4-20mA和RS485的區別

RS485是通訊(xun)接口(kou)，4-20mA是模擬量輸出。

儀表(biao)輸(shu)(shu)出(chu)信號(hao)為什么選擇4-20mA電(dian)(dian)(dian)流的原因(yin)？變(bian)送器的傳統輸(shu)(shu)出(chu)直流電(dian)(dian)(dian)信號(hao)有(you)0-5V、0-10V、1-5V、0-20mA、4-20mA等，目前廣泛采用的是(shi)用4~20mA電(dian)(dian)(dian)流來傳輸(shu)(shu)模擬(ni)量。工業上廣泛采用的是(shi)用4~20mA電(dian)(dian)(dian)流來傳輸(shu)(shu)模擬(ni)量。采用電(dian)(dian)(dian)流信號(hao)的原因(yin)是(shi)不(bu)容易受干擾。并(bing)且電(dian)(dian)(dian)流源內阻無窮大，導線電(dian)(dian)(dian)阻串(chuan)聯在回路中不(bu)影響(xiang)精度，在普通(tong)雙絞線上可以傳輸(shu)(shu)數百米。

上限(xian)取(qu)20mA是因(yin)為防爆的(de)(de)要求：20mA的(de)(de)電(dian)(dian)流(liu)通斷引(yin)起的(de)(de)火花能量(liang)(liang)不是以引(yin)燃瓦斯(si)。下(xia)限(xian)沒有取(qu)0mA的(de)(de)原因(yin)是為了能檢測(ce)斷線(xian)(xian)：正常(chang)工(gong)作(zuo)時不會低(di)于4mA，當(dang)(dang)傳(chuan)輸(shu)線(xian)(xian)因(yin)故障斷路(lu)(lu)，環路(lu)(lu)電(dian)(dian)流(liu)降為0。常(chang)取(qu)2mA作(zuo)為斷線(xian)(xian)報警(jing)值。電(dian)(dian)流(liu)型變(bian)(bian)(bian)送(song)(song)器(qi)(qi)(qi)將物理量(liang)(liang)轉(zhuan)換(huan)成4~20mA電(dian)(dian)流(liu)輸(shu)出，必(bi)然要有外電(dian)(dian)源(yuan)為其供電(dian)(dian)。典型的(de)(de)是變(bian)(bian)(bian)送(song)(song)器(qi)(qi)(qi)需要兩(liang)根(gen)(gen)電(dian)(dian)源(yuan)線(xian)(xian)，加上兩(liang)根(gen)(gen)電(dian)(dian)流(liu)輸(shu)出線(xian)(xian)，總共要接4根(gen)(gen)線(xian)(xian)，稱之(zhi)為四線(xian)(xian)制(zhi)(zhi)(zhi)變(bian)(bian)(bian)送(song)(song)器(qi)(qi)(qi)。當(dang)(dang)然，電(dian)(dian)流(liu)輸(shu)出可(ke)以與(yu)電(dian)(dian)源(yuan)公(gong)用(yong)根(gen)(gen)線(xian)(xian)(公(gong)用(yong)VCC或者(zhe)GND)，可(ke)節省一(yi)(yi)根(gen)(gen)線(xian)(xian)，稱之(zhi)為三(san)線(xian)(xian)制(zhi)(zhi)(zhi)變(bian)(bian)(bian)送(song)(song)器(qi)(qi)(qi)。其實4-20mA電(dian)(dian)流(liu)本身就可(ke)以為變(bian)(bian)(bian)送(song)(song)器(qi)(qi)(qi)供電(dian)(dian)。變(bian)(bian)(bian)送(song)(song)器(qi)(qi)(qi)在(zai)電(dian)(dian)路(lu)(lu)中相當(dang)(dang)于一(yi)(yi)個(ge)特殊的(de)(de)負載(zai)，特殊之(zhi)處在(zai)于變(bian)(bian)(bian)送(song)(song)器(qi)(qi)(qi)的(de)(de)耗電(dian)(dian)電(dian)(dian)流(liu)在(zai)4~20mA之(zhi)間根(gen)(gen)據傳(chuan)感(gan)器(qi)(qi)(qi)輸(shu)出而變(bian)(bian)(bian)化(hua)，顯(xian)示儀表只(zhi)需要串在(zai)電(dian)(dian)路(lu)(lu)中即(ji)可(ke)。這種變(bian)(bian)(bian)送(song)(song)器(qi)(qi)(qi)只(zhi)需外接2根(gen)(gen)線(xian)(xian)，因(yin)而被(bei)稱為兩(liang)線(xian)(xian)制(zhi)(zhi)(zhi)變(bian)(bian)(bian)送(song)(song)器(qi)(qi)(qi)。工(gong)業電(dian)(dian)流(liu)環標準下(xia)限(xian)為4mA，因(yin)此只(zhi)要在(zai)量(liang)(liang)程(cheng)范圍內，變(bian)(bian)(bian)送(song)(song)器(qi)(qi)(qi)至(zhi)少有4mA供電(dian)(dian)。這使(shi)得(de)兩(liang)線(xian)(xian)制(zhi)(zhi)(zhi)傳(chuan)感(gan)器(qi)(qi)(qi)的(de)(de)設計成為可(ke)能。在(zai)工(gong)業應用(yong)中，測(ce)量(liang)(liang)點(dian)一(yi)(yi)般(ban)在(zai)現場，而顯(xian)示設備(bei)或者(zhe)控(kong)制(zhi)(zhi)(zhi)設備(bei)一(yi)(yi)般(ban)都(dou)在(zai)控(kong)制(zhi)(zhi)(zhi)室或控(kong)制(zhi)(zhi)(zhi)柜上。兩(liang)者(zhe)之(zhi)間距(ju)離可(ke)能數十至(zhi)數百(bai)米(mi)。按一(yi)(yi)百(bai)米(mi)距(ju)離計算，省去2根(gen)(gen)導線(xian)(xian)意味著成本降低(di)近百(bai)元。因(yin)此在(zai)實際(ji)使(shi)用(yong)中兩(liang)線(xian)(xian)制(zhi)(zhi)(zhi)傳(chuan)感(gan)器(qi)(qi)(qi)得(de)到越來越多(duo)的(de)(de)應用(yong)。

