機(jī)電一體化畢業(yè)設(shè)計外文翻譯

1、<p><b>　　PLC</b></p><p>　　1 .About Programmable Logic Controllers (PLCs)</p><p>　　PLCs (programmable logic controllers) are the control hubs for a wide variety of automated syst

2、ems and processes. They contain multiple inputs and outputs that use transistors and other circuitry to simulate switches and relays to control equipment. They are programmable via software interfaced via standard comput

3、er interfaces and proprietary languages and network options. Programmable logic controllers I/O channel specifications include total number of points, number of inputs and outputs,</p><p>　　2. PLC hardw

4、are </p><p>　　PLC hardware mainly has the central processing unit (CPU), memory, input unit, output unit, the communication interface, expansion interface power components. Among them, the CPU is the core, i

5、nput unit of PLC with output unit is connected site input/output devices to the CPU, interface circuit between the communication interface used with programmer, connecting peripherals such as the PC. </p><p>

6、;　　2.1 The central processing unit (CPU) </p><p>　　The same sort of microcomputer is same, the CPU is the core of PLC. PLC deployed by the CPU in different and different, with models used into three categori

7、es: general microprocessor (such as 8086, Z80, 80286, etc.), single chip microprocessor (such as 8031, etc) and a piece 8096 microprocessors (such as AMD29W, etc.). Small PLC is used mostly eight general microprocessor a

8、nd single chip microprocessor; Medium PLC is used mostly 16 general microprocessor or single chip microprocessor; Large PLC </p><p>　　2.2 memory </p><p>　　Memory basically has two kinds: one kin

9、d is read/write operation of random access memory RAM, another kind is read only memory (ROM, PROM, an EPROM and EEPROM. In PLC, memory is mainly used for storage system programming, the user program and job data. </p

10、><p>　　System programming is written by PLC manufacturer, and PLC hardware related, complete system diagnosis, command interpretation, function subroutine calling, management, logic operations, communications a

11、nd various parameters, and other functions, provide PLC operation platform. System programming related to the performance, but also in PLC PLC in use process fluctuant, so is not directly by manufacturer in read-only mem

12、ory ROM, curing PROM or an EPROM, users can't access and modification. </p><p>　　The user program with PLC control object is determined by users, according to the object of production process control req

13、uirements and prepare application. In order to facilitate read, check and modification, the user program generally exist in CMOS static RAM, use of lithium battery as backup power, to ensure off when power is not lost in

14、formation. In order to prevent interference in the destruction of RAM, when the user program after operating normally, not program, but will need to change it</p><p>　　2.3 Input/output unit </p><p

15、>　　Input/output unit usually also say I/O unit or an I/O modules, PLC and industrial production site is the connection between the components. PLC via input interface can detect various controlled object data to these

16、 data to be controlled object as PLC to control the basis; Meanwhile PLC and through the output interface will deal with the results gave controlled object, in order to realize the control purpose. </p><p>　

17、　Due to external input device, and output equipment required the signal level is varied, and internal CPU processing of PLC information is only standard level, so I/O interface to realize the conversion. The I/O interfac

18、e generally have photoelectric isolation and filter function, in order to improve the PLC anti-jamming ability. In addition, I/O interface usually a state instructions, working condition, intuitive, and easy to maintain.

19、 </p><p>　　PLC offers a variety of operation level and drive ability of I/O interface, there are various functions of I/O interface for customers to choose. The I/O interface is the main type of: the digital

20、 quantity (the switch quantity) input, the digital quantity (the switch quantity) output, analogue inputs, analog output, etc. </p><p>　　2.4 Communication interface </p><p>　　PLC with various co

21、mmunication interface, the communication interface with communications processor commonly. PLC through these communication interface can and monitors, printers, and other PLC, computers and other devices communicate. PLC

22、 and printer connections, but will process information, system parameters, such as output print; And monitor connection, but will control process image display; Connect with other PLC, can form a multiple machine system

