Terminology

AT: Direct representation of a variable.

FUNCTION BLOCK: Consists of a function that requires an instance.

CONFIGURATION: Organization of the software at a higher level. Can contain multiple resources.

FUNCTION: Block responsible for executing a behavior or action based on parameters (for example: VAR_IN, VAR_IN_OUT, VAR_OUT).

INSTANCE: Memory area occupied for use by a Function Block.

LD (Ladder Diagram): Graphical language based on electrical diagrams (contacts and coils) that follows the flow of power between elements.

OVERFLOW: Occurs when the result of a calculation exceeds the limits of the result's data type.

POU: Program Organization Unit — can be a Program, Function Block (FB), or Function.

PROGRAM: Logical grouping of elements and constructs necessary for signal processing to control a machine or process.

RESOURCE: Element with processing capacity responsible for program execution.

SCAN: Program scan cycle.

STACK: Program ladder stack — memory area used to execute program logic.

TAG: Variable name.

TASK: Controls program execution, periodically or triggered by events.

DATATYPE: Informs the compiler of the space occupied by a variable and its format.

VARIABLE: Memory location capable of holding and representing a value or expression. May have scope:

Local: Automatic position calculated by the compiler.
Global: Located in a specific memory area (e.g., inputs/outputs), accessible throughout the configuration.

A variable can be:

Retentive: Stores the value after powering down.
Volatile: Initializes with the value of the initial value field when the device is powered up.
Constant: Fixed value, non-changeable.

VAR: Variables for internal use in a User Block (USERFB) — equivalent to local scope.

VAR_INPUT: Input argument of a USERFB. Variables here are read only within the USERFB.

VAR_IN_OUT: Input and output arguments of a USERFB. Does not allow constants, physical inputs (%I_), network inputs (%I_), or read-only system markers (%S_).

VAR_OUTPUT: Output argument of a USERFB. Does not allow constants, physical inputs (%I_), network inputs (%I_), or read-only system markers (%S_).

WATCHDOG: Manufacturer's mechanism to take specific actions if system integrity is violated.