可編程邏輯控制器畢業(yè)論文外文翻譯

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1、Programmable Logic Controllers (PLCS) 1 About Programmable Logic Controllers (PLCs) PLCs (programmable logic controllers) are the control hubs for a wide variety of automated systems and processes. They contain multiple inputs and outputs that use transistors and other circuitry to simulate switches

2、 and relays to control equipment. They are programmable via software interfaced via standard computer interfaces and proprietary languages and network options. Programmable logic controllers I/O channel specifications include total number of points, number of inputs and outputs, ability to expand, a

3、nd maximum number of channels.  Number of points is the sum of the inputs and the outputs. PLCs may be specified by any possible combination of these values.  Expandable units may be stacked or linked together to increase total control capacity.  Maximum number of channels refers to t

4、he maximum total number of input and output channels in an expanded system.  PLC system specifications to consider include scan time, number of instructions, data memory, and program memory.  Scan time is the time required by the PLC to check the states of its inputs and outputs.  Ins

5、tructions are standard operations (such as math functions) available to PLC software.  Data memory is the capacity for data storage.  Program memory is the capacity for control software. Available inputs for programmable logic controllers include DC, AC, analog, thermocouple, RTD, frequenc

6、y or pulse, transistor, and interrupt inputs.  Outputs for PLCs include DC, AC, relay, analog, frequency or pulse, transistor, and triac.  Programming options for PLCs include front panel, hand held, and computer. Programmable logic controllers use a variety of software programming languag

7、es for control.  These include IEC 61131-3, sequential function chart (SFC), function block diagram (FBD), ladder diagram (LD), structured text (ST), instruction list (IL), relay ladder logic (RLL), flow chart, C, and Basic.  The IEC 61131-3 programming environment provides support for fiv

8、e languages specified by the global standard: Sequential Function Chart, Function Block Diagram, Ladder Diagram, Structured Text, and Instruction List. This allows for multi-vendor compatibility and multi-language programming.  SFC is a graphical language that provides coordination of program s

9、equences, supporting alternative sequence selections and parallel sequences.  FBD uses a broad function library to build complex procedures in a graphical format. Standard math and logic functions may be coordinated with customizable communication and interface functions.  LD is a graphic

10、language for discrete control and interlocking logic. It is completely compatible with FBD for discrete function control.  ST is a text language used for complex mathematical procedures and calculations less well suited to graphical languages.  IL is a low-level language similar to assembl

11、y code. It is used in relatively simple logic instructions.  Relay Ladder Logic (RLL), or ladder diagrams, is the primary programming language for programmable logic controllers (PLCs). Ladder logic programming is a graphical representation of the program designed to look like relay logic. 

12、; Flow Chart is a graphical language that describes sequential operations in a controller sequence or application. It is used to build modular, reusable function libraries.  C is a high level programming language suited to handle the most complex computation, sequential, and data logging tasks.

13、 It is typically developed and debugged on a PC.  BASIC is a high level language used to handle mathematical, sequential, data capturing and interface functions. Programmable logic controllers can also be specified with a number of computer interface options, network specifications and features

14、.  PLC power options, mounting options and environmental operating conditions are all also important to consider.2 INTRODUCTIONFor 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 VonNeuma

15、n computer processes one instruction at a time. Most computers operate this way, although they appear to be doing many things at once. Consider the computer components shown in Figure 1.Figure 1 Simplified Personal Computer ArchitectureInput is obtained from the keyboard and mouse, output is sent to

16、 the screen, and the disk and memory are used for both input and output for storage. (Note: the directions of these arrows are very important to engineers, always pay attention to indicate where information is flowing.) This figure can be redrawn as in Figure 2 to clarify the role of inputs and outp

17、uts.Figure 2 An Input-Output Oriented ArchitectureIn this figure the data enters the left side through the inputs. (Note: most engineering diagrams 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 ci

18、rcuits. Memory and disks are used for storage of data that is not 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 keyboard.A PLC is also a computer controlling a pro

19、cess. When fully integrated into an application the analogies become;inputs - the keyboard is analogous to a proximity switchinput -circuits - the serial input chip is like a 24Vdc input cardcomputer - the 686 CPU is like a PLC CPU unitoutput - circuits - a graphics card is like a triac output cardo