23、or network, realize the more massive join</p><p>　　2.5. Intelligence interface module </p><p>　　Intelligent interface module is an independent computer system, it has its own CPU, system program

24、, storage and PLC system bus connected interface. It as a module, PLC system by bus and PLC is linked together, exchange data, and the coordinated management in PLC under work independently. PLC intelligent interface mod

25、ule is a lot of motion, such as: high-speed counting module, closed-loop control module, motion control module, interrupt control module, etc. </p><p>　　2.6. Programming device </p><p>　　Program

26、ming device's role is to edit, commissioning, input user program, and also can be online monitoring PLC internal state and parameters, and PLC man-machine dialogue. It is the development, application and maintenance

27、PLC indispensable tools. Programming device can be special programmer, also can be equipped with a special programming packages general computer system. Special programmer is by PLC specializes in this factory, some manu

28、facturers use PLC, and it mainly consists of product keyb</p><p>　　Simple type programmer can only online programming, and cannot directly enter and edit ladder-diagram program, should be ladder-diagram prog

29、ram into instruction table program can input. Simple programmer small volume, price cheap, it can directly interpolated in PLC programming socket, or use special cables and PLC is linked together, to facilitate the progr

30、amming and debugging. Some simple programming device used to with storage box, store the user program, such as mitsubishi FX - 20P - E simpl</p><p>　　2.7. Power </p><p>　　PLC is equipped with sw

31、itch power, for internal circuit use. Compared with ordinary power, PLC power good stability, strong anti-jamming capability. The power supply provided for grid stability requirement is not high, average allowed in the p

32、ower supply voltage rating the range of the plus or minus 15%. Many PLC also provide outward, used in dc 24V external sensor power supply. </p><p>　　2. 8.Other external equipment </p><p>　　In ad

33、dition to the above mentioned components and equipment outside, PLC and many external devices, such as an EPROM writing implement, external storage, man/machine interface device etc. An EPROM writing implement user progr

34、am can be used to cure to an EPROM memory of a PLC external equipment. In order to make the debugging good user program with an EPROM not easily lost, often written within the RAM save to an EPROM PLC. </p><p&

35、gt;　　3. INTRODUCTION</p><p>　　For simple programming the relay model of the PLC is sufficient. As more complex functions are used the more complex VonNeuman model of the PLC must be used. A VonNeuman compute

36、r processes one instruction at a time. Most computers operate this way, although they appear to be doing many things at once. Input is obtained from the keyboard and mouse, output is sent to the screen, and the disk and

37、memory are used for both input and output for storage. (Note: the directions of these arrows are very i</p><p>　　In this figure the data enters the left side through the inputs. (Note: most engineering diagr

38、ams have inputs on the left and outputs on the right.) It travels through buffering circuits before it enters the CPU. The CPU outputs data through other circuits. Memory and disks are used for storage of data that is no

39、t destined for output. If we look at a personal computer as a controller, it is controlling the user by outputting stimuli on the screen, and inputting responses from the mouse and the k</p><p>　　4. OPERATIO

40、N SEQUENCE</p><p>　　All PLCs have four basic stages of operations that are repeated many times per second. Initially when turned on the first time it will check it’s own hardware and software for faults. If

41、there are no problems it will copy all the input and copy their values into memory, this is called the input scan. Using only the memory copy of the inputs the ladder logic program will be solved once, this is called the

42、 logic scan. While solving the ladder logic the output values are only changed in temporary m</p><p>　　SELF TEST - Checks to see if all cards error free, reset watch-dog timer, etc. (A watchdog timer will ca

43、use an error, and shut down the PLC if not reset within a short period of time - this would indicate that the ladder logic is not being scanned normally).</p><p>　　INPUT SCAN - Reads input values from the ch

44、ips in the input cards, and copies their values to memory. This makes the PLC operation faster, and avoids cases where an input changes from the start to the end of the program (e.g., an emergency stop). There are specia

