Abstract—The development of adaptive communication systems for industrial applications is crucial for achieving flexibility in the so-called Industrie 4.0. In this paper we present the design and implementation of a cognitive radio (CR) prototype for a wireless extension of the tethered process fieldbus (PROFIBUS) in the 2.4 GHz industrial, scientific and medical (ISM) band. The prototype integrates an adaptive filter bank-based physical layer (PHY) with a cognitive medium access control layer (MAC) in order to meet the strict quality-of-service (QoS) requirements of factory automation in terms of delay and packet loss. The paper also presents a hardware-efficient implementation of computationally intensive PHY and MAC algorithms and functionalities. To the best of our knowledge, the presented prototype is the first practical implementation of a CR system for wireless fieldbus communication in an industrial automation scenario.