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"PLC Fundamentals: Distinguishing Between Controller and Program Scope"
Time:Dec 16,2025
Understand the variables in the PLC
A variable is a block of memory that stores binary values, which can be represented by a single bit, integer, floating-point number, or ASCII character. Nowadays, most PLC variables are label-based, which means that each variable is assigned a data type that defines the representation of the variable. It can be a standard integer, a double-precision integer, a byte, a word or a string, all of which are given a name.
Compared with some standards of previous generations (Rockwell's SLC 500), labels used to be typically located at fixed memory addresses, such as N7:0.1 or B3:5.0, with letters indicating specific memory blocks for different types of variables.
What is a definition label?
Defining the location where the tags exist is an important issue. There are mainly two locations: they can be defined in the controller scope or the program scope. The scope determines their scope of use. Global variables can be used by any program, routine or subroutine at any time, so they can pass information between programs and functions. Local variables can only be used within the program where they are defined.
What are the tag definitions in Studio 5000 and TIA Portal?
When creating labels in the Rockwell Studio 5000 Logix Designer IDE, you can choose to create labels that are accessible to the entire controller project, or create labels that are accessible only within a single program.
For Siemens PLC, labels can be created in data blocks or defined in function blocks and function calls. When labels are created in a program or function block/function call, only that program or function can access these labels. Building ladder diagram logic code in this way can achieve a compact label list and a modular code structure.
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