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So you want a Layout that runs all the time with another Layout to run for x duration every 15 minutes over a specified period of time.
Your Always configuration is ok, with your Repeat configuration first make sure that the Start and End date is just for one day (you can set an end period on the Repeat tab to set the needed time frame). You also need to set Start and end Times here, as this is where you are setting for how long you want your second Layout to show, every 15 minutes.
You can see in my example above I have selected one day and have that set to start at 8am, for a 5 minute duration. So at 8:06 my Always Layout will resume.
Now I want to Repeat this behaviour using the Repeats tab. Set as Per minute and the frequency and then include the date and time you want this behaviour to end:
Your schedule should then look like this with just one record of the Repeat behaviour until the end date, which in this case is the 31st July
A really good way to check what you have scheduled will play as intended is to use the Agenda View, you can use the slider to pinpoint a time in the day as to what will be shown on a particular display / display group. So below is the Agenda view at 7.59 when we see only the Always Layout:
If we then move the slider to 8:00, we can see the Always Layout and the Repeat Layout to be shown every 15 minutes:
You can scroll through and check the timings are correct in this way: