CODESYS™ and PLCopen™ are the base for many PLC programming environments that share programming languages and development environment.
However Object programming in these environments is oriented towards C++ object inheritance and other characteristics but limits access to the inside of program Blocks to IO variables.
IEC DUTs (Data Unit Type) called UDT (User Defined Data Type) by Rockwell or PLC Data Types (Siemens) have to be used to create global hierarchical structures.
We use, the different branches of this structures to govern functions to which we pass IN/OUT (By Reference) variables. We do not longer use Function blocks but standard functions that have a single IN/OUT variable. We can call this program structure as IO-Less as IO for any function is reduced to a single variable.
