This paper describes a protocol by which network security can be included in existing Fieldbus systems. The protocol makes use of the 56-bit DES cipher for data encryption. It also includes a scheme for symmetric Key exchange and automatic Key Update at specific time intervals. In addition, the protocol includes a new idea called Key Refresh. The Key Refresh is a simple scheme that can be an alternative to operating the cipher in processor intensive chaining or feedback modes. The industrial field devices are equipped with the TMS series of Digital Signal Processors to perform DES encryption/decryption. Since these processors are already present embedded in many of the state-of-art field devices, they were chosen so that the protocol would not require additional hardware. The protocol is generic and can run over any of the commercial Fieldbus networks. ©2006 IEEE.