# Process Model

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TestStand stores its sequences in so called sequence files. A sequence file usually has a so called process model attached. TestStand ships with 3 standard process models: sequential model (default), parallel model and batch model. In TestStand the normal sequence file is also called **client sequence file**, whereas the process model is called process model sequence file. For the LTT we call the client sequence file **testplan**. The testplan represents the project specific collection of sequences.
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![Process Model Selected](https://1704635928-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FH5etzy4TvPRLxLRNfHlb%2Fuploads%2Fgit-blob-82fd14b5dfeb97098d7bbe3c6a5fb57f1ef27b55%2FTS_ProcModel_Selected.png?alt=media)

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Each LTT testplan needs to have the LTT process model assigned. It is located at `\50_LTT_SW\LTT TS\model\LTT Model.seq`. **Do not modify the LTT process model** as it gets overwritten by the next update.
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The **LTT process model** specifies the **frame sequence** for each LTT testplan. It determines the order, in which each of those [standard sequences](https://docs.irs.systems/ltt/documentation/ltt-gui-manual/teststand/predefinedsequences) are called and also provides some execution logic, initialization, error handling and so on. The LTT process model is the same for every LTT testplan, hence it is not project specific.

The LTT model is derived from the **standard TestStand parallel model**. It is therefore able to handle multiple DUT's in parallel. The LTT model also contains features of the standard TestStand batch model.

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When running multiple DUT's in parallel, you need to be aware that parallel execution has some limitations / drawbacks.
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* You cannot access hardware simultaneously, which exists only once for all DUT's. Therefore you need some kind of [locking](https://docs.irs.systems/ltt/documentation/ltt-gui-manual/teststand/synchronization) to make sure that the hardware is accessed serially.
* Since parallel DUT's run independently, some are faster than others. At some point in your testplan you might need to wait for all DUT's before you continue. You will need this kind of [synchronization](https://docs.irs.systems/ltt/documentation/ltt-gui-manual/teststand/synchronization) especially if you want to change the voltage level of your power supply, if it supplies more than one DUT.

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Use the [SYNC sequence](https://docs.irs.systems/ltt/documentation/ltt-gui-manual/teststand/synchronization) to change your DUT's voltage level and/or wait for the other DUT's to continue.
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