When a CompactLogix PLC displays a solid red OK indicator, it typically indicates a major fault resulting in the program being cleared from memory. However, in a recent case with a 1769-L16ER-BB1B series B PLC running firmware v24.011, the program was running normally despite the red OK light. The PLC had been removed from a malfunctioning machine and sent for inspection. Upon inspection, the PLC had indeed encountered a powerup fault and cleared its program. After clearing the fault, redownloading the program, and power cycling the PLC, it resumed normal operation with outputs blinking as expected and inputs functioning properly. Despite the solid red OK light, the PLC was executing the code normally. All other indicators (NS, NET, RUN, I/O, etc) were functioning as expected, and the PLC was running its user program without any issues. Even after multiple power cycles, the OK indicator remained red, contradicting the documentation's explanation of a major fault. After contacting technical support, the sudden change occurred where the OK indicator turned solid green, signaling normal operation. Subsequent power cycles also resulted in the OK indicator behaving normally. The PLC was eventually returned to the customer with a note of the unusual behavior. While the issue ultimately resolved itself, it leaves room for curiosity and the question of whether others have encountered similar occurrences or can provide further insights.
To conduct a basic diagnostic, consider using GetAttributeSingle on instance 1 attribute 5 (Status, WORD/INT) or attribute 8 (State, USINT) of Identity (0x01). This method can provide valuable information for troubleshooting.
That's really interesting, I always thought that a solid red OK light was a sure sign of a major issue. I work a lot with 1769-L16ER-BB1B CompactLogix PLCs and I've never come across this issue before. One possible explanation could be a glitch in the OK indicator's firmware. But it's strange that it rectified itself without any firmware update or significant intervention. It's also baffling that the PLC continued to operate perfectly with a solid red OK light. This makes it even more essential to thoroughly inspect and not completely rely on fault indications when troubleshooting any issues. Thanks for sharing your experience. It definitely adds a new layer of complexity to understanding PLC behaviors.
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Answer: A solid red OK indicator on a CompactLogix PLC usually indicates a major fault that results in the program being cleared from memory.
Answer: Yes, in some cases, a CompactLogix PLC can still run its program normally even with a solid red OK light. This was observed in a specific case with a 1769-L16ER-BB1B series B PLC running firmware v24.011.
Answer: To restore normal operation after a powerup fault and program clearance, the fault needs to be cleared, the program redownloaded, and the PLC power cycled. This process allowed the PLC mentioned in the discussion to resume normal operation.
Answer: The reason for the OK indicator remaining red despite the PLC executing the code normally was not explicitly mentioned in the discussion. However, it was noted that the behavior contradicted the documentation's explanation of a major fault.
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