指令表畢業(yè)論文外文翻譯

1、外文翻譯第 6章指令表、順序結(jié)構(gòu)圖和結(jié)構(gòu) 文本的程序設(shè)計方法 本章繼續(xù)接著上一章討論另外幾種標(biāo)準(zhǔn)編程語言，例如，指令表（il）、順序結(jié)構(gòu)圖（sfc）和結(jié)構(gòu)文本（st）。 6.1指令表 指令表是一種采用文本形式進(jìn)行梯形圖程序設(shè)計的編程方法。指令表的程序由一系列的指令組成，每條指令占一行。程序指令由操作碼及的一個至多個操作數(shù)組成，執(zhí)行對象是操作碼。梯形圖中操作碼被看作梯形圖的構(gòu)元素。每條指令都可以使用或更改存在記憶寄存器中的信息。為此，使用了助記符代碼，每個代碼對應(yīng)一個操作碼梯形圖元素。盡管標(biāo)準(zhǔn) iec 1131-3 已經(jīng)被提出并廣泛使用，但是不同制造商使用的指令一定程度上并不一樣。表6.1 給出

2、一些廠家所使用的指令，及本章所提議的標(biāo)準(zhǔn)指令（后面章節(jié)將講述指令的其他功能。） 下面給出使用iec 113-1 標(biāo)準(zhǔn)操作碼的例子： ld a(*加載a*) and b (*與b*) st q (*存儲結(jié)果q,輸出q*) 在程序的第一行，ld 是操作碼，a 是操作數(shù)，在程序末尾括號中*之間的內(nèi)容是評論，解釋這行指令的作用，它是 plc 指令的一部分。ld a 這條指令表示將a 加載至寄存器中在之后的操作中可以調(diào)用它。第二行是 a 與b 的布爾運算中的與操作。最后一行是將結(jié)果存儲在 q 中，即向q 輸出。 標(biāo)號可以用來標(biāo)識不同的進(jìn)入程序的入口，在程序跳轉(zhuǎn)時十分有用。標(biāo)號在指令的前面被冒號分隔開。因

3、此有： pump_ok : ld c (*加載c*) 在程序之前面有一條指令，如果某個特殊的條件滿足，那么程序就跳轉(zhuǎn)執(zhí)行pump_ok。 采用iec 113-1 標(biāo)準(zhǔn)的操作碼，在值后面加上字符n 表示取反。例如： ld a （*加載a*） andn b （*and not b*） 其中，andn 操作把梯形圖觸點先“與”后“取反”。 6.1.1梯形圖程序設(shè)計及指令表 下面看看梯形圖的條件，當(dāng)一個梯級開始工作，必須要使用一個“開始梯級代碼 它以是ld 或l，用于指示梯級以常開觸點開始?；蛘呤莑di、ldn、ld not、an 或 ln,用來指示梯級以常閉觸點開始。所有梯級必須以一個輸出或存儲結(jié)果

4、的編碼結(jié)束。它可以是 out、=或 st。下圖描述了一個梯形圖中各獨立梯級是如何輸入的，其中與門使用三菱的記憶，如圖6.1a 所示。 此梯形圖以操作碼ld開始，因為它開始于常開觸點。由于輸入地址是x400，所以加載指令為 ldx400，其后還有一個常開觸點作為輸入，所以下一行程序指令用and 指令對元件地址邏輯與操作，相應(yīng)語句為：and x401。因為，這個梯級要以輸出終止，所以用輸出指令 out 產(chǎn)生輸出，即 out y430。因此，梯形圖的單個梯級可以用程序指 令描述為： ld x400 and x401 out y430 同樣的功能，采用西門子符號，如圖 6.1b 所示。 a i0.1

5、a i0.2 = q2.0 再看另一個與門的例子，如圖 6.2a 所示，采用的是三菱的符號。 梯形圖以常開觸點開始，指令為ld x400， 與x401并聯(lián)，因此下一條指令為or x401。最后一條是輸出，所以輸出為 out y430。指令表如下： ld x400 or x401 out y430 如圖6.2b 所示的是西門子符號的或門，指令表如下： a i0.1 o i0.2 = q2.0 字母 i 用來表示對這條指令的取反。緊接著串聯(lián)一個常閉觸點，其指令為 ani，i 再次表示對這條與指令的取反，當(dāng)然 i 也是一個非門的指令。所梯形圖中或非門的指令可以寫成： ldi x400 ani x40

