Fault Model Tutorial:
The following table attempts to make the simulator's representation of faults clearer. Generally, faults can be categorized into "stuck-at's" "brigdes"and "opens". It is also necessary to simulate unknown conditions.  A stuck-at fault results from a wire being accidentally connected to either ground (Gnd) or a voltage source (Vcc). When attached to ground, a wire's value will be stuck at 0 (_0_ in the simulator) because it cannot hold a charge. Similarly, when a wire is mistakenly connected to a voltage source, that wire's value will always be interpreted as a 1 (_1_) by the simulator.  Bridge faults can occur when wires are physically connected. The resulting values on the wires depend on the origional values. Generally, such bridges act similarly to either AND gates or OR gates. For example, if the bridge is an AND-Bridge, then if either (or both) of the wires have a value of 0, the resulting value on the bridge will be 0. In the case of an OR-Bridge,  if either of the wires had a value of 1, the resulting value on the bridge would be 1. A different type of bridge-fault occurs when two wires are very close in proximity to one another. Depending on the envionment and the specific conditions of the circuit, a current going through one wire can create a magnetic field which would interfere or induce a current in any near-by wire. Currently, this type of fault is not supported by the simulator. If a wire is broken, an stuck-open  fault can occur. The result of stuck-open faults is that the previous value on the wire slowly dissipates, much like the charge in a capacitor. A transistor-level description of the circuit is required to calculate the exact value at any given time. As a result, if the circuit description is on a large-scale or behavorial level, the simulator gives a propagation value _Z_ to the wire. Unknown conditions such as an uninitialized input are given a value of _X_ by the simulator.

Fault Type:	Simulator Representation:	Description:	Graphical   Representation:
Stuck at 0	_0_	Wire is attached to ground (Gnd)
Stuck at 1	_1_	Wire is attached to voltage source (Vcc)
Bridge	_1_ or _0_	Wires are touching or crossed. New values depend on the strength of previous values on wires.
Induced	Not  Simulated	Wires are very close, causing magnetic interference / inductance effect.
Open (unintentional)	Large:        _Z_   Small:   _1_ or _0_	A wire broken, forming  an open circuit. On a high (behavorial) level, recieves a _Z_ (propagation) value. However, when analyzed on a smaller scale, the simulator resolves_Z_ to an 1 or 0.
Unknown	_X_	Used to represent an uninitialized startup state or unknown value.

 For more information on fault models, try the following sites:

 http://www.cedcc.psu.edu/ee497i/rassp_43/sld017.htm

 http://www-micrel.deis.unibo.it/~michele/research/testing/testing/node1.html