LTT Limit Tests
For this chapter we assume the following configuration:
In TestStand you can place a LTT Limit Test step. A dialog will appear.
A dialog will appear where you:
Select a signal from the drop down box
Select a limit from the drop down box
See a preview of the selected limit for the selected signal for each DUT type
Provide a data source, typically a TestStand variable holding the value to be tested
Optionally provide unit, precision and comparator, just like in the standard TestStand numeric limit test
Check the "Rename Step?" (recommended) and click OK
The inserted step will be renamed to the selected signal/limit. It will also contain the unique error number which you configured for that signal/limit.
A limit test step name in a LTT testplan—no matter if standard numeric limit test or LTT limit test—should always contain an error number in [square brackets] at the beginning of the step name. Optionally the step name could further follow the format {signal name} @ {limit name}.
If you select a LTT limit step and click on the edit button or press Ctrl + E, the limit selection dialog will appear again. You can choose another signal and/or limit and the step will adapt to your settings.
Dynamic Limits
By pressing the "tool button" to the right of the signal or limit drop down boxes, a TestStand expression editor window will open.
There you can select a TestStand variable from Locals, Parameters, FileGlobals, etc. and/or specify a more complex expression which determines the signal and/or limit name dynamically. As a result, the limit preview will not show all possible limits, since now the step does not specify one exact limit.
LTT limit steps containing expressions for signal and/or limit name are called dynamic limits. The step name will not contain an error number but a [???] instead, since many are possible. The step name will also contain <variable> for that part which is an expression. However, when you run the testplan the step will update to the correct limit, name and error number, depending on the value of the provided expression.
If the expression for a dynamic limit does not evaluate to an existing signal/limit name you will get a limit not found error and the station will stop.
Besides the LTT limit test you can—of course—still use all other TestStand standard limit tests.
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