您好，4-20mA是(shi)(shi)三線(xian)(xian)(xian)式傳(chuan)(chuan)輸(shu)，儀器(qi)的(de)(de)信(xin)號(hao)(hao)傳(chuan)(chuan)輸(shu)和(he)(he)儀器(qi)的(de)(de)供電(dian)一起用三根(gen)線(xian)(xian)(xian)來完成(cheng)(cheng)(cheng)，電(dian)源(yuan)負極(ji)和(he)(he)信(xin)號(hao)(hao)負極(ji)共用一根(gen)線(xian)(xian)(xian)，另外兩(liang)(liang)根(gen)線(xian)(xian)(xian)分別是(shi)(shi)電(dian)源(yuan)正極(ji)和(he)(he)信(xin)號(hao)(hao)線(xian)(xian)(xian)正極(ji)，其優點(dian)儀器(qi)連接到控(kong)制(zhi)(zhi)器(qi)上時(shi)不(bu)需要通訊協議，很大程度上方便了不(bu)同廠家的(de)(de)儀器(qi)在一個控(kong)制(zhi)(zhi)系(xi)統中集成(cheng)(cheng)(cheng)，而且(qie)不(bu)需要任何軟(ruan)件調試。RS485是(shi)(shi)一種(zhong)數字信(xin)號(hao)(hao)的(de)(de)傳(chuan)(chuan)輸(shu)方式，必須用4根(gen)線(xian)(xian)(xian)，兩(liang)(liang)根(gen)電(dian)源(yuan)線(xian)(xian)(xian)，兩(liang)(liang)根(gen)信(xin)號(hao)(hao)線(xian)(xian)(xian)，其優點(dian)是(shi)(shi)測(ce)量結果(guo)精確傳(chuan)(chuan)輸(shu)，沒有任何傳(chuan)(chuan)輸(shu)誤差，采(cai)用總線(xian)(xian)(xian)的(de)(de)方式集成(cheng)(cheng)(cheng)儀器(qi)，一個控(kong)制(zhi)(zhi)器(qi)上可以(yi)掛接128或者(zhe)255臺儀器(qi)，節省大量布(bu)線(xian)(xian)(xian)成(cheng)(cheng)(cheng)本，儀器(qi)可以(yi)接受控(kong)制(zhi)(zhi)器(qi)指令被遠程控(kong)制(zhi)(zhi)。