儀表(biao)輸出信號(hao)(hao)為(wei)(wei)(wei)什么選擇4-20mA電(dian)流(liu)的(de)原(yuan)因：遠(yuan)傳(chuan)信號(hao)(hao)用電(dian)流(liu)源(yuan)(yuan)優(you)于電(dian)壓(ya)源(yuan)(yuan)的(de)原(yuan)因：因為(wei)(wei)(wei)現場與(yu)控制室之(zhi)間的(de)距離較遠(yuan)，連接電(dian)線的(de)電(dian)阻較大時(shi)，如果用電(dian)壓(ya)源(yuan)(yuan)信號(hao)(hao)遠(yuan)傳(chuan)，由于電(dian)線電(dian)阻與(yu)接收(shou)儀表(biao)輸入電(dian)阻的(de)分壓(ya)，將產生較大的(de)誤差，而用恒電(dian)流(liu)源(yuan)(yuan)信號(hao)(hao)作為(wei)(wei)(wei)遠(yuan)傳(chuan)，只要傳(chuan)送(song)回(hui)路(lu)不出現分支，回(hui)路(lu)中的(de)電(dian)流(liu)就不會(hui)隨電(dian)線長短而改變，從而保證了傳(chuan)送(song)的(de)精度。

信號大電(dian)(dian)(dian)流選擇(ze)20mA的(de)原因(yin)(yin)(yin)：大電(dian)(dian)(dian)流20mA的(de)選擇(ze)是基于：安全、實(shi)用(yong)、功(gong)(gong)耗、成本的(de)考(kao)慮(lv)。安全火花(hua)儀表只能(neng)采(cai)用(yong)低電(dian)(dian)(dian)低電(dian)(dian)(dian)流綜合考(kao)慮(lv)生(sheng)產現(xian)場(chang)儀表之(zhi)間的(de)連接距離，所帶(dai)負載等(deng)因(yin)(yin)(yin)素；還(huan)有(you)就是功(gong)(gong)耗及(ji)成本問題，對(dui)電(dian)(dian)(dian)子(zi)元(yuan)件的(de)要(yao)(yao)求，供(gong)電(dian)(dian)(dian)功(gong)(gong)率的(de)要(yao)(yao)求等(deng)因(yin)(yin)(yin)素。信號起(qi)點(dian)(dian)電(dian)(dian)(dian)流選擇(ze)4mA的(de)原因(yin)(yin)(yin)：4-20mA變送(song)器(qi)(qi)兩線制的(de)居多，兩線制即電(dian)(dian)(dian)源(yuan)、負載串聯(lian)在一(yi)起(qi)，有(you)一(yi)公(gong)共點(dian)(dian)，而(er)現(xian)場(chang)變送(song)器(qi)(qi)與控(kong)制室儀表之(zhi)間的(de)信號聯(lian)絡及(ji)供(gong)電(dian)(dian)(dian)僅用(yong)兩根電(dian)(dian)(dian)線為什么起(qi)點(dian)(dian)信號不(bu)是0mA？這是基于兩點(dian)(dian)：1.變送(song)器(qi)(qi)電(dian)(dian)(dian)路沒有(you)靜(jing)態工(gong)作(zuo)電(dian)(dian)(dian)流將無法工(gong)作(zuo)，信號起(qi)點(dian)(dian)電(dian)(dian)(dian)流4mA.DC就是變送(song)器(qi)(qi)的(de)靜(jing)態工(gong)作(zuo)電(dian)(dian)(dian)流。2.同時儀表電(dian)(dian)(dian)氣(qi)零點(dian)(dian)為4mA.DC，不(bu)與機械零點(dian)(dian)重合，這種“活零點(dian)(dian)"有(you)利于識別(bie)斷電(dian)(dian)(dian)和(he)斷線等(deng)故障。

