Hi
There is no stop/start button. The intended use for the TSW14xxx/HSDC Pro was to only allow a pattern to be loaded and then looped infinitely for DAC evaluation. If you are concerned about spuriours outputs you can always shut the DAC output off (DAC register) while new patterns are loaded.
If you wish to just stop the loop - you can use the software reset (Reset Board) in the Instrument Options menu. This is equivalent to pushing the hardware reset button.
Ken.