Abstract: In a programmable logic controller data is exchanged between a data communication section and a sequential control section. Within the sequential control section a communication task is activated for a given period during each PLC cycle. Subsequently, inter alia a user task is activated; this user task controls a machine or a process inter alia on the basis of the data exchanged. By making the ratio of the period in which the communication task is active to that in which the user task is active adjustable, very time-critical processes with a very short response time can be controlled as well as large amounts of data can be exchanged in few PLC cycles during less time-critical phases of operation of the machine or process to be controlled.