In Profibus master stations poll data from slave stations using the cyclic service-CSRD (Cyclic Send and Request Data with Reply). Because the polling-interval determines the data sampling interval, it is important for the Profibus user to be able to know the maximum polling-interval of a cyclic service. In this paper we develop a formula for determining the maximum polling-interval of a cyclic service. Furthermore, if the polling-interval of a cyclic service cannot be met, then we can repeat the corresponding cyclic service in a poll list to reduce the polling-interval. However, how are the repeated cyclic services to be positioned in the poll list? The approach we use is an equi-distance one.