45、l PLC functions that read the inputs directly, and avoid the input tables.</p><p>　　LOGIC SOLVE/SCAN - Based on the input table in memory, the program is executed 1 step at a time, and outputs are updated. T

46、his is the focus of the later sections.</p><p>　　OUTPUT SCAN - The output table is copied from memory to the output chips. These chips then drive the output devices.</p><p>　　The input and outpu

47、t scans often confuse the beginner, but they are important. The input scan takes a snapshot of the inputs, and solves the logic. This prevents potential problems that might occur if an input that is used in multiple plac

48、es in the ladder logic program changed while half way through a ladder scan. Thus changing the behaviors of half of the ladder logic program. This problem could have severe effects on complex programs that are developed

49、later in the book. One side effect of the</p><p>　　When the PLC is initially turned on the normal outputs will be turned off. This does not affect the values of the inputs.</p><p>　　4.1 The Inpu

50、t and Output Scans</p><p>　　When the inputs to the PLC are scanned the physical input values are copied into memory. When the outputs to a PLC are scanned they are copied from memory to the physical outputs.

51、 When the ladder logic is scanned it uses the values in memory, not the actual input or output values. The primary reason for doing this is so that if a program uses an input value in multiple places, a change in the inp

52、ut value will not invalidate the logic. Also, if output bits were changed as each bit was changed, in</p><p>　　4.2 The Logic Scan</p><p>　　Ladder logic programs are after relay logic. In relay l

53、ogic each element in the ladder will switch as quickly as possible. But in a program elements can only be examines one at a time in a fixed sequence. Consider the ladder logic in Figure 4, the ladder logic will be interp

54、reted left-to-right, top-to-bottom. In the figure the ladder logic scan begins at the top rung. At the end of the rung it interprets the top output first, then the output branched below it. On the second rung it solves b

55、ranch</p><p>　　Ladder Logic Execution Sequence</p><p>　　The logic scan sequence become important when solving ladder logic programs which use outputs as inputs. It also becomes important when co

56、nsidering output usage. Consider Figure 5, the first line of ladder logic will examine input A and set output X to have the same value. The second line will examine input B and set the output X to have the opposite value

57、. So the value of X was only equal to A until the second line of ladder logic was scanned. Recall that during the logic scan the outputs are o</p><p>　　A Duplicated Output Error</p><p><b>

58、　　PLC</b></p><p><b>　　1．PLC介紹</b></p><p>　　PLCS（可編程邏輯控制器）是用于各種自動控制系統(tǒng)和過程的可控網(wǎng)絡(luò)集線器。他們包含多個輸入輸出，輸入輸出是用晶體管和其它電路，模擬開關(guān)和繼電器來控制設(shè)備的。PLCS用軟件接口，標(biāo)準(zhǔn)計算器接口，專門的語言和網(wǎng)絡(luò)設(shè)備編程。</p><p>　　可

59、編程邏輯控制器I/O通道規(guī)則包括所有的輸入觸點(diǎn)和輸出觸點(diǎn)，擴(kuò)展能力和最大數(shù)量的通道。觸點(diǎn)數(shù)量是輸入點(diǎn)和輸出點(diǎn)的總和。PLCS可以指定這些值的任何可能的組合。擴(kuò)展單元可以被堆棧或互相連接來增加總的控制能力。最大數(shù)量的通道是在一個擴(kuò)展系統(tǒng)中輸入和輸出通道的最大總數(shù)量。PLC系統(tǒng)規(guī)則包括掃描時間，指令數(shù)量，數(shù)據(jù)存儲和程序存儲。掃描時間是 PLC需要的用來檢測輸入輸出模塊的時間。指令是用于PLC軟件（例如數(shù)學(xué)運(yùn)算）的標(biāo)準(zhǔn)操作。數(shù)據(jù)存儲是存儲數(shù)據(jù)

