Programs

Programs, also known as program organization units (POUs), are logical programming units describing operations between variables of a process. Programs describe either sequential or cyclic operations. Cyclic programs are executed at each target system cycle. Sequential programs, representing sequential operations, are grouped together. The execution of sequential programs has a dynamic behavior.

Programs before and after sequential programs describe cyclic operations. These are cyclic programs which are not time-dependent. Cyclic programs are systematically executed at the beginning of each run time cycle. Main sequential programs (at the top of the hierarchy) are executed according to their respective dynamic behavior.

Programs located at the beginning of a cycle (before sequential programs) typically describe preliminary operations on input controllers to build high level filtered variables. Sequential programs frequently use these variables. Programs located at the end of the cycle (after sequential programs) typically describe security operations on the variables operated on by sequential programs, before sending values to output controllers.

Programs are described using the available graphic or literal languages. Specify the programming language when creating a program; the programming language cannot be changed for an existing program.

POUs defined as programs are executed on the target system respecting the order shown in the Programs section.

Respect the hierarchy of programs. Programs are linked together in a hierarchical tree. Those placed at the top of the hierarchy are activated by the system. Child-programs (lower level of the hierarchy) are activated by their parent.

Projects can contain up to 65 536 programs.

For programs within a project and the global library, names are limited to 128 characters beginning with a letter followed by letters, digits, and single underscores. The last character for a program name must be a letter or digit; program names cannot end with an underscore character. Reserved words, defined words, or data types such as, elementary, structures, or arrays, are not valid names. The same type of elements within a scope must have unique names.