CompactLogix 5380 major fault Type 4 Code 20 after download?
I'm getting a CompactLogix 5380 major fault, Type 4 Code 20, right after downloading a new routine. The controller immediately went into a faulted mode. How can I find the cause of this fault?
A Type 4 Code 20 major fault on a CompactLogix 5380 controller, especially after a new routine download, points to a program error. This typically means an array subscript is too large or there's an invalid .POS or .LEN value within a CONTROL structure. Focus your troubleshooting on the recently added logic.
Likely causes and checks
1Array subscript too large
- Points to it
- The fault code T04:C20 directly indicates 'Array subscript too large'. Since the fault occurred right after a new routine download, the new routine likely contains logic that attempts to access an array element using an index that exceeds the array's defined bounds.
- Check
- Review the new routine for any indirect addressing or calculations that determine array indices. Monitor the value of the index tag and compare it against the array's defined dimensions at the point of the fault.
2CONTROL structure .POS or .LEN invalid
- Points to it
- The fault code T04:C20 also indicates 'CONTROL data type POS or LEN invalid'. If the new routine uses instructions like FFL/FFU, COP, or sequencer instructions that rely on a CONTROL tag, an incorrect .POS or .LEN value could cause this fault.
- Check
- Identify any FFL/FFU, COP-like file, or sequencer instructions in the new routine that use a CONTROL tag. Compare the .LEN and .POS values of these CONTROL tags with the size of the array they are addressing.
Next check: If the above checks don't resolve the issue, consider isolating the new routine by temporarily disabling it or testing it in a controlled environment to confirm it's the source of the fault.
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