The LAN which connects sensors, adjusters and controllers is well known as the fieldbus. Foundation Fieldbus(FF) which is currently one of the most popular fieldbuses adopts release/subscribe communication relationship to sustain its real-time communication. In the artide the mechaism of the FF's MAC and its real-time token characteristic are discussed. The minimum Schedule Spacing time of two continous CD tokens are calculated and the result is validated by the Rosemount Co's Deltav System.