什么(me)是4-20mA？一(yi)般儀器儀表的(de)(de)(de)信(xin)號(hao)電(dian)(dian)流都為(wei)(wei)4-20mA，指小(xiao)電(dian)(dian)流為(wei)(wei)4mA大(da)電(dian)(dian)流為(wei)(wei)20mA。傳輸(shu)(shu)信(xin)號(hao)時候，要考(kao)慮(lv)到(dao)導線(xian)上也(ye)有電(dian)(dian)阻，如果用(yong)電(dian)(dian)壓傳輸(shu)(shu)則會在(zai)導線(xian)的(de)(de)(de)產生一(yi)定(ding)的(de)(de)(de)壓降，那接收端的(de)(de)(de)信(xin)號(hao)就會產生一(yi)定(ding)的(de)(de)(de)誤(wu)差了(le)！所以(yi)使用(yong)電(dian)(dian)流信(xin)號(hao)作為(wei)(wei)變送(song)器的(de)(de)(de)標(biao)準傳輸(shu)(shu)！雙(shuang)絞(jiao)線(xian)特(te)抗是50歐(ou)姆(mu)(mu)左右(you)，相隔1cm寬的(de)(de)(de)0.2平方毫米(mi)的(de)(de)(de)導線(xian)特(te)性阻抗300歐(ou)姆(mu)(mu)左右(you)，所以(yi)負載(zai)電(dian)(dian)阻選擇50-300歐(ou)姆(mu)(mu)比較(jiao)(jiao)理想，為(wei)(wei)了(le)AD轉(zhuan)換方便(bian)，負載(zai)電(dian)(dian)阻上的(de)(de)(de)信(xin)號(hao)大(da)量程(cheng)值一(yi)般5-10V比較(jiao)(jiao)合(he)適，那么(me)權衡所有，負載(zai)電(dian)(dian)阻250歐(ou)姆(mu)(mu)，電(dian)(dian)流20mA，負載(zai)壓降5V比較(jiao)(jiao)滿(man)意。

那么為什么選(xuan)擇(ze)4-20mA而(er)不(bu)是(shi)(shi)(shi)0-20mA呢？為了(le)減少接線(xian)(xian)的(de)(de)復雜(za)性，傳感器(qi)選(xuan)擇(ze)2線(xian)(xian)要(yao)比多線(xian)(xian)簡(jian)單(dan)的(de)(de)多，2線(xian)(xian)既(ji)要(yao)傳輸(shu)信號(hao)，又要(yao)給(gei)傳感器(qi)供電(dian)(dian)，所以(yi)設計(ji)者從中盜竊4mA電(dian)(dian)流給(gei)傳感器(qi)放大電(dian)(dian)路供電(dian)(dian)，這(zhe)樣(yang)4-20mA的(de)(de)標準就(jiu)確定了(le)。RS485：智能儀(yi)表(biao)(biao)是(shi)(shi)(shi)隨著80年代(dai)初單(dan)片機(ji)技術的(de)(de)成熟而(er)發展起來的(de)(de)，現在世界(jie)儀(yi)表(biao)(biao)市場基本被智能儀(yi)表(biao)(biao)所壟斷。究(jiu)其原因就(jiu)是(shi)(shi)(shi)企業信息化的(de)(de)需要(yao)，企業在儀(yi)表(biao)(biao)選(xuan)型時其中的(de)(de)一個必要(yao)條件就(jiu)是(shi)(shi)(shi)要(yao)具(ju)有聯網通信接口。初是(shi)(shi)(shi)數據模(mo)擬(ni)信號(hao)輸(shu)出(chu)簡(jian)單(dan)過程(cheng)量(liang)也(ye)就(jiu)是(shi)(shi)(shi)4-20mA，后來儀(yi)表(biao)(biao)接口是(shi)(shi)(shi)RS232接口，這(zhe)種接口可以(yi)實現點對(dui)點的(de)(de)通信方(fang)式(shi)(shi)，但這(zhe)種方(fang)式(shi)(shi)不(bu)能實現聯網功能。隨后出(chu)現的(de)(de)R5485解決了(le)這(zhe)個問題(ti)。下面我們就(jiu)簡(jian)單(dan)介紹RS485。

