Here each module is tested to confirm that it functions as required by the Module Specification.
This can be a very tedious process, but can be greatly improved by various methods.
First it is important to understand what is being tested and what the possible faults might be.
Testing for example a P&ID loop could take a long time were it not for the fact that the loop algorithm is (nearly always) a standard piece of code that has been developed by the System supplier. What is important is that each loop is tested to confirm that it has the right connections and modes.