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　　中央處理單元(CPU)是PLC控制器的控制中枢。它按照PLC控制器系统程序赋予的功能接收并存储从编程器键入的用户程序和数据;检查电源、存储器、I/O以及警戒定时器的状态，并能诊断用户程序中的语法错误。当PLC控制器投入运行时，首先它以扫描的方式接收现场各输入装置的状态和数据，并分别存入I/O映象区，然后从用户程序存储器中逐条读取用户程序，经过命令解释后按指令的规定执行逻辑或算数运算的结果送入I/O映象区或数据寄存器内。等所有的用户程序执行完毕之后，最后将I/O映象区的各输出状态或输出寄存器内的数据传送到相应的输出装置，如此循环运行，直到停止运行。

　　爲了進一步提高PLC控制器的可靠性，近年來對大型PLC還采用雙CPU構成冗余系統，或采用三CPU的表決式系統。這樣，即使某個CPU出現故障，整個系統仍能正常運行。

　　存儲器

　　存放系統軟件的存儲器稱爲系統程序存儲器。

　　存放應用軟件的存儲器稱爲用戶程序存儲器。

　　電源

　　PLC控制器的電源在整個系統中起著十分重要得作用。如果沒有一個良好的、可靠的電源系統是無法正常工作的，因此PLC的制造商對電源的設計和制造也十分重視。一般交流電壓波動在+10%(+15%)範圍內，可以不采取其它措施而將PLC控制器直接連接到交流電網上去。

　　程式輸入裝置

　　負責提供操作者輸入、修改、監視程式用作的功能

　　輸入\輸出回路

　　負責接收外部輸入元件信號和負責接收外部輸出元件信號。

　　PLC維修現在是工業領域的核心部分，在工業當中起著很重要的作用，它起源于美國，中國引進來的，用了好幾十年還很適用，但是隨著網絡技術的不斷增強，它也在不斷的改善。

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.gy2sw.com.cn
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