A real time environment model is built to simulate IEC/ISA Fieldbus protocol behavior in the industrial applications. The closed loop control system is one of these important applications. Extensive study for the Fieldbus different variables and parameters as well as its behavior under noisy channels lead to the calculation of an average Fieldbus time delay that encounter any frame on the link. The effect of this Fieldbus time delay over closed loop control system stability is studied using the discrete root locus analysis. The paper used MATLAB real time environment to focus on the second order closed loop system communicating through the IEC/ISA Fieldbus network.