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What does error code E21 on a Series three PLC mean?

November 27th, 2018

Error code E21 signifies a memory parity failure and is one of more frequently seen error codes on the Series Three PLC. This error is displayed when the program data stored in RAM has become corrupt and is no longer valid. It is common for this condition to occur following a temporary power outage in cases where the battery is too weak to retain the program stored in memory. The only way to resolve an E21 error is to reload or reenter the program. In rare circumstances, E21 can also indicate a hardware fault within the CPU itself. Our technical support staff can help you determine whether or not hardware replacement is warranted. We can be reached by phone at or through email to . For information on other error codes, please refer to "Series Three Error Code Definitions" available on the Series Three documentation page.

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