Models meant for logic verification and simulation are often used for ATPG. For custom digital circuits, these models contain many tristate devices, which leads to lower fault coverage. Unlike other research in the literature, the modeling algorithms presented in this paper analyze each channel connected component in the context of its environment, thereby capturing the relationship among its input signals. This reduces the number of tristates and increases the modeling efficiency, as measured by fault coverage. Experimental results demonstrate the superiority of this approach.