We define the expected behavior of a system by specifying output Signal patterns. If the Actual Output Signal patterns match their Expected patterns, then a TestCase is considered to have passed. The question arises as to how close the Actual Signal pattern has to match the Expected Signal pattern. We need to specify some kind of tolerance level to apply when we perform the comparison. There are 3 kinds of tolerances: Time, Value, and Count. They can be assigned to:
•The time that something occurs
•The value that it has at a point in time
•The number of changes (or transitions)
•The time between transitions (for Periodic Signals)
Which tolerances are applicable and what the tolerances mean in terms of the Pass/Fail Judgment depends whether the Signal is Numeric, Message, or Event, and for Numeric Signals, whether it is continuous or discrete. Normally, discrete Signals do not have a value tolerance. See Signal Properties.
•Tolerances have two component values:
oUpper or Pos Tolerance - An allowable positive delta from the nominal value
oLower or Neg Tolerance - An allowable negative delta from the nominal value (The negative delta is entered as a positive number.)
Note: The deltas do not have to be the same.
•Value and count tolerances are inclusive. For example, if the nominal expected value is 3 and the positive tolerance is 2, then an actual value of 5 would be within tolerance.
•Lower (Neg) time tolerances are inclusive. If the actual transition time is greater than or equal to) the Expected Time - Lower Tolerance, the transition is within tolerance and passes. For example, if the expected time is 3 seconds the lower tolerance is 200 ms, then an actual time of 2.8 seconds would be within tolerance.
•Upper (Pos) time tolerances are exclusive. If the actual transition time is less than (but not equal to) the Expected Time + Upper Tolerance, the transition is within tolerance. For example, if the expected time is 3 seconds the upper tolerance is 200 ms, then an actual time of 3.2 seconds would not be within tolerance.
In MxSpecIt, there is an object hierarchy (TestCase > Signal > DataBlock). There is an inheritance mechanism that allows you to specify a Tolerance at a higher level, and if need be override it at a lower level. Inheritance mechanisms exist as follows:
•Time Tolerance - The default Time Tolerance is set at the TestCase level. See TestCase Properties. It can be overridden at the DataBlock level.
•Transition Count Tolerance - The Transition Count tolerance is set at the DataBlock level. See DataBlock Properties.
To display visible Tolerance Bands in the TestCase window, click in the Signal Toolbar.