6、1 out y430 圖 6.3b 是西門子的或非門符號。注意指令后面的字母 n 的作用是取反。指令表如下： ln i0.1 an i0.2 = q2.0 圖6.4a 為三菱符號表示的與非門。 梯形圖以常觸點 x400 開始，指令語句為 ldi x400。下一條對應(yīng)于一個并聯(lián)的常閉觸點，其指令為 or1 x401。最后一步是輸出，指令為 out y430。因此，指令表如下： ldi x400 ori x401 out y430 圖6.4b 為采用西門子符號的異或門。指令表如下： an i0.1 on i0.2 = y430 6.1.2分支代碼 如圖 6.5a 所示的采用三菱符號的異或門，它有兩

7、條并聯(lián)的分支，每條分支分別由與關(guān)系構(gòu)成 圖中三菱公司使用orb 指令來表示或和并聯(lián)分支的關(guān)系。第一條指令對應(yīng)一個常開觸點x400。下一條指令對應(yīng)與之串聯(lián)的常閉觸點 x401，指令為ani x401。在讀取前兩條指令之后，第三條指令開始于新的一行。另起一行是因為它開始于指令 ldi，所有新的一行指令語句都是以 ld 或 ldi 開始。但是第一行不能以輸出結(jié)束，所以plc 把與其并聯(lián)的那行包含在第二行，這樣讀取以上所列內(nèi)容直到orb 指令。助記碼 orb（分支或模塊間的或運算）用于將前兩行及第三、四行的指令執(zhí)行結(jié)果進(jìn)行邏輯或運算。指令表以輸出 out y430 終止。指令表如下： ld x400

8、ani x401 ldi x400 and x401 orb out y430 圖 6.5b 是西門子的異或門。括弧用來表示將特殊指令結(jié)合在一起作為一個區(qū)塊執(zhí)行。 這種用法和數(shù)學(xué)方程中的括弧用法相同。例如，（2+3)/4表示2 和3相加的結(jié)果再除以 4。 因此，西門子指令表中從第 0 步的指令 a 開始，括弧到第三步結(jié)束，表示第 0 步的a 只能允許在第一步和第二步指令執(zhí)行結(jié)束后才能夠執(zhí)行。 iec 1131-3 標(biāo)準(zhǔn)應(yīng)用于此段程序就是使用上述西門子例子中的括弧來實現(xiàn)的。 圖 6.6a 的電路圖，采用三菱符號，可看成是兩個分支模進(jìn)行與操作。所采用的指令是anb。指令表如下： 圖 6.6b 是用

9、西門子符號來表示的同一個電路。本程序中的指令表使用了括弧。第0 步的指令a 應(yīng)用到第1、2 步的結(jié)果中。第4 步中的指令a 應(yīng)用到第5、6 步的結(jié)果中。所以程序的指令表如下： 6.1.3多個梯級 圖 6.7a 是用三菱符號表示的梯形圖，它有兩層梯級。在編寫指令表時，按順序?qū)懗雒啃械闹噶?。指?ld 或ldi 表示plc 新的一層梯級開始。指令表如下： ld x400 out y430 ldi x400 out y431 此系統(tǒng)表明，當(dāng)x400 沒有被觸發(fā)時，y431 有輸出，y430 沒有輸出。當(dāng) x400被觸發(fā)時，y430 有輸 出，y431 沒有輸出。 圖 6.7b 用西門子符號描述了同樣

10、的程序。=指令表示一行結(jié)束。a 或 an 指令可以表示一層梯級的開始。但不是必需的，因為還可以用 and 和and not 表示該指令。則指令表如下： a i0.0 = q2.0 an i0.0 = q2.1 6.1.4程序舉例 下面介紹本節(jié)所述程序設(shè)計方法的具體應(yīng)用，以 5.7 節(jié)的梯形圖和功能塊為例。對照5.7 節(jié)中梯形圖的解釋，在此只講述和程序相關(guān)的指令表。 如果水泵在運行狀態(tài)，且滿足一定壓力條件或指示燈開關(guān)閉合，那么信號燈亮。圖6.8 給出了梯形圖和相關(guān)的指令表。 對于一個用來提升負(fù)載的閥，當(dāng)泵在運行并且提升開關(guān)閉合或有開關(guān)指示負(fù)載在底部提升槽中而沒有被提升，那么閥工作。圖 6.9 是

