Chapter - Software Engineering Fail safe design problems Component jams Operator detected failure Erroneous input Unsafe modes Programming errors Sabotage Random failure

Debugging Programs A fail-safe design Proper programming techniques Modular programs Predictable, non-configured programs Inaccessible to unauthorized persons Check for system OK at start-up. Built in functions for error and failure detection. People Clear and current documentation Training for new users and engineers

Debugging Minimized by good design techniques

Troubleshooting 1. Look at the process 2. PLC error lights HALT - something has stopped the CPU RUN - the PLC thinks it is OK (and probably is) ERROR - a physical problem has occurred with the PLC 3. Indicator lights on I/O cards and sensors 4. Consult the manuals, or use software if available 5. Call for help

Process Modeling

Function Diagrams

Design steps for verification 1. Structured design 2. Modular testing 3. Code review 4. Modular building 5. Design confirmation 6. Error proofing 7. Burn-in

Documentation

Commissioning 1. Visual inspection 2. Mechanical installation 3. Electrical installation 4. Functional tests 5. Process verification 6. Contract/specification verification 7. Put into production