This paper describes the controller functions, types of control units and from which all components can be composed. Next part is about explaining how to program the controllers can take place, what are the types of controlling and communication protocols that are most frequently used for connection to field devices and supervisor. The whole interpretation is conducted through first theoretical introduction followed by practical example. The whole paper is written exclusively on control units, which are used in building automation and belonging to the group DDC (Direct Digital Control) systems. These controllers are most often used to HVAC systems (Heat, Ventilation and Air Condition). The aim of these systems is to achieve thermal comfort in buildings. The whole industry is then called Measurement and Control. Specific application is e.g. controlling of Air handling unit according to pre-defined project.