11、其梯形圖及與之對應(yīng)的指令表 當(dāng)四個傳感器中的任意一個有輸出時，系統(tǒng)都沒有輸出產(chǎn)生，否則有輸出產(chǎn)生，圖6.10 給出了梯形圖程序及與之對應(yīng)的指令表。 6.2 順序功能圖 如果想描述一個交通燈序列。一種方法就是把它看成是一系列的功能或狀態(tài)，例如紅燈 狀態(tài)、綠燈狀態(tài)，然后考慮每個狀態(tài)的輸人和輸出。 如圖 6.11 就是對此的描述。狀態(tài) 0 的輸人在綠燈持續(xù)亮一分鐘后觸發(fā)，然后以紅燈亮 作為輸出。狀態(tài) i 的輸人在紅燈持續(xù)亮一分鐘后觸發(fā)，然后以綠燈亮作為輸出。 順序功能圖是一種系統(tǒng)操作的圖形表示，它描述了一系列操作中的事件順序。順序功能圖遵循以下規(guī)則： 1.這種操作用互相獨立、相繼連接的狀態(tài)或動作來描

12、述，用矩形方框來表示狀態(tài)，每 個方框代表被控系統(tǒng)的某個狀態(tài)。程序的初始化動作與其他動作的表示方法有所不同。圖 6.12 給出了它的表示法。 2.在兩個狀態(tài)之間的連接線上有一條水平的短線，表示系統(tǒng)從一個狀態(tài)到下一個狀態(tài) 需要滿足的轉(zhuǎn)換條件。兩個動作之間不能直接連接，它們中間必須加轉(zhuǎn)換條件。兩個轉(zhuǎn)換條 件之間也不能直連，中間必須有動作狀態(tài)。 3.下一狀態(tài)的轉(zhuǎn)換條件滿足時，程序中的下一個狀態(tài)或動作就開始執(zhí)行。 4.從一個狀態(tài)到下一個狀態(tài)的程序進(jìn)程將會持續(xù)執(zhí)行直到一個完整的機(jī)器周期結(jié)束。 5.任一狀態(tài)的輸出/動作都由水平連接的方框表示，當(dāng)狀態(tài)轉(zhuǎn)換條件滿足時，產(chǎn)生輸出。 圖6.13 描述了部分順序功能圖

13、及其等價的梯形圖。 為了說明順序功能圖的規(guī)則，以庭洗衣機(jī)的部分洗漆周期為例。洗衣機(jī)滾桶內(nèi)注滿水 后便打開加熱器開關(guān)，直到溫度達(dá)到求的標(biāo)準(zhǔn)，然后滾桶轉(zhuǎn)動一段特定時間。用一系列順 序狀態(tài)表示，如圖6.14 所示 6.2.1分支和匯聚 如圖6.15 所示，當(dāng)跳轉(zhuǎn)條件滿足時，選擇性分支便轉(zhuǎn)移至相應(yīng)的狀態(tài)。 并聯(lián)分支如圖 6.16 所示，由一對平行線表示，可同時執(zhí)行兩個或多個不同的狀態(tài)。 圖 6.17 和圖 6.18 給出了順序功能圖中匯聚的表示方法。在圖 6.17 中，若in4 發(fā)生， 執(zhí)行順序從狀態(tài)2 轉(zhuǎn)移到狀態(tài)4，或若in5 發(fā)生，可從狀態(tài)3 轉(zhuǎn)移到狀態(tài)4。在圖6.18 中， 若in4 發(fā)生，執(zhí)行

14、順序可同時從狀態(tài) 2、3 轉(zhuǎn)移到狀態(tài)4 6.2.2動作 對于某個狀態(tài)來說，可以執(zhí)行一個或多個動作。所謂“動作”就像上面例子中的輸出，用方框表示。 動作的行為可以用梯形圖、功能塊、指令表和結(jié)構(gòu)文本來描述。所以，當(dāng)使用梯形圖 時，動作的行為由方框中的梯形圖表示。當(dāng)有電流輸人“動作框圖”中時，方所表示的 動作就會被激活。如圖 6.20 所示。 在動作框圖前通常附一個限定符，用以說明動作執(zhí)行的條件。若有限定符或限定符 在相關(guān)狀態(tài)激活時，所要求動作就不能被存儲及連續(xù)執(zhí)行。限定符 p用于脈沖動作，當(dāng)某一程序步驟激活時它只執(zhí)行一次。限定符 d 用于延時動作，在一段時間之后相應(yīng)動作才會激發(fā)。限定符 l 用于限