60、的能力。程序存儲是控制軟件的能力。</p><p>　　用于可編程邏輯控制器的輸入設(shè)備包括DC，AC，中間繼電器，熱電偶，RTD，頻率或脈沖，晶體管和中斷信號輸入；輸出設(shè)備包括DC，AC，繼電器，中間繼電器，頻率或脈沖，晶體管，三端雙向可控硅開關(guān)元件；PLC的編程設(shè)備包括控制面板，手柄和計算機(jī)。</p><p>　　可編程邏輯控制器用各種軟件編程語言來控制。這些語言包括IEC61131-3

61、，順序執(zhí)行表（SFC），動作方塊圖（FBD），梯形圖（LD），結(jié)構(gòu)文本（ST），指令序列（IL），繼電器梯形圖（RIL），流程圖，C語言和Basic語言。IEC61131-3編程環(huán)境能支持五種語言，用國際標(biāo)準(zhǔn)加以規(guī)范，分別為SFC，F(xiàn)BD，LD，ST和IL。這便允許了多賣主兼容性和多種語言編程。SFC是一種圖表語言，它提供了編程順序的配合，就能支持順序選擇和并列選擇，二者擇其一即可。FBD用一種大的運(yùn)行庫，以圖表形式建立了一些復(fù)雜的過程

62、。標(biāo)準(zhǔn)數(shù)學(xué)和邏輯運(yùn)行可以與用戶交流和接口運(yùn)行相結(jié)合。LD是適用于離散控制和互鎖邏輯的圖表語言。它在離散控制上與FBD是完全兼容的。ST是一種文本語言，用于復(fù)雜的數(shù)學(xué)過程和計算，不太適用于圖表語言。IL是與組合編碼相似的低級語言。它用在相對比較簡單的邏輯指令。繼電器梯形圖或梯形圖是適用于可編程邏輯控制器的重要的編程語言。梯形圖編程是設(shè)計成繼電器邏輯程序的圖表表示法。流程圖是一種圖表語言，用于在一個控制器或應(yīng)用軟件中描述順序操作，它用于建立

63、有標(biāo)準(zhǔn)組件的可循環(huán)使用的運(yùn)行庫。C語言是一種高</p><p>　　可編程邏輯控制器也規(guī)范了許多計算機(jī)接口設(shè)備，網(wǎng)絡(luò)規(guī)則和特色。PLC能源設(shè)備和運(yùn)行環(huán)境也是非常重要的。</p><p><b>　　2、PLC硬件</b></p><p>　　PLC的硬件主要有中央處理器（CPU）、存儲器、輸入單元、輸出單元、通信接口、擴(kuò)展接口電源等部分組成。其

64、中，CPU是PLC的核心，輸入單元與輸出單元是連接現(xiàn)場輸入/輸出設(shè)備與CPU之間的接口電路，通信接口用于與編程器、上位計算機(jī)等外設(shè)連接。</p><p>　　2、1．中央處理單元（CPU）</p><p>　　同一般的微機(jī)一樣，CPU是PLC的核心。PLC中所配置的CPU 隨機(jī)型不同而不同，常用有三類：通用微處理器（如Z80、8086、80286等）、單片微處理器（如8031、8096等）

65、和位片式微處理器(如AMD29W等) 。小型PLC大多采用8位通用微處理器和單片微處理器；中型PLC大多采用16位通用微處理器或單片微處理器；大型PLC大多采用高速位片式微處理器。</p><p><b>　　2、2．存儲器</b></p><p>　　存儲器主要有兩種：一種是可讀/寫操作的隨機(jī)存儲器RAM，另一種是只讀存儲器ROM、PROM 、EPROM 和EEPR

66、OM。在PLC中，存儲器主要用于存放系統(tǒng)程序、用戶程序及工作數(shù)據(jù)。</p><p>　　系統(tǒng)程序是由PLC 的制造廠家編寫的，和PLC的硬件組成有關(guān)，完成系統(tǒng)診斷、命令解釋、功能子程序調(diào)用管理、邏輯運(yùn)算、通信及各種參數(shù)設(shè)定等功能，提供PLC運(yùn)行的平臺。系統(tǒng)程序關(guān)系到PLC的性能，而且在PLC使用過程中不會變動，所以是由制造廠家直接固化在只讀存儲器ROM、PROM或EPROM中，用戶不能訪問和修改。</p&g

