Controllability is defined as the ability to establish a specific signal logic value either low or high at each node in a circuit by setting values to determine the signal value at any node in circuit by controlling the circuits inputs and observing its outputs. The controllability of a chip is to set the node to a 1 or 0 state. This measure is of importance because when assessing the difficulty of testing a particular signal within a circuit can be easily tested. Controllability should be the aim of a well-designed circuit to have all nodes easily controllable. For digital systems, stimulus is in the form of logic 1’s and 0’s known as vectors. Having the ability to apply the stimulus to the unit under test is generally referred to as controllability.