15、時動作，在執(zhí)行一段時間以后才停止相應(yīng)動作，如圖 6.21 所示。 6.3結(jié)構(gòu)文本 結(jié)構(gòu)文本是一種和 pascal 極其相似的編程語言。程序由一系列被分號隔開的陳述語句構(gòu)成。這些語句使用預(yù)先定義的語句和子函數(shù)來改變變量，變量可以是已定義的值或內(nèi)部存儲值或輸入/輸出值。 賦值語句用于對欲改變的變量進(jìn)行賦值。例如： light:=switcha； 表示變量light 的值將被改變，當(dāng)開關(guān) a 閉合或斷開時，變量light 的值也隨之變化。賦值語句的一般格式如下：x:=y ; 其中，x 是自變量，y 是因變量。再看另一個例子： light:switcha or switchb； 表示燈的開關(guān)由開關(guān) a

16、 或b 控制。 表6.2 給出了一些結(jié)構(gòu)文本中的操作符， 如上面語句中的or。并且也列出了它們在 程序運行時的優(yōu)先級。例如， inputa=6 inputb=4 inputc=2 outputq:=inputa/3+inputb/(3-inputc) 其中，(3-inputc)的值作為函數(shù)的除數(shù)，所以計算輸出 q 語句的第二步是4/(3-2)=4。除法比加法優(yōu)先，所以第一步在計算加法之前計算 6/3。因此，輸出outputq 的值是2+1=3。 結(jié)構(gòu)文本對字符大小寫不敏感，因此為了增強(qiáng)可讀性可任意使用大小寫字母。同樣，也可使用空格鍵或空行來增強(qiáng)可讀性。所有表示變量的等式都以字符%開始，在其后面

17、是一個或兩個字母碼用以標(biāo)識其存儲單元和其相應(yīng)數(shù)據(jù)類型，其中存儲單元包括輸入/輸出存儲器或內(nèi)部存儲器，數(shù)據(jù)類型包括位、字節(jié)、字。例如： %ix100(*input memory bit 100*) %id200(*input memory word200*) %qx100(*output memory bit 100*) 第一個字母 i 是輸入存儲單元，q 是輸出存儲單元，m 是內(nèi)部存儲器。第二個字母x 表示位，8 表示字節(jié)i(8 位)，w 表示字(16 位)，d 表示雙字(32 位)，l表示長字(64 位)。 at 用來確定變量的存儲單元。因此有： input at %ix100; (*inp

18、ut is located at input memory bit 100*) 英文原文6 il, sfc and st programming methods this chapter continues from the previous chapter and discusses the other iec 1131-3 (international electrotechnical commission) programming languages, i.e. instruction lists (il), sequential function charts (sfc), and s

19、tructured text (st). 6.1 instruction lists a programming method, which can be considered to be the entering of a ladder program using text, is instruction lists (il). instruction list gives programs which consist of a series of instructions, each instruction being on a new line. an instruction consi

20、sts of an operator followed by one of more operands, i.e. the subjects of the operator. in terms of ladder diagrams an operator may be regarded as a ladder element. each instruction may either use or change the value stored in a memory register. for this, mnemonic codes are used, each code correspon

21、ding to an operator/ladder element. the codes used differ to some extent from manufacturer to manufacturer, though a standard iec 1131-3 has been proposed and is being widely adopted. table 6.1 shows some of the codes used by manufacturers, and the proposed standard, for instructions used in this ch

22、apter (see later chapters for codes for other functions). table 6.1 instruction code mnemonics store result register an output into operand ld a (*load a*) and b (*and b*) st q (* store result in q, i.e. output to q*) in the first line of the program, ld is the operator, a the operand, and the words

23、 at the ends of program lines and in brackets and preceded and followed by * are comments added to explain what the operation is and are not part of the program operation instructions to the plc. ld a is thus the instruction to load the a into the memory register. it can then later be called on for

24、further operations. the next line of the program has the boolean operation and performed with a and b. the last line has the result stored in q, i.e. outputted to q. labels can be used to identify various entry points to a program, useful as we will find later for jumps in programs, and these preced

