On the Safe IOCOS relation for Testing Safety PLC Code

In this paper, limitations of the IOCOS testing relation in regard to testing safety PLC code is examined and a modification of the current IOCOS relation, called safe-IOCOS is proposed. In the IOCOS testing relation, an implementation is IOCOS with respect to a specification, if it emits a subset of the specified outputs and a super-set of the specified inputs after the execution of each trace in the specification. However, for testing safety PLC code, the IOCOS relation is not detailed enough as the subset requirement on the respective inputs and outputs could allow some safety behaviors to go untested. These limitations of the IOCOS relation may thus pose threats to humans. So the notion of safe-IOCOS is defined, which strengthens IOCOS to require equality between the implementation and the specification in relation to the inputs and outputs, respectively. An example shows these shortcomings of IOCOS and how the proposed safe-IOCOS relation is better suited for testing safety PLC code.