RS485采(cai)(cai)用差分信(xin)號(hao)負邏輯，+2V~+6V表示“0"，-6V~2V表示“1"RS485有兩線(xian)(xian)制(zhi)和(he)四線(xian)(xian)制(zhi)兩種接(jie)(jie)線(xian)(xian)，四線(xian)(xian)制(zhi)只(zhi)能實現點(dian)(dian)對點(dian)(dian)的(de)通信(xin)方式，現很少采(cai)(cai)用，現在多(duo)采(cai)(cai)用的(de)是兩線(xian)(xian)制(zhi)接(jie)(jie)線(xian)(xian)方式，這種接(jie)(jie)線(xian)(xian)方式為(wei)總線(xian)(xian)式拓樸(pu)結構(gou)在同一總線(xian)(xian)上多(duo)可以掛接(jie)(jie)32個結點(dian)(dian)。

在RS485通信(xin)(xin)網絡中一(yi)般采(cai)用的是主(zhu)從(cong)通信(xin)(xin)方式，即一(yi)個主(zhu)機帶多個從(cong)機很(hen)多情況下，連接RS485通信(xin)(xin)電路時只需簡單地用一(yi)對雙絞(jiao)線將各個接口的“A"、“B"端連接起來。

在(zai)使用(yong)RS485接口時，對(dui)于特定的(de)(de)(de)傳(chuan)(chuan)輸(shu)(shu)線路，從(cong)RS485接口到(dao)負載其數據(ju)(ju)信(xin)號(hao)(hao)傳(chuan)(chuan)輸(shu)(shu)所允許的(de)(de)(de)大電(dian)纜長度(du)(du)與(yu)信(xin)號(hao)(hao)傳(chuan)(chuan)輸(shu)(shu)的(de)(de)(de)波特率(lv)成反(fan)比(bi)，這(zhe)個長度(du)(du)數據(ju)(ju)主要是受信(xin)號(hao)(hao)失真及噪聲等(deng)影(ying)響所影(ying)響。理(li)論(lun)上RS485的(de)(de)(de)長傳(chuan)(chuan)輸(shu)(shu)距(ju)(ju)離(li)能(neng)達到(dao)1200米(mi)，但在(zai)實際(ji)應(ying)用(yong)中(zhong)傳(chuan)(chuan)輸(shu)(shu)的(de)(de)(de)距(ju)(ju)離(li)要比(bi)1200米(mi)短，具體能(neng)傳(chuan)(chuan)輸(shu)(shu)多(duo)遠視周圍環境而定。在(zai)傳(chuan)(chuan)輸(shu)(shu)過程(cheng)中(zhong)可(ke)(ke)(ke)以采用(yong)增加中(zhong)繼(ji)的(de)(de)(de)方法對(dui)信(xin)號(hao)(hao)進(jin)行放大，多(duo)可(ke)(ke)(ke)以加八個中(zhong)繼(ji)，也(ye)就是說理(li)論(lun)上RS485的(de)(de)(de)大傳(chuan)(chuan)輸(shu)(shu)距(ju)(ju)離(li)可(ke)(ke)(ke)以達到(dao)9.6公理(li)。如果真需(xu)要長距(ju)(ju)離(li)傳(chuan)(chuan)輸(shu)(shu)，可(ke)(ke)(ke)以采用(yong)光(guang)(guang)纖為傳(chuan)(chuan)播介質，收發兩(liang)端各加一(yi)個光(guang)(guang)電(dian)轉換器，多(duo)模(mo)光(guang)(guang)纖的(de)(de)(de)傳(chuan)(chuan)輸(shu)(shu)距(ju)(ju)離(li)是5~10公里(li)，而采用(yong)單模(mo)光(guang)(guang)纖可(ke)(ke)(ke)達50公里(li)的(de)(de)(de)傳(chuan)(chuan)播距(ju)(ju)離(li)。