25、e the instruction and are separated from it by a colon. thus we might have: pump_ok: ld c (* load c*) with there being the instruction earlier in the program to jump to pump_ok if a particular condition is realised. with the iec 113-1 operators, an n after it is used to negate its value. for example

26、, if we have: ld a (*load a*) andn b (*and not b*) thus the andn operator inverts the value of ladder contacts and ands the result. 6.1.1 ladder programs and instruction lists when looked at in terms of ladder diagrams, whenever a rung is started, it must use a start a rung code. this might be ld, o

27、r perhaps a or l, to indicate the rung is starting with open contacts, or ldi, or perhaps ldn or ld not or an or ln, to indicate it is starting with closed contacts. all rungs must end with an output or store result code. this might be out or = or st. the following shows how individual rungs on a la

28、dder are entered using the mitsubishi mnemonics for the and gate, shown in figure 6.1(a). the rung start with ld because it is starting with open contacts. since the address of the input is x400, the instruction is ld x400. this is followed by another open contacts input and so the next program line

29、 involves the instruction and with the address of the element, thus the instruction is and x401. the rung terminates with an output and so the instruction out is used with the address of the output, i.e. out y430. the single rung of a ladder would thus be entered as: ld x400 and x401 out y430 for th

30、e same rung with siemens notation (figure 6.1(b) we have: a i0.1 a i0.2 = q2.0 consider another example, an or gate. figure 6.2(a) shows the gate with mitsubishi notation. the instructions for the rung start with an open contact is ld x400. the next item is the parallel or set of contacts x401. thus

31、 the next instruction is or x401. the last step is the output, hence out y430. the instruction list would thus be: ld x400 or x401 out y430 figure 6.2(b) shows the siemens version of the or gate. the following is the siemens instruction list: a i0.1 o i0.2 = q2.0 figure 6.3(a) shows the ladder syste

32、m for a nor gate in mitsubishi notation. the rung starts with normally closed contacts and so the instruction is ldi. i when added to mitsubishi instruction is used to indicate the inverse of the instruction. the next step is a series normally closed contact and so ani, again the i being used to mak

33、e an and instruction the inverse. i is also the instruction for a not gate. the instructions for the nor gate rung of the ladder would thus be entered as: ldi x400 ani x401 out y430 figure 6.3(b) shows the nor gate with siemens notation. note that n added to an instruction is used to make the invers

34、e. the instruction list then becomes: ln i0.1 an i0.2 = q2.0 consider the rung shown in figure 6.4(a) in mitsubishi notation, a nand gate. it starts with the normally closed contacts x400 and so starts with the instruction ldi x400. the next instruction is for a parallel set of normally closed conta

35、cts, thus the instruction is ori x401. the last step is the output, hence out y430. the instruction list is thus: ldi x400 ori x401 out y430 figure 6.4(b) shows the nand gate in siemens notation. the instruction list is then: an i0.1 on i0.2 = q2.0 6.1.2 branch codes the exclusive or (xor) gate show

36、n in figure 6.5(a) in mitsubishi notation has two parallel arms with an and situation in each arm. in such a situation mitsubishi uses an orb instruction to indicate or together parallel branches. the first instruction is for a normally open pair of contacts x400. the next instruction is for a serie

37、s set of normally closed contacts x401, hence ani x401. after reading the first two instructions, the third instruction starts a new line. it is recognised as a new line because it starts with ldi, all new lines starting with ld or ldi. but the first line has not been ended by an output. the plc thu

38、s recognises that a parallel line is involved for the second line and reads together the listed elements until the orb instruction is reached. the mnemonic orb (or branches/blocks together) indicates to the plc that it should or the results of the first and second instructions with that of the new b

39、ranch with the third and fourth instructions. the list concludes with the output out y430. the instruction list would thus be entered as: ld x400 ani x401 ldi x400 待添加的隱藏文字內(nèi)容3and x401 orb out y430 figure 6.5(b) shows the siemens version of xor gate. brackets are used to indicate that certain instruc

40、tions are to be carried out as a block. they are used in the same way as brackets in any mathematical equation. for example (2 + 3)/4 means that the 2 and 3 must be added before dividing by 4. thus with the siemens instruction list we have in step 0 the instruction a(. the brackets close in step 3.

