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大家可能會非常熟悉RS232，RS485，CAN等工業上常用的總線，他們都是傳輸數字信號的方式。那麽，我們用什麽方式來傳輸模擬信號呢？工業上普遍需要測量各類非電物理量，例如溫度、壓力、速度、角度等，這些都需要轉換成模擬量電信號才能傳輸到幾百米外的控制室或顯示設備上。工業上最廣泛采用的是用4~20mA電流來傳輸模擬量。

采用電流信號的原因是不容易受幹擾，因爲工業現場的噪聲電壓的幅度可能達到數V，但是噪聲的功率很弱，所以噪聲電流通常小于nA級別，因此給4－20mA傳輸帶來的誤差非常小；電流源內阻趨于無窮大，導線電阻串聯在回路中不影響精度，因此在普通雙絞線上可以傳輸數百米；由于電流源的大內阻和恒流輸出，在接收端我們只需放置一個250歐姆到地的電阻就可以獲得0－5V的電壓，低輸入阻抗的接收器的好處是nA級的輸入電流噪聲只産生非常微弱的電壓噪聲。
上限取20mA是因为防爆的要求：20mA的电流通断引起的火花能量不足以引燃瓦斯。下限没有取0mA的原因是为了能检测断线：正常工作时不会低于4mA，当传输线因故障断路，环路电流降为0。常取2mA作为断线报警值。电流型变送器将物理量转换成4~20mA电流输出，必然要有外电源为其供电。最典型的是变送器需要两根电源线，加上两根电流输出线，总共要接4根线，称之为四线制变送器。当然，电流输出可以与电源公用一根线（公用VCC或者GND），可节省一根线，所以现在基本上将四线制变送器称之为三线制变送器。其实大家可能注意到， 4-20mA电流本身就可以为变送器供电，变送器在电路中相当于一个特殊的负载，这种变送器只需外接2根线，因而被称为两线制变送器。工业电流环标准下限为4mA，因此在量程范围内，变送器通常只有24V，4mA供电（因此，在轻负载条件下高效率的DC/DC电源（TPS54331,TPS54160），低功耗的傳感器和信号链产品、以及低功耗的处理器（如MSP430）对于两线制的4-20mA收发非常重要）。这使得两线制傳感器的设计成为可能而又富有挑战。
一般需要設計一個VI轉換器，輸入0-3.3v，輸出4mA-20mA，可采用運放LM358，供電+12v。

For an Oriental city Tokio has remarkably wide streets, and some of them are laid out with all the care of Western engineering. In the course of their morning ride the party came to Sakuradu Avenue, which Fred recognized from a drawing by a native artist, who had taken pains to preserve the architecture of the buildings on each side with complete fidelity. The foundations of the houses were of irregular stones cut in the form of lozenges, but not with mathematical accuracy. The boys had already noticed this form of hewing stone in the walls of the castles, where some very large blocks were piled. They were reported to have been brought from distant parts of the empire, and the cost of their transportation must have been very great. Few of the houses were of more than two stories, and the great majority were of only one. Along Sakuradu Avenue they were of two stories, and had long and low windows with paper screens, so that it was impossible for a person in the street to see what was going on inside. The eaves projected far over the upright sides, and thus formed a shelter that was very acceptable in the heat of summer, while in rainy weather it had many advantages. These yashikis were formerly the property of Daimios, but are now occupied by the Foreign Office and the War Department. Inside the enclosure there are many shade-trees, and they make a cooling contrast to the plain walls of the buildings. The Japanese rarely paint the interior or the exterior of their buildings. Nearly everything is finished in the natural color of the wood, and very pretty the wood is too. It is something like oak in appearance, but a trifle darker, and is[Pg 120] susceptible of a high polish. It admits of a great variety of uses, and is very easily wrought. It is known as keyaki-wood; and, in spite of the immense quantity that is annually used, it is cheap and abundant. "Shut it," said Charlotte, with a sigh like that which had risen when the lead first struck her. "If I could be moved ever so little,--" she said. "That's his look-out, madam. If the sick lady isn't Charlotte Oli'--" The truth is that Doctor Allingham had not been able to summon the courage to make a further examination of the Clockwork man; and he had permitted himself to assume that there would be no immediate developments. So far as was possible he had allowed himself that very necessary relaxation, and he had insisted upon Gregg sharing it with him. The Clockwork man was not quite what either of them had, alternatively, hoped or feared. From Allingham's point of view, in particular, he was not that bogey of the inhuman fear which his original conduct had suggested. True, he was still an unthinkable monstrosity, an awful revelation; but since the discovery of the printed instructions it had been possible to regard him with a little more equanimity. The Clockwork man was a figment of the future, but he was not the whole future. Skip this postscript if you don't want your sensibilities all Larry took a swift, sharp look around the enclosure. His railroad friend again came up with some hot coffee and broiled meat, and explained that after the train had reached a station some miles out it got orders to run back and clear the track for some trains of troops from the Army of the Potomac which were being rushed through. The Deacon's heart almost sank in despair, but he took the coffee and meat, and helped the boys to it. As they were all eating they heard a voice outside which struck on the chords of their memories: "Whose head'll you knock off?" said a burly man, thrusting himself in front of the lawyer, with his fist doubled. "Why," said Si, "I supposed that they was too tired to do anything but lay down and go to sleep. What'd they do?" "It isn't me. I only asked for a little time to think it over, and then you go and cool off." These tricks were never unkind, for David and William were the most benevolent little boys. They saw life through a golden mist, it smelt of milk and apples, it was full of soft lowings and bleatings and cheepings, of gentle noses to stroke and little downy things to hold. For the first time since it became Reuben's, Odiam made children happy. The farm which had been a galley and a prison to those before them, was an enchanted land of adventure to these two. Old Beatup, who remembered earlier things, would sometimes smile when he saw them trotting hand in hand about the yard, playing long hours in the orchard, and now and then[Pg 385] pleading as a special favour to be allowed to feed the chickens, or help fetch the cows home. He seemed to see the farm peopled by little ghosts who had never dared trot about aimlessly, or had time to play, and had fed the fowls and fetched the cows not as a treat and an adventure, but as a dreary part of the day's grind ... he reflected that "the m?aster had learned summat by the others, surelye." "My lord," said the intrepid yeoman, "I had a right to defend my dwelling against unlawful assault." Here Calverley's quick ear caught the sound of the tramping of a horse—his heart beat quick—it might be a traveller journeying to Gloucester, but it was more probable that it was the messenger. He threw the bridle of his horse over the branch of a tree, sprang to the end of the path, and, concealing himself behind the under-wood, discovered in a moment, by the dark medley hue of the rider's dress, that it was the man he expected. He hurried back, and, mounting his steed, waited till the echo of the horse's hoofs could no longer be distinguished; and then, giving the impulse to his own spirited animal, he was the next moment bounding at full speed after the messenger, followed at a distance by his accomplice. No reply was given, but the door was instantly unclosed by Holgrave. Black Jack stood in the shade, just beyond the light that streamed from within, but so close that Holgrave, without crossing the threshold, merely leant his head forward, and heard him say, "Stephen Holgrave, do you remember the cross-roads and Hailes church-yard?" HoME 欧美videosgratis浓毛ENTER NUMBET 002www.shequnet.com.cn
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