20、utputs - a monitor is like a lightstorage - memory in PLCs is similar to memories in personal computersIt is also possible to implement a PLC using a normal Personal Computer, although this is not advisable. In the case of a PLC the inputs and outputs are designed to be more reliable and rugged for

21、harsh production environments.3 OPERATION SEQUENCEAll PLCs have four basic stages of operations that are repeated many times per second. Initially when turned on the first time it will check its own hardware and software for faults. If there are no problems it will copy all the input and copy their

22、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 logic scan. While solving the ladder logic the output values are only changed in temporary memory. When the ladder scan is done the outputs wil

23、l be updated using the temporary values in memory, this is called the output scan. The PLC now restarts the process by starting a self check for faults. This process typically repeats 10 to 100 times per second as is shown in Figure 3.Figure 3 PLC Scan CycleSELF TEST - Checks to see if all cards err

24、or free, reset watch-dog timer, etc. (A watchdog timer will cause 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).INPUT SCAN - Reads input values from the chips in the input cards, and copies their

25、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 special PLC functions that read the inputs directly, and avoid the input tables.LOGIC SOLVE/SCAN - Based on the input table in

26、memory, the program is executed 1 step at a time, and outputs are updated. This is the focus of the later sections.OUTPUT SCAN - The output table is copied from memory to the output chips. These chips then drive the output devices.The input and output scans often confuse the beginner, but they are i

27、mportant. 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 places in the ladder logic program changed while half way through a ladder scan. Thus changing the behaviors of half of the ladder log

28、ic program. This problem could have severe effects on complex programs that are developed later in the book. One side effect of the input scan is that if a change in input is too short in duration, it might fall between input scans and be missed.When the PLC is initially turned on the normal outputs

29、 will be turned off. This does not affect the values of the inputs.31 The Input and Output ScansWhen 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. When the ladder logic is

30、 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 input value will not invalidate the logic. Also, if output bits were changed as each bit was changed, instea

31、d of all at once at the end of the scan the PLC would operate much slower.32 The Logic ScanLadder logic programs are modelled after relay logic. In relay logic 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 seque

32、nce. Consider the ladder logic in Figure 4, the ladder logic will be interpreted 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 branch

33、es, before moving along the ladder logic rung.Figure 4 Ladder Logic Execution SequenceThe logic scan sequence become important when solving ladder logic programs which use outputs as inputs. It also becomes important when considering output usage. Consider Figure 5, the first line of ladder logic wi

34、ll 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. 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 only changed in

35、 memory, the actual outputs are only updated when the ladder logic scan is complete. Therefore the output scan would update the real outputs based upon the second line of ladder logic, and the first line of ladder logic would be ineffective.Figure 5 A Duplicated Output Error4 PLC STATUSThe lack of k

36、eyboard, and other input-output devices is very noticeable on a PLC. On the front of the PLC there are normally limited status lights. Common lights indicate;power on - this will be on whenever the PLC has powerprogram running - this will often indicate if a program is running, or if no program is r

37、unningfault - this will indicate when the PLC has experienced a major hardware or software problemThese lights are normally used for debugging. Limited buttons will also be provided for PLC hardware. The most common will be a run/program switch that will be switched to program when maintenance is be

38、ing conducted, and back to run when in production. This switch normally requires a key to keep unauthorized personnel from altering the PLC program or stopping execution. A PLC will almost never have an on-off switch or reset button on the front. This needs to be designed into the remainder of the s

39、ystem.The status of the PLC can be detected by ladder logic also. It is common for programs to check to see if they are being executed for the first time, as shown in Figure 6. The first scan input will be true on the very first time the ladder logic is scanned, but false on every other scan. In thi

40、s case the address for first scan in a PLC-5 is S2:1/14. With the logic in the example the first scan will seal on light, until clear is turned on. So the light will turn on after the PLC has been turned on, but it will turn off and stay off after clear is turned on. The first scan bit is also refer

41、red to at the first pass bit.Figure 6 An program that checks for the first scan of the PLC5 MEMORY TYPESThere are a few basic types of computer memory that are in use today.RAM (Random Access Memory) - this memory is fast, but it will lose its contents when power is lost, this is known as volatile m

