When making minor changes to a PLC program the PLC started to operate the plant in the wrong way. Valves opened at the wrong time, the batch sequence changed and the operator was unable to control his plant. The PLC in question used a number of subroutines that over time had changed. The PLC manufacturer’s software used the Microsoft Windows compiler to compile the code; however, this compiler failed to pick up the fact that there were too many variables being passed to the subroutine.

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