67、t;<p>　　用戶程序是隨PLC的控制對象而定的，由用戶根據(jù)對象生產(chǎn)工藝的控制要求而編制的應(yīng)用程序。為了便于讀出、檢查和修改，用戶程序一般存于CMOS靜態(tài)RAM中，用鋰電池作為后備電源，以保證掉電時不會丟失信息。為了防止干擾對RAM中程序的破壞，當(dāng)用戶程序經(jīng)過運(yùn)行正常，不需要改變，可將其固化在只讀存儲器EPROM中?，F(xiàn)在有許多PLC直接采用EEPROM作為用戶存儲器。</p><p>　　2、3．

68、輸入/輸出單元</p><p>　　輸入/輸出單元通常也稱I/O單元或I/O模塊，是PLC與工業(yè)生產(chǎn)現(xiàn)場之間的連接部件。 PLC通過輸入接口可以檢測被控對象的各種數(shù)據(jù)，以這些數(shù)據(jù)作為PLC對被控制對象進(jìn)行控制的依據(jù)；同時PLC又通過輸出接口將處理結(jié)果送給被控制對象，以實(shí)現(xiàn)控制目的。</p><p>　　由于外部輸入設(shè)備和輸出設(shè)備所需的信號電平是多種多樣的，而PLC內(nèi)部CPU的處理的信息只能

69、是標(biāo)準(zhǔn)電平，所以I/O接口要實(shí)現(xiàn)這種轉(zhuǎn)換。I/O接口一般都具有光電隔離和濾波功能，以提高PLC的抗干擾能力。另外，I/O接口上通常還有狀態(tài)指示，工作狀況直觀，便于維護(hù)。</p><p>　　PLC提供了多種操作電平和驅(qū)動能力的I/O接口，有各種各樣功能的I/O接口供用戶選用。I/O接口的主要類型有：數(shù)字量（開關(guān)量）輸入、數(shù)字量（開關(guān)量）輸出、模擬量輸入、模擬量輸出等。</p><p>&l

70、t;b>　　2、4．通信接口</b></p><p>　　PLC配有各種通信接口，這些通信接口一般都帶有通信處理器。PLC通過這些通信接口可與監(jiān)視器、打印機(jī)、其它PLC、計算機(jī)等設(shè)備實(shí)現(xiàn)通信。PLC與打印機(jī)連接，可將過程信息、系統(tǒng)參數(shù)等輸出打印；與監(jiān)視器連接，可將控制過程圖像顯示出來；與其它PLC連接，可組成多機(jī)系統(tǒng)或連成網(wǎng)絡(luò)，實(shí)現(xiàn)更大規(guī)模控制。 與計算機(jī)連接，可組成多級分布式控制系統(tǒng)，實(shí)現(xiàn)控制

71、與管理相結(jié)合。遠(yuǎn)程I/O系統(tǒng)也必須配備相應(yīng)的通信接口模塊。</p><p>　　2、5．智能接口模塊</p><p>　　智能接口模塊是一獨(dú)立的計算機(jī)系統(tǒng)，它有自己的CPU、系統(tǒng)程序、存儲器以及與PLC系統(tǒng)總線相連的接口。它作為PLC系統(tǒng)的一個模塊，通過總線與PLC相連，進(jìn)行數(shù)據(jù)交換，并在PLC的協(xié)調(diào)管理下獨(dú)立地進(jìn)行工作。PLC的智能接口模塊種類很多，如：高速計數(shù)模塊、閉環(huán)控制模塊、運(yùn)動控

