Figure 5. Forward Implication and D-frontier

Figure 5 illustrates examples of forward implications. The first gate in the figure shows how D is propagated to output and then D-frontier removes the gate from the list. Other cases can be explained in a similar manner.  

Procedure 5: X-path
An X-path is a path of consecutive nets in a circuit all of whose values are X. Let A be a gate in a D-frontier. The faults on the inputs of A can be propagated to a primary output O only if there is an X-path from A to O. In Figure 6 there is only one X-path from a to output—a-c-d.

Figure 6. Example of an X-path

3. D-Algorithm

In this section we will first present the steps involved in D-algorithm. Following that we will elaborate the algorithm using an example.

a. D-Algorithm

Input: Circuit netlist and one stuck at fault with its location.
Output: Primary input values and expected values at the primary outputs

Step-1: Initialize all the signal values of the circuit to X.
Step-2: Sensitize the fault location, i.e., if s-a-0 is the fault, apply D in the fault
location, else if s-a-1 is the fault, apply in the fault  location.