Siemens alarm 12640: Channel %1 block %2 invalid nesting of control structures

What it means

Control structures (IF/ELSE/ENDIF, LOOP/ENDLOOP, WHILE/ENDWHILE, FOR/ENDFOR, REPEAT/UNTIL) are interleaved incorrectly, e.g. an ENDIF closes what WHILE opened. Each structure must open and close in strict last-in-first-out order. Re-indenting the program usually exposes the misplaced terminator.

Likely causes

• Missing or duplicated ENDWHILE/ENDIF terminator
• Jumping out of or into a loop body with GOTOF/GOTOB
• Structure spanning PROC subroutine boundaries