On-Site Debugging Experience: Observing Siemens CPU LED Status Lights to Identify Faults

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 How to Identify Faults through Siemens CPU LED Status Lights？



When performing on-site debugging, various issues such as CPU errors, valve island errors, and network issues are commonly encountered. Newcomers to the field may not know where to start in resolving these problems. In this article, I will share some of my on-site debugging experiences – how to quickly identify the source of the problem by observing LED status lights. Although PLCs and their connected distributed modules won't pop up a dialog box telling you where the problem is, the numerous LED lights they possess can provide valuable information. With keen observation, you can swiftly pinpoint the root cause of the problem. The diagram below illustrates the on-site status of a CPU 317-2PN/DP:




At the upper-left corner of the CPU, there is a column of LED lights indicating the CPU's status from top to bottom:

  1. SF Light (System Fault): The topmost light, glowing red when lit, indicates system faults, which can be hardware malfunctions, discrepancies between CPU hardware configuration and reality, lack of power to a distributed module, or errors in IO diagnostics. Software programming errors can also trigger the SF light.

  2. BF Light (Bus Fault): Glowing red when lit, it signifies bus faults. For CPU 317-2PN/DP supporting two buses (DP and PN), there are separate BF1 and BF2 lights. BF1 indicates faults in the Profibus DP bus, while BF2 indicates faults in the Profinet bus. If a bus encounters a fault, the corresponding fault light will illuminate.

  3. MAINT Light (Maintenance): Lit in yellow, it indicates that the CPU requires maintenance.

  4. DC5V Light: Glowing green when lit, it signifies the normal operation of the CPU and bus's 5V DC power supply. If this light is not lit during normal operation, there may be an issue with the CPU's power supply.

  5. FRCE Light (Force): Lit in yellow when forced, indicating that certain variables in the CPU are in a forced state. The force function ignores the running logic of variables and outputs with specified values. For details: How to Use the Forced Function of Variables for Emergency Production?

  6. RUN Light (Run): Lit in green, it indicates that the CPU is in the running state. The CPU only refreshes the input/output buffer when in the running state.

  7. STOP Light (Stop): Lit in yellow, it indicates that the CPU is in the stopped state. Seeing the STOP light during on-site debugging can be nerve-wracking, as it signifies a serious error. However, there are various reasons for the CPU entering the stop mode, such as calling non-existent addresses or missing required data blocks in the program. Downloading specific organization blocks (OB) can prevent the CPU from entering stop mode, as the CPU calls the corresponding OB when detecting an error. The OB can contain no code, and the CPU won't stop as long as it exists.

With modern industrial control relying heavily on networks, network connectivity issues are also commonly encountered during on-site debugging. Let's now examine the LED status lights of the network module.


The above image is a field view of CP343, similar to the CPU. It has a column of LED status lights in the upper-left corner, mostly similar to the CPU, except for the RX/TX lights. RX/TX denotes the receive/transmit lights, which light up in yellow when CP module is sending or receiving data, indicating data transmission activity. Below, there are three additional LED lights: the MAINT light (Maintenance), which is the same as introduced in the previous CPU section. Now, let's focus on X1P1 and X1P2 lights.

X1P1 is the indicator light for network interface 1 (Port 1), and X1P2 is the indicator light for network interface 2 (Port 2). Each light has two operational states: green and yellow. Green signifies a successful network connection (linked), while yellow indicates data transmission (RX/TX). If the network is not disconnected, the interface LED lights should be solid green with occasional yellow blinking. I have encountered situations where only the yellow light occasionally blinked, and the green light was not on. In such cases, the transmitted data packets could not be received. The photo below of the CPU's network interface might provide a clearer view:



The discussion on diagnosing issues through LED status lights concludes here. Next time you are on-site for debugging, take a close look at the LED status lights for more insights. Good luck!