【正文】 s brain and calculators, with the mode that person39。t guarantee the request of the higher quantity and high new the image of the technique business enterprise. The people see in produce practice, automate brought the tremendous convenience and the product quantities for people up of assurance, also eased the personnel39。 the operation that list depends the artificial has already can39。 Inputs the keyboard is analogous to a proximity switch. Input circuits the serial input chip is like a 24Vdc input card. Computer the 686 CPU is like a PLC CPU unit. Output circuits a graphics card is like a triac output card. Outputs a monitor is like a light. Storage memory in PLC is similar to memories in personal puters. It 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 harsh production environments. 3. OPERATION SEQUENCE All PLC 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 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 will 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 Cycle SELF TEST Checks to see if all cards error free, reset watchdog timer, etc. (A watchdog timer will 可編程邏輯控制器（ PLC ） 18 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 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 (., 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 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 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 places in the ladder logic program changed while half way through a ladder scans. This problem could have severe effects on plex 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 will be turned off. This does not affect the values of the inputs. 4． The Input and Output Scans 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. 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 input value will not invalidate the logic. Also, if output bits were changed as each bit was changed, instead of all at once at the end of the scan the PLC would operate much slower. 5． The Logic Scan Ladder logic programs are modelled after relay logic. In relay logic each element in the ladder will 可編程邏輯控制器（ PLC ） 19 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 interpreted lefttoright, toptobottom. 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 branches, before moving along the ladder logic rung. Figure 4 Ladder Logic Execution Sequence It also bees important when considering 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. 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 memory, the actual outputs are only updated when the ladder logic scan is plete. 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. 可編程邏輯控制器（ PLC ） 20 Figure 5 A Duplicated Output Errors 6. PLC STATUS The lack of keyboard and other inputoutput devices is very noticeable on a PLC. On the front of the PLC there are normally limited status lights. Common lights indicate。 PLC 的出現(xiàn)已經(jīng)足足影響了幾代人，我們也從上一輩的經(jīng)驗(yàn)中獲取了更多的知識(shí)和教訓(xùn)，來(lái)不斷的發(fā)展 PLC 技術(shù)，將它推向更高浪潮。組態(tài)軟件可以將所有的這些硬件連接起來(lái)，通過(guò)更直觀的動(dòng)畫(huà)圖片來(lái)進(jìn)行控制，并可以通過(guò)互聯(lián)網(wǎng)在異地進(jìn)行控制，像神舟五號(hào)的發(fā)射就是采用這種辦法來(lái)使飛船升空。程序重新開(kāi)始運(yùn)行，但對(duì)中斷不會(huì)進(jìn)行破壞。 你也許聽(tīng)過(guò)太多的這個(gè)詞 :“死機(jī)”，大致的意思是 CPU 工作量過(guò)大，內(nèi)部資源不足等情況造成程序無(wú)法運(yùn)行。它在 PLC 執(zhí)行程序時(shí)一旦發(fā)現(xiàn)高級(jí)計(jì)數(shù)器對(duì)應(yīng)的中斷，就會(huì)立即放下手上的工作。 當(dāng)我們?cè)诠ぷ魍暌粋€(gè)工件時(shí)，給 PLC 一個(gè)信號(hào)，將 PLC 的內(nèi)部計(jì)數(shù)