72、制模塊、中斷控制模塊等。</p><p><b>　　2、6．編程裝置</b></p><p>　　編程裝置的作用是編輯、調(diào)試、輸入用戶程序，也可在線監(jiān)控PLC內(nèi)部狀態(tài)和參數(shù)，與PLC進(jìn)行人機(jī)對話。它是開發(fā)、應(yīng)用、維護(hù)PLC不可缺少的工具。編程裝置可以是專用編程器，也可以是配有專用編程軟件包的通用計算機(jī)系統(tǒng)。專用編程器是由PLC廠家生產(chǎn)，專供該廠家生產(chǎn)的某些PLC產(chǎn)品

73、使用，它主要由鍵盤、顯示器和外存儲器接插口等部件組成。專用編程器有簡易編程器和智能編程器兩類。</p><p>　　簡易型編程器只能聯(lián)機(jī)編程，而且不能直接輸入和編輯梯形圖程序，需將梯形圖程序轉(zhuǎn)化為指令表程序才能輸入。簡易編程器體積小、價格便宜，它可以直接插在PLC的編程插座上，或者用專用電纜與PLC相連，以方便編程和調(diào)試。有些簡易編程器帶有存儲盒，可用來儲存用戶程序，如三菱的FX-20P-E簡易編程器。</

74、p><p><b>　　2、7.電源</b></p><p>　　PLC配有開關(guān)電源，以供內(nèi)部電路使用。與普通電源相比，PLC電源的穩(wěn)定性好、抗干擾能力強(qiáng)。對電網(wǎng)提供的電源穩(wěn)定度要求不高，一般允許電源電壓在其額定值±15% 的范圍內(nèi)波動。許多PLC還向外提供直流24V穩(wěn)壓電源，用于對外部傳感器供電。</p><p>　　2、8.其它外部設(shè)

75、備</p><p>　　除了以上所述的部件和設(shè)備外，PLC還有許多外部設(shè)備，如EPROM寫入器、外存儲器、人/機(jī)接口裝置等。EPROM寫入器是用來將用戶程序固化到EPROM存儲器中的一種PLC外部設(shè)備。為了使調(diào)試好用戶程序不易丟失，經(jīng)常用EPROM寫入器將PLC內(nèi)RAM保存到EPROM中。</p><p><b>　　3、指令</b></p><p

76、>　　對于簡單的編程，繼電器型PLC是有效的。隨著功能的復(fù)雜化，復(fù)雜的VonNeaman型PLC就必須被采用。一個VonNeaman計算機(jī)一次只能執(zhí)行一個指令，他們是這樣運(yùn)行的，盡管許多計算機(jī)看上去一次在做許多事情。輸入是通過鍵盤和鼠標(biāo)得到的。輸出被送到屏幕。磁盤和存儲器用于輸入和輸出存儲（注意：這些箭頭的方向?qū)τ谠O(shè)計者是非常重要的，要注意表明信息是流向哪里的。）</p><p>　　在這個圖表中數(shù)據(jù)通過輸

77、入設(shè)備進(jìn)入左邊。（注意：大多數(shù)設(shè)計圖表都是左邊輸入，右邊輸出的。）在進(jìn)入CPU之前，它穿過緩沖電路。CPU通過其他回路輸出數(shù)據(jù)。存儲器和磁盤用語存儲要輸出的數(shù)據(jù)。如果我們把個人計算機(jī)看作一個控制器，它通過在屏幕上輸出激勵和輸入來自鼠標(biāo)和鍵盤的響應(yīng)來控制用戶。</p><p><b>　　4、運(yùn)行順序</b></p><p>　　所有的PLC系統(tǒng)有每秒鐘重復(fù)多次的四種基

78、本運(yùn)行階段。最初被第一次接通時，它會檢測它的硬件和軟件是否有錯誤。如果沒有錯誤，它會把所有輸入和輸入值復(fù)制到存儲器，這叫輸入掃描。只用復(fù)制了輸入值的存儲器，梯形邏輯圖將被解決一個，這叫邏輯掃描。在解決梯形圖期間，輸出值只在臨時存儲器中被改變。當(dāng)梯形圖掃描完成后，輸出將用存儲器中臨時值修正，這叫做輸出掃描。PLC此時將從自我檢測開始重新啟動這個過程，這個過程很明顯地每秒鐘重復(fù)10到100次，正如圖所示</p><p&g

