linuxcnc with ethercat

I want to use linuxcnc as the ethercat master station and the servo drive as the slave station. I have a few questions that are very confusing. First, what data do I need to pass from the motion control library or to the slave, is it motor-pos-cmd? Or something else? Second, I want to know the period in which ethercat reads data from the motion control library. thanks!

You send motor-pos-cmd and need to rad back into motor-pos-fb

thanks，How is the period of reading data determined?