42、emory. Every PLC uses this memory for the central CPU when running the PLC.ROM (Read Only Memory) - this memory is permanent and cannot be erased. It is often used for storing the operating system for the PLC.EPROM (Erasable Programmable Read Only Memory) - this is memory that can be programmed to b

43、ehave like ROM, but it can be erased with ultraviolet light and reprogrammed.EEPROM (Electronically Erasable Programmable Read Only Memory) This memory can store programs like ROM. It can be programmed and erased using a voltage, so it is becoming more popular than EPROMs.All PLCs use RAM for the CP

44、U and ROM to store the basic operating system for the PLC. When the power is on the contents of the RAM will be kept, but the issue is what happens when power to the memory is lost. Originally PLC vendors used RAM with a battery so that the memory contents would not be lost if the power was lost. Th

45、is method is still in use, but is losing favor. EPROMs have also been a popular choice for programming PLCs. The EPROM is programmed out of the PLC, and then placed in the PLC. When the PLC is turned on the ladder logic program on the EPROM is loaded into the PLC and run. This method can be very rel

46、iable, but the erasing and programming technique can be time consuming. EEPROM memories are a permanent part of the PLC, and programs can be stored in them like EPROM. Memory costs continue to drop, and newer types (such as flash memory) are becoming available, and these changes will continue to imp

47、act PLCs.6 SOFTWARE BASED PLCSThe dropping cost of personal computers is increasing their use in control, including the replacement of PLCs. Software is installed that allows the personal computer to solve ladder logic, read inputs from sensors and update outputs to actuators. These are important to

48、 mention here because they dont obey the previous timing model. For example, if the computer is running a game it may slow or halt the computer. This issue and others are currently being investigated and good solutions should be expected soon.7 SUMMARY A PLC and computer are similar with inputs, out

49、puts, memory, etc. The PLC continuously goes through a cycle including a sanity check, input scan, logic scan, and output scan. While the logic is being scanned, changes in the inputs are not detected, and the outputs are not updated. PLCs use RAM, and sometime EPROMs are used for permanent programs

50、.8 PRACTICE PROBLEMS1. Does a PLC normally contain RAM, ROM, EPROM and/or batteries?2. What are the indicator lights on a PLC used for?3. A PLC can only go through the ladder logic a few times per second. Why?4. What will happen if the scan time for a PLC is greater than the time for an input pulse?

51、 Why?5. What is the difference between a PLC and a desktop computer?6. Why do PLCs do a self check every scan?7. Will the test time for a PLC be long compared to the time required for a simple program?8. What is wrong with the following ladder logic? What will happen if it is used?9. What is the add

52、ress for a memory location that indicates when a PLC has just been turned on?9 PRACTICE PROBLEM SOLUTIONS1. Every PLC contains RAM and ROM, but they may also contain EPROM or batteries.2. Diagnostic and maintenance3. Even if the program was empty the PLC would still need to scan inputs and outputs,

53、and do a self check.4. The pulse may be missed if it occurs between the input scans5. Some key differences include inputs, outputs, and uses. A PLC has been designed for the factory floor, so it does not have inputs such as keyboards and mice (although some newer types can). They also do not have ou

54、tputs such as a screen or sound. Instead they have inputs and outputs for voltages and current. The PLC runs user designed programs for specialized tasks, whereas on a personal computer it is uncommon for a user to program their system.6. This helps detect faulty hardware or software. If an error we

55、re to occur, and the PLC continued operating, the controller might behave in an unpredictable way and become dangerous to people and equipment. The self check helps detect these types of faults, and shut the system down safely.7. Yes, the self check is equivalent to about 1ms in many PLCs, but a sin

56、gle program instruction is about 1 micro second.8. The normal output Y is repeated twice. In this example the value of Y would always match B, and the earlier rung with A would have no effect on Y.9. S2:1/14 for micro logy, S2:1/15 for PLC-5附錄3 中文翻譯可編程邏輯控制器1 PLC介紹PLCS（可編程邏輯控制器）是用于各種自動(dòng)控制系統(tǒng)和過(guò)程的可控網(wǎng)絡(luò)集線器

