙絞線將各個(gè)接口的“A”、“B”端連接起來(lái)。而忽略了信號(hào)地的連接,這種連接方法在許多場(chǎng)合是能正常作的,但卻埋下了很大的隱患,
這有二個(gè)原因：
(1)共模干擾問(wèn)題： RS-485接口采用差分方式傳輸信號(hào)方式,并不需要相對(duì)于某個(gè)參照點(diǎn)來(lái)檢測(cè)信號(hào),系統(tǒng)只需檢測(cè)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(gè)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(shí)就會(huì)影響通信的穩(wěn)定可靠,甚至損壞接口。
(2)EMI問(wèn)題：發(fā)送驅(qū)動(dòng)器輸出信號(hào)中的共模部分需要個(gè)返回通路,如沒(méi)有個(gè)低阻的返回通道（信號(hào)地）,就會(huì)以輻射的形式返回源端,整個(gè)總線就會(huì)像個(gè)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(shí),請(qǐng)注意如下幾點(diǎn)：
（1）采用條雙絞線電纜作總線,將各個(gè)節(jié)點(diǎn)串接起來(lái),從總線到每個(gè)節(jié)點(diǎn)的引出線長(zhǎng)度應(yīng)盡量短,以便使引出線中的反射信號(hào)對(duì)總線信號(hào)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(zhǎng)或通信速率的提高,其不良影響會(huì)越來(lái)越嚴(yán)重,主要原因是信號(hào)在各支路末端反射后與原信號(hào)疊加,會(huì)造成信號(hào)質(zhì)量下降。
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