Quite often, function blocks for logic, compare and calculaction functions have fault bit Der (Derived) set at their output(s). Derived fault bit indicates that one or more of the inputs of the block have one or more fault bits set. Forementioned function blocks do not pass individual fault bits to their outputs but, instead, set just the Derived fault bit.

Case 1: no faults at the inputs

As the inputs are free from faults, outputs are also fault free.

Case 2: one faulty input

In this example, the first inputs have fault bits Inv (Invalid data) and Old (Old data) set, but the outputs have Derived fault with Der bit set. Inv and Old fault bits indicate that, after downloading the module (or resetting the process station), the function block input has not been written to yet. Usually, the reason for this is that the I/O module (or even the I/O card) where the signal is supposed to come from, is missing. Another possible reason for the fault is that the input signal is connected to a Direc access or Interface port that has not been written to at all (yet) or has been written to with a faulty value.

Case 3: several faulty inputs with different kinds of fault bits set

Even though there are several faulty bits set at the inputs, the output fault status is set to Derived fault, only.