先軟后硬的檢修原則首先檢查用戶程序是否出現問題。S5的用戶程序儲存在PLC的RAM中，是掉電易失性的，當后備電池故障系統電源發生閃失時，程序丟失或紊亂的可能性就很大，當然強烈的電磁干擾也會引起程序出錯。有EPROM存儲卡及插槽的PLC恢復程序就相當簡單，將EPROM卡上的程序拷回PLC后一般都能解決問題；沒有EPROM子卡的用戶就要利用PG的聯機功能將正確的程序發送到PLC上。需要特別說明的是，有時簡單的程序覆蓋不能解決問題，這時在重新拷貝程序前總清一下RAM中的用戶程序是相當必要的。通過將PLC上的“RUN"“ST"開關按RUN---ST---RUN---ST---RUN的順序一遍或在PG上執行“Object—Blocks—Delete---inPLC—allblocks---overall—Reset"功能就完成了RAM中程序的總清。 另外，保存在EPROM中的程序并不是的，過分相信EPROM上的程序有時會給檢修帶來困惑。所以經常性的檢查核對EPROM中的程序，特別是PG中的備份程序就顯的尤為重要?！　z修實例：一次，本廠一臺停機兩個月的設備出現問題，上電后無法啟動。檢修人員在檢查后認為程序出錯，很自然地將EPROM卡插入PLC中，總清后拷貝程序，完成后重啟，故障依舊，由于程序不大，逐條把EPROM上的程序讀出，與手冊上的指令核對后發現一樣，重復拷貝無效后認為是PLC硬件故障。我們用PG將備份程序調出，與EPROM上的程序進行比對，結果語句指令表相同，但程序存放地址發生了變化，把備份程序發送到PLC后設備運行正常。可見EPROM上的程序也出現了錯誤，用紫外線擦除后重新寫入問題解決?！　《LC硬件故障　　PLC的硬件故障較為直觀地就能發現，維修的基本方法就是更換模塊。根據故障指示燈和故障現象判斷故障模塊是檢修的關鍵，盲目的更換會帶來不必要的損失。　?。?）電源模塊故障。一個工作正常的電源模塊，其上面的工作指示燈如“AC"、“24VDC"、“5VDC"、“BATT"等應該是綠色長亮的，哪一個燈的顏色發生了變化或閃爍或熄滅就表示那一部分的電源有問題?！癆C"燈表示PLC的交流總電源，“AC"燈不亮時多半無工作電源，整個PLC停止。這時就應該檢查電源保險絲是否熔斷，更換熔絲是應用同規格同型號的保險絲，無同型號的進口熔絲時要用電流相同的快速熔絲代替。如重復燒保險絲說明電路板短路或損壞，更換整個電源?！?VDC"、“24VDC"燈熄滅表示無相應的直流電源輸出，當電源偏差超出正常值5%時指示燈閃爍，此時雖然PLC仍能工作，但應引起重視，必要時停機檢修。“BATT"變色燈是后備電源指示燈，綠色正常，黃色電量低，紅色故障。黃燈亮時就應該更換后備電池，手冊規定兩到三年更換鋰電池一次，當紅燈亮時表示后備電源系統故障，也需要更換整個模塊?！?（2）I/O模塊故障。

　　輸入模塊一般由光電耦合電路組成；輸出模塊根據型號不同有繼電輸出、晶體管輸出、光電輸出等。每一點輸入輸出都有相應的發光二極管指示。有輸入信號但該點不亮或確定有輸出但輸出燈不亮時就應該懷疑I/O模塊有故障。輸入和輸出模塊有6到24個點，如果只是因為一個點的損壞就更換整個模塊在經濟上不合算。通常的做法是找備用點替代，然后在程序中更改相應的地址。但要注意，程序較大是查找具體地址有困難。特別強調的是，無論是更換輸入模塊還是更換輸出模塊，都要在PLC斷電的情況下進行，S5帶電插拔模塊是不允許的?！　。?）CPU模塊故障。

　　通用型S5 PLC的CPU模塊上往往包括有通信接口、EPROM插槽、運行開關等，故障的隱蔽性更大，因為更換CPU模塊的費用很大，所以對它的故障分析、判斷要尤為仔細。

表1-1例出了CPU模塊常見故障及處理方法。

to be transferred between the loading and unloading equipment and the central control system, and ensures real-time and accurate data as the control network uses the most popular producer/consumer model. For loading and unloading equipment fully automatic operation to improve the strong guarantee. Single computer system hardware configuration figure 4.1 ControlLogix5555 controller (1756-L1) : The system control core, according to the preloaded procedure, carries on the logic judgment and the arithmetic computation to each kind of signal which the input module provides, and according to the Central Control Room or the operator control command, controls the output module. 1756-L1 mainly has the following functions: 4.1.1 RS232(channel a) and D H + (channel B) interface: Because rs-232 serial port communication speed is slow, this system adopts ethernet module to connect with programmer through network medium (network line) to realize on-line programming. Through 1747-ASB adapter and communication cable, the remote i/o Channel B interface is connected with the operator's desk and touch screen respectively to realize manual operation and semi-automatic operation.4.1.2 LED light indication: displays the various status of the controller, including RUN (RUN) , Flt (failure) , Batt (battery) , FORCE (forced input/output indication) , DH + and RS232 channel indication. 4.1.3 key switch: a 4.2 input module for RUN, PROG, and REM modes can be selected by flipping the key switch: It is used to receive and collect input signals from field devices, including by the button, select switch, stroke switch, relay contacts, proximity switch, photoelectric switch, digital dial switch, and so on the switch input signal and potentiometer, tachometer generator and various transmitters sent by the continuous change of analog input signal. High-speed counting module: 1756-HSCE counts the pulse input signal of the walking encoder accurately, calculates the walking distance of the large computer, and then transmits to the central control system through optical cable to realize full automatic operation. 4.3 output module: used to output control signals to various actuators, including digital output signals to contactors, solenoid valves, indicator lights, switches, etc. and analog output signals to control valves, speed regulators, etc. , then drive the field actuator to achieve the goal of automatic control