79、t;　　自我檢測—檢測是否所有的卡沒有錯誤，把時間繼電器復(fù)零等。（如果在很小一段時間內(nèi)沒有復(fù)零，時間繼電器會引起錯誤，關(guān)閉PLC系統(tǒng)?！@會表明梯形圖沒有被正常掃描。）</p><p>　　輸入掃描—從芯片上的輸入卡讀取輸入值，并把輸入值復(fù)制到存儲器，這能使PLC更快速地運(yùn)行，并且避免從程序開始到結(jié)束輸入變化。（例如：意外停止）有一些特殊的PLC功能，能直接讀取輸入值，避免了輸入表格。</p>&l

80、t;p>　　邏輯處理/掃描—基于存儲器的輸入表格，程序被一次執(zhí)行一步，同時輸出值也被修正，這是其它節(jié)的集中。</p><p>　　輸出掃描—輸出表格從存儲器復(fù)制到輸出芯片，這些芯片然后驅(qū)動輸出儀器。</p><p>　　輸入輸出掃描經(jīng)常會令初學(xué)者感到迷惑，但是他們是很重要的。輸入掃描是輸入值的快照，并且解決邏輯關(guān)系。在一個梯形圖掃描期間，如果一個輸入在梯形圖的多個地方被用到，它就會起

81、變化，潛在問題就可能發(fā)生，而輸入掃描卻避免了這些問題。這個邊境效應(yīng)是如果在一段持續(xù)時間內(nèi)如果一個輸入變化太短，它可能在輸入掃描之間會減少或者丟失。</p><p>　　當(dāng)PLC最初被啟動時，通常的輸出會被關(guān)閉，這不會影響輸入值。</p><p>　　4、1 輸入輸出掃描</p><p>　　當(dāng)輸入值被掃描到PLC時，自然輸入值被復(fù)制到存儲器。當(dāng)輸出值被掃描到PLC時

82、，他們將從存儲器復(fù)制到自然輸出設(shè)備。當(dāng)梯形圖被掃描時，它將用存儲器中的值，并不是實(shí)際的輸入輸出值。這樣做的主要原因是如果一個程序在多個地方用一個輸入值，那么輸入值的變化將使其邏輯關(guān)系無效。而且，如果隨著每塊的變化，輸出模塊也變化，在掃描結(jié)束時PLC的運(yùn)行速度將大大減慢。</p><p><b>　　4、2 邏輯掃描</b></p><p>　　梯形邏輯程序圖是模仿繼電

83、器邏輯圖的。在繼電器邏輯圖中，程序的每個元件將盡可能快地開關(guān)。但是在一個程序中，元件只能按固定的順序一次檢測一個。如圖所示，梯形圖將按從左到右，從上到下的順序被解釋。在圖中，梯形邏輯掃描將從最高層開始。在底層，它將先解釋高層輸出，然后輸出它下面的分支。在第二層，沿著梯形邏輯圖移動之前，將先解釋分支。</p><p><b>　　梯形圖邏輯執(zhí)行順序</b></p><p&g

84、t;　　解決梯形邏輯程序時，邏輯掃描順序會變得非常重要。梯形圖輸出作為輸入，考慮輸出應(yīng)用時，它也變得非常重要。如圖5所示，梯形圖第一行將檢測輸入并把輸出X置1，得到相同的值。第二行將檢測輸入B并把輸出X置1，得到相反的值。因此，直到梯形圖的第二行被掃描時X值才能與A相等。在邏輯掃描期間，輸出值只能在存儲器中被改變，只有當(dāng)梯形邏輯掃描完成時，實(shí)際的輸出才能被修正。因此，在第二行的基礎(chǔ)上，輸出掃描將修正實(shí)際輸出值。并且梯形圖的第一行將無效。