Generates simulation data, as if the logic analyzer was collecting real world data. If a Saleae Logic was connected to the PC, then this button would say Start. After the capture has started, you can stop it before it finishes, which will retain the data to that point. If the Start button is gray and cannot be clicked, that means you are not on the capture tab (See #4). Ensure you are on the capture tab so you can take a capture.