41、this means that the a in step 0 is applied only after the instructions in steps 1 and 2 have been applied. the iec 1131-3 standard for such programming is to use brackets in the way used in the above siemens example. figure 6.6(a) shows a circuit, in mitsubishi notation, which can be considered as t

42、wo branched and blocks. the instruction used here is anb. the instruction list is thus: figure 6.6(b) shows the same circuit in siemens notation. such a program is written as an instruction list using brackets. the a instruction in step 0 applies to the result of steps 1 and 2. the a instruction in

43、step 4 applies to the result of steps 5 and 6. the program instruction list is thus: figure 6.7(a) shows a ladder, in mitsubishi notation, with two rungs. in writing the instruction list we just write the instructions for each line in turn. the instruction ld or ldi indicates to the plc that a new r

44、ung is starting. the instruction list is thus: ld x400 out y430 ldi x400 out y431 the system is one where when x400 is not activated, there is an output from y431 but not y430. when x400 is activated, there is then an output from y430 but not y431. figure 6.7(b) shows the same program in siemens not

45、ation. the = instruction indicates the end of a line. the a or an instruction does not necessarily indicate the beginning of a rung since the same instruction is used for and and and not. the instruction list is then: a i0.0 = q2.0 an i0.0 = q2.1 6.1.4 programming examples the following tasks are in

46、tended to illustrate the application of the programming techniques given in this section and are the examples for which ladder diagrams and function block diagrams were derived in section 5.7. see that section for an explanation of the ladder diagrams; here we just show the instruction lists relatin

47、g to the programs. a signal lamp is required to be switched on if a pump is running and the pressure is satisfactory, or if the lamp test switch is closed. figure 6.8 shows the ladder program and the related instruction list. for a valve which is to be operated to lift a load when a pump is running

48、and either the lift switch is operated or a switch operated indicating that the load has not already been lifted and is at the bottom of its lift channel, figure 6.9 shows the ladder program and the related instruction list. for a system where there has to be no output when any one of four sensors g

49、ives an output, otherwise there is to be an output, figure 6.10 shows the ladder program and the instruction list. 6.2 sequential function if we wanted to describe a traffic lamp sequence, one way we could do this would be to represent it as a sequence of functions or states such as red light state

50、and green light state and the inputs and outputs to each state. figure 6.11 illustrates this. state 0 has an input which is triggered after the green light has been on for 1 minute and an output of red light on. state 1 has an input which is triggered after the red light has been on for 1 minute and

51、 an output of green light on. the term sequential function chart (sfc) is used for a pictorial representation of a systems operation to show the sequence of the events involved in its operation. sfc charts have the following elements: 1 the operation is described by a number of separate sequentially

52、 connected states or steps which are represented by rectangular boxes, each representing a particular state of the system being controlled. the initial step in a program is represented differently to the other steps, figure 6.12 shows its representation. 2 each connecting line between states has a h

53、orizontal bar representing the transition condition that has to be realised before the system can move from one state to the next. two steps can never be directly connected, they must always be separated by a transition. two transitions can never directly follow from one to another, they must alway

54、be separated by a step. 3 when the transfer conditions to the next state are realised then thenext state or step in the program occurs. 4 the process thus continues from one state to the next until the complete machine cycle is completed. 5 outputs/actions at any state are represented by horizontall

55、y linked boxes and occur when that state has been realised. as an illustration, figure 6.13 shows part of an sfc and its equivalent ladder diagram. figure 6.13 sfc and equivalent ladder diagram as an illustration of the principles of sfc, consider the situation with, say, part of the washing cycle o

56、f a domestic washing machine where the drum is to be filled with water and then when full a heater has to be switched on and remain on until the temperature reaches the required level. then the drum is to be rotated for a specified time. we have a sequence of states which can be represented in the m

57、anner shown in figure 6.14. 6.2.1 branching and convergence selective branching is illustrated in figure 6.15 and allows for different states to be realised depending on the transfer condition that occurs. parallel branching (figure 6.16), represented by a pair of horizontal lines, allows for two or

58、 more different states to be realised and proceed simultaneously. figures 6.17 and 6.18 show how convergence is represented by an sfc. in figure 6.17 the sequence can go from state 2 to state 4 if in 4 occurs or from state 3 to state 4 if in 5 occurs. in figure 6.18 the sequence can go simultaneously from both state 2 and state 3 to state 4 if in 4 occurs. 6.2.2 actions with sta