Allen-Bradley Micro820 Fault — User Program Error
What This Fault Means
A user program error on the Micro820 means the controller detected an invalid instruction or runtime error in the user program. The controller stops executing and all outputs go to their fault state.
Common Causes
- Division by zero in math instruction
- Array index out of bounds
- Invalid pointer or indirect address reference
- Program corruption after incomplete download
Recommended Fix
- Connect with Connected Components Workbench (CCW) and check the fault log for the specific instruction address.
- Review the flagged rung for division operations — add a pre-check for zero divisors.
- Verify array sizes match the index range used in your logic.
- Re-download the program from your saved project file.
- Clear the fault in CCW and switch to Run mode.
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