PLC检查是否有与编程器和计算机的通信请求，若有则进行相应处理，如接收由编程器送来的程序、命令和各种数据，并把要显示的状态、数据、出错信息等发送给编程器进行显示。如果有与计算机等的通信请求，也在这段时间完成数据的接受和发送任务。
下面以FX系列PLC为例，来说明根据LED指示灯状况以诊断PLC故障原因的方法。
在输入处理阶段，CPU将SBl，SB2和FR的常开触点的状态读入相应的输入映像寄存器，外部触点接通时存入寄存器的是二进制数l，反之存入0。
比较下二个程序的异同：
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不管输出单元的LED灯亮还是灭，如果负载不能进行ON或OFF时，主要是由于过载、负载短路或容量性负载的冲击电流等，引起继电器输出接点粘合，或接点接触面不好导致接触不良
用户程序存储器用于存放用户载入的PLC应用程序，载入初期的用户程序因需修改与调试，所以称为用户调试程序，存放在可以随机读写操作的随机存取存储器RAM内以方便用户修改与调试。
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PLC checks whether there is a communication request with the programmer and computer, and if so, it will handle it accordingly, such as receiving the program, command and various data sent by the programmer, and sending the status, data and error information to be displayed to the programmer for display. If there is a communication request with the computer, etc., the task of receiving and sending data is also completed during this period.
The following takes FX series PLC as an example to illustrate the method of diagnosing the cause of PLC fault according to the status of LED indicator.
In the input processing stage, the CPU reads the status of the normally open contacts of SBL, SB2 and fr into the corresponding input image register. When the external contact is connected, the binary number L is stored in the register, and vice versa.
Compare the similarities and differences between the following two procedures:
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No matter whether the LED light of the output unit is on or off, if the load cannot be on or off, it is mainly due to overload, load short circuit or impulse current of capacitive load, which causes the bonding of the relay output contact, or poor contact caused by poor contact surface
The user program memory is used to store the PLC application program loaded by the user. The user program at the initial stage of loading needs to be modified and debugged, so it is called user debugging program, and is stored in the random access memory RAM that can be read and written randomly to facilitate user modification and debugging.
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