57、。他們包含多個(gè)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(lái)控制設(shè)備的。PLCS用軟件接口，標(biāo)準(zhǔn)計(jì)算器接口，專門的語(yǔ)言和網(wǎng)絡(luò)設(shè)備編程?？删幊踢壿嬁刂破鱅/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(lái)增加總的控制能力。最大數(shù)量的通道是在一個(gè)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(shí)間，指令數(shù)量，數(shù)據(jù)存儲(chǔ)和程序存儲(chǔ)。掃描時(shí)間是 PLC需要的用來(lái)檢測(cè)輸入輸出模塊的時(shí)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(chǔ)是存儲(chǔ)數(shù)據(jù)的能力。程序

58、存儲(chǔ)是控制軟件的能力。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(hào)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(jì)算機(jī)。可編程邏輯控制器用各種軟件編程語(yǔ)言來(lái)控制。這些語(yǔ)言包括IEC61131-3，順序執(zhí)行表（SFC），動(dòng)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(yǔ)言和Basic語(yǔ)言。IEC61131-3編程環(huán)境能支持五種語(yǔ)言，用國(guó)際標(biāo)準(zhǔn)加以規(guī)范，分別為SFC，F(xiàn)BD，LD，ST和IL。這便允

59、許了多賣主兼容性和多種語(yǔ)言編程。SFC是一種圖表語(yǔ)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(kù)，以圖表形式建立了一些復(fù)雜的過(guò)程。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(yǔ)言。它在離散控制上與FBD是完全兼容的。ST是一種文本語(yǔ)言，用于復(fù)雜的數(shù)學(xué)過(guò)程和計(jì)算，不太適用于圖表語(yǔ)言。IL是與組合編碼相似的低級(jí)語(yǔ)言。它用在相對(duì)比較簡(jiǎn)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(yǔ)言。梯形圖編程是設(shè)計(jì)成繼電器邏輯程序的圖表表示法。流程圖是一種圖表語(yǔ)言，用于在一個(gè)控制器或應(yīng)用軟件中描述順序

60、操作，它用于建立有標(biāo)準(zhǔn)組件的可循環(huán)使用的運(yùn)行庫(kù)。C語(yǔ)言是一種高級(jí)編程語(yǔ)言，適用于處理最復(fù)雜的計(jì)算，連續(xù)的數(shù)據(jù)采集任務(wù)。它典型地在PC機(jī)上運(yùn)行調(diào)試。BASIC語(yǔ)言是用于處理數(shù)據(jù)的連續(xù)的數(shù)字采集和接口運(yùn)行的高級(jí)語(yǔ)言?？删幊踢壿嬁刂破饕惨?guī)范了許多計(jì)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。2指令對(duì)于簡(jiǎn)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(gè)VonNeaman計(jì)算機(jī)一次只能執(zhí)行一個(gè)指令，他們是這樣運(yùn)行的，盡管許多計(jì)算機(jī)看上去一次在做許多事情。正如圖1所示的計(jì)算機(jī)組成。圖 1 簡(jiǎn)化個(gè)人計(jì)算機(jī)結(jié)構(gòu)圖 輸入是通過(guò)鍵盤和

61、鼠標(biāo)得到的。輸出被送到屏幕。磁盤和存儲(chǔ)器用于輸入和輸出存儲(chǔ)（注意：這些箭頭的方向?qū)τ谠O(shè)計(jì)者是非常重要的，要注意表明信息是流向哪里的。）這個(gè)圖表可以像圖2那樣能被重新擬訂來(lái)闡明輸入設(shè)備和輸出設(shè)備的作用。圖2輸入輸出示意圖 在這個(gè)圖表中數(shù)據(jù)通過(guò)輸入設(shè)備進(jìn)入左邊。（注意：大多數(shù)設(shè)計(jì)圖表都是左邊輸入，右邊輸出的。）在進(jìn)入CPU之前，它穿過(guò)緩沖電路。CPU通過(guò)其他回路輸出數(shù)據(jù)。存儲(chǔ)器和磁盤用語(yǔ)存儲(chǔ)要輸出的數(shù)據(jù)。如果我們把個(gè)人計(jì)算機(jī)看作一個(gè)控制器，它通過(guò)在屏幕上輸出激勵(lì)和輸入來(lái)自鼠標(biāo)和鍵盤的響應(yīng)來(lái)控制用戶。 PLC也是一個(gè)控制過(guò)程的計(jì)算機(jī)。當(dāng)與應(yīng)用程序完全結(jié)合起來(lái)時(shí)，類似之處變成：輸入設(shè)備鍵盤與接近開關(guān)相

