Distributed control systems are widely deployed to all areas in industrial automation. However, a systematic approach to the design and programming of distributed automation and control systems is not clearly defined. This paper analyzes the differences between two control system architectures defined in IEC Standards - IEC 61131-3 for centralized control with programmable controllers (PLCs) vs. IEC 61499 for the use of function blocks (FBs) in distributed automation and control - in an attempt to identify the essential elements - the sine qua non - for such programming. Through this analysis, two indispensable elements - software modularity and the use of communication services - are identified. Guidelines are given for the use of these elements in both PLC and FB systems, and the application of these guidelines is illustrated in a case study of airport baggage handling systems