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Safety Instrumented Systems (SIS) are designed to prevent accidents, avoid undesirable situations and guarantee continuous operation of oil and gas production systems. An interruption in the operation can be caused by faults in sensors and/or actuators. Hence, SIS are integrated to the supervisory control system in order to use the information from sensors to prevent such undesirable situations. In this scenario, it is important to be able to validate the SIS implementation against its specification in order to increase the reliability of the system. In this work a technique to obtain a timed automata from function block diagram (FBD) implementation of a SIS is presented and applied to a case study provided by Petrobras (Brazilian oil company). This is a step towards automatic conformance testing between specification in ISA 5.2 and the implementation in FBD for a SIS to improve its dependability.