在40年代，過(guò)程控制是基于3～15PSI的氣動(dòng)標(biāo)準(zhǔn)信號(hào)。其后，由于4～20mA模擬信號(hào)的使用，使得模擬控制器得到了廣泛應(yīng)用，但是并不是所有的傳感儀表和驅(qū)動(dòng)裝置都使用統(tǒng)一的4～20mA信號(hào)。70年代，由于在檢測(cè)、模擬控制和邏輯控制領(lǐng)域率先使用了計(jì)算機(jī)，從而產(chǎn)生了集中控制。進(jìn)入80年代，由于微處理器的出現(xiàn)，促使工業(yè)儀表進(jìn)入了數(shù)字化和智能化的時(shí)代，4～20mA模擬信號(hào)傳輸逐步被數(shù)字化通信代替，加之分布式控制以及網(wǎng)絡(luò)技術(shù)的迅速發(fā)展，促進(jìn)了控制、調(diào)度、優(yōu)化、決策等功能一體化的發(fā)展。然而由于檢測(cè)、變送、執(zhí)行等機(jī)構(gòu)大都采用模擬信號(hào)連接，其傳送方式是一對(duì)一結(jié)構(gòu)，這使得接線復(fù)雜，工程費(fèi)用高，維護(hù)困難，而信號(hào)傳輸精度底，易受干擾，儀表互換性差，這都阻礙了上層系統(tǒng)的功能發(fā)揮。另一方面，由于智能儀表的功能遠(yuǎn)遠(yuǎn)超過(guò)了現(xiàn)場(chǎng)模擬儀表，如對(duì)量程和零點(diǎn)進(jìn)行遠(yuǎn)方設(shè)定，儀表工作狀態(tài)實(shí)現(xiàn)自診斷，能進(jìn)行多參數(shù)測(cè)量和對(duì)環(huán)境影響的補(bǔ)償?shù)取Ｓ纱丝梢?jiàn)，智能儀表和控制系統(tǒng)的發(fā)展，都要求上層系統(tǒng)和現(xiàn)場(chǎng)儀表實(shí)現(xiàn)數(shù)字通信。
.控制網(wǎng)絡(luò)的發(fā)展,其基本趨勢(shì)是逐漸趨向于開(kāi)放性、透明的通訊協(xié)議。上述出現(xiàn)的問(wèn)題,根本原因在于現(xiàn)場(chǎng)總線的開(kāi)放性是有條件的、不徹底的。以太網(wǎng)具有傳輸速度高、低耗、易于安裝和兼容性好等方面的優(yōu)勢(shì),由于它支持幾乎所有流行的網(wǎng)絡(luò)協(xié)議,所以在商業(yè)系統(tǒng)中被廣泛采用。近些年來(lái),隨著網(wǎng)絡(luò)技術(shù)的發(fā)展,以太網(wǎng)進(jìn)入了控制領(lǐng)域,形成了新型的以太網(wǎng)控制網(wǎng)絡(luò)技術(shù)。這主要是由于工業(yè)自動(dòng)化系統(tǒng)向分布化、智能化控制方面發(fā)展,開(kāi)放的、透明的通訊協(xié)議是必然的要求?，F(xiàn)場(chǎng)總線由于種類(lèi)繁多，互不兼容，尚不能滿(mǎn)足這一要求。而以太網(wǎng)的TCP/IP協(xié)議的開(kāi)放性使得在工控領(lǐng)域通訊這一關(guān)鍵環(huán)節(jié)具有無(wú)可比擬的優(yōu)勢(shì)
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TA6307H09 TAPER BUSH REDUCER 906003 DODGE-RELIANCE
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Tabor Electronics WS8251 Single-Channel Arbitrary / Function Generator -
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