The paper presents a methodology of designing control logic that is implemented by industrial programmable logic controllers. The approach is based on discrete-event model of a plant to be controlled and a set of interlock and sequential specification models. Supervisory control theory is used to test the controllability of the specifications and in the final stage, to derive a model of the admissible behaviour of the system which serves as a specification for the sequential part of the controller. A laboratory scale modular assembly line case study is presented to illustrate the practical issues of the approach.