# TestStand

URL: https://irs.systems/en/docs/ltt/documentation/ltt-gui-manual/teststand

**National Instrument TestStand** is the underlying test execution system used by the LTT GUI.

> This chapter is by no means an introduction or documentation of National Instruments TestStand itself. At this point you should already have basic knowledge about basic TestStand concepts such as _sequence files_, _sequences_, _steps_ and _variables_. If not, maybe you want to read the [TestStand help document](https://www.ni.com/docs/en-US/bundle/teststand/page/user-manual-welcome.html).

Within the LTT folder structure all TestStand related files are located below `[Install Location]\50_LTT_SW\LTT TS\`. The TestStand sequence file which represents your **LTT testplan** must be located there. There is a **LTT sequence file template** (`\50_LTT_SW\LTT TS\LTT Template.seq`) if you want or need to begin from scratch. This sequence file template contains all necessary structures needed for a LTT testplan.

> The LTT template sequence file is different for **MesSy II** or **MesSy II FD**. Depending on what you have selected during installation, the template file is for that MesSy version. If you want to switch to the other MeSy version read chapter [MesSy Version Setup](/en/docs/ltt/documentation/ltt-gui-manual/configuration/messyconfiguration/changing-messy-version).

> A **TestStand sequence file** must meet certain criteria in order to be used for LTT. We call TestStand sequence file a **LTT testplan**, when it fulfills all required criteria.

The required criteria for a TestStand sequence file to be a LTT testplan are as follows:

* it need to have a specific [process model](../teststand/processmodel/)
* it need to contain some [predefined sequences](/en/docs/ltt/documentation/ltt-gui-manual/teststand/predefinedsequences)
* it need to contain some [predefined variables](/en/docs/ltt/documentation/ltt-gui-manual/teststand/predefinedvariables)

During this chapter the terms _execution_, _socket_, _DUT_ and (_test_)station may be used. They are interchangeable in most cases. Execution or socket are NI TestStand jargon. An execution / socket means a single DUT / test station.