62、類比。輸入電路連續(xù)輸入芯片就像一個(gè)直流24V的輸入卡。計(jì) 算 機(jī)686CPU就像一個(gè)PLC的CPU模塊。輸出電路圖形卡就像一個(gè)三相開關(guān)輸出卡。輸出設(shè)備監(jiān)控器就像指示燈。存 儲(chǔ) 器PLC的存儲(chǔ)器與個(gè)人計(jì)算機(jī)的存儲(chǔ)器相似。用普通個(gè)人計(jì)算機(jī)可以運(yùn)行PLC，雖然則并不被提倡做。就PLC來(lái)說(shuō)，輸入和輸出設(shè)備設(shè)計(jì)得更加可靠，更加粗糙，更適合惡劣的制造環(huán)境。3運(yùn)行順序所有的PLC系統(tǒng)有每秒鐘重復(fù)多次的四種基本運(yùn)行階段。最初被第一次接通時(shí)，它會(huì)檢測(cè)它的硬件和軟件是否有錯(cuò)誤。如果沒(méi)有錯(cuò)誤，它會(huì)把所有輸入和輸入值復(fù)制到存儲(chǔ)器，這叫輸入掃描。只用復(fù)制了輸入值的存儲(chǔ)器，梯形邏輯圖將被解決一個(gè)，這叫邏輯掃描。在解決梯形

63、圖期間，輸出值只在臨時(shí)存儲(chǔ)器中被改變。當(dāng)梯形圖掃描完成后，輸出將用存儲(chǔ)器中臨時(shí)值修正，這叫做輸出掃描。PLC此時(shí)將從自我檢測(cè)開始重新啟動(dòng)這個(gè)過(guò)程，這個(gè)過(guò)程很明顯地每秒鐘重復(fù)10到100次，正如圖3所示 圖3 PLC掃描循環(huán)自我檢測(cè)檢測(cè)是否所有的卡沒(méi)有錯(cuò)誤，把時(shí)間繼電器復(fù)零等。（如果在很小一段時(shí)間內(nèi)沒(méi)有復(fù)零，時(shí)間繼電器會(huì)引起錯(cuò)誤，關(guān)閉PLC系統(tǒng)。這會(huì)表明梯形圖沒(méi)有被正常掃描。）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(chǔ)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。邏輯處理/掃描基于存儲(chǔ)器的

64、輸入表格，程序被一次執(zhí)行一步，同時(shí)輸出值也被修正，這是其它節(jié)的集中。輸出掃描輸出表格從存儲(chǔ)器復(fù)制到輸出芯片，這些芯片然后驅(qū)動(dòng)輸出儀器。輸入輸出掃描經(jīng)常會(huì)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(gè)梯形圖掃描期間，如果一個(gè)輸入在梯形圖的多個(gè)地方被用到，它就會(huì)起變化，潛在問(wèn)題就可能發(fā)生，而輸入掃描卻避免了這些問(wèn)題。這個(gè)邊境效應(yīng)是如果在一段持續(xù)時(shí)間內(nèi)如果一個(gè)輸入變化太短，它可能在輸入掃描之間會(huì)減少或者丟失。當(dāng)PLC最初被啟動(dòng)時(shí)，通常的輸出會(huì)被關(guān)閉，這不會(huì)影響輸入值。31 輸入輸出掃描 當(dāng)輸入值被掃描到PLC時(shí)，自然輸入值被復(fù)制到存儲(chǔ)器。當(dāng)輸出值被掃描到PLC

65、時(shí)，他們將從存儲(chǔ)器復(fù)制到自然輸出設(shè)備。當(dāng)梯形圖被掃描時(shí)，它將用存儲(chǔ)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(gè)程序在多個(gè)地方用一個(gè)輸入值，那么輸入值的變化將使其邏輯關(guān)系無(wú)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(shí)PLC的運(yùn)行速度將大大減慢。32 邏輯掃描梯形邏輯程序圖是模仿繼電器邏輯圖的。在繼電器邏輯圖中，程序的每個(gè)元件將盡可能快地開關(guān)。但是在一個(gè)程序中，元件只能按固定的順序一次檢測(cè)一個(gè)。如圖4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(dòng)之前，將先解釋分支。