An industrial controller has an I/O scanner that may scan I/O modules at differing rates depending on the intrinsic bandwidth of the controlled process variable. The user defines an update rate for each I/O module reflecting the bandwidth of its associated variable. The scanner continuously updates a table of I/O events for each I/O module or I/O rack listing permissive and mandatory I/O execution times associated with each event. Using this event table, the I/O scanner determines the next I/O module to be serviced. If the event cannot be serviced on schedule, indication of an event slip is reported to the controller.