Windows Compiler Causes PLC Code to Crash PLC

Event Year: 2001 Reliability: Likely But Unconfirmed
Country: United Kingdom
Industry Type: Food & Beverage

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.


The organisation was unable to change the program on the PLC for a number of months and the problem was discovered by accident. An additional 3 PLCs had a similar style of programming and were experiencing the same problem.

Action Description: Working with the PLC manufacturer, a patch to their software was installed to prevent this from happening again.