Faults and Fault Detection
Last Edit July 22, 2001
Formation Rules for the Existence Function
The inputs and outputs for a vector represent a point on the Existence
function map that has a value of one. The set of all points of value equal
to one is the Existence Function.
A Test Sequence is a sequence of tests or input vectors represented by
a selected sequencing through the points of the Existence Function. A
Minimal Test Sequence may not necessarily use all Existence Function points
(minterms, vectors). The existence function for the sample circuit of
Figure 92 is shown in Figure 93.
Figure 9.3 Sample Circuit (2Stage NAND)
Forming Links
The points on the Existence Function, treated as minterms, are linked
following the linkage rule:
Two points connect if and only if:
 The input variables are logical distance one
 A primary output variable toggles 01 or 10
Logical distance one for the input variables means that only one input
variable may change state when traversing the link from one minterm to
another. The primary output must be an observable output (as shown in
Figure 94.)
The requirement that only one input change per vector is designed to
reduce the instance of hazards and race conditions in the test vector
set. Hazards are introduced during test when multiple inputs change state
due to differences in the tester lead connections. The parametric vectors
for the gate tree in the last chapter are written using the Minimal Test
Sequence for the tree.
Figure 94 Adding The LogicalDistanceOne Edges
Selecting a Chain
When all the possible links have been formed, there will be one or more
observable chains or sets of links. The longest chain defines the desired
Test Sequence.
