Industrial Engineer · phase 4 of 16
Local / Global Tag
Tag scoped to one program or made accessible across the controller project.
The Explain card
- Plain English
- A global tag is visible to every program in the controller, and a local tag belongs to one program and is invisible outside it. The scope decides who can read and write it.
- Example
- Plant_Estop is global because every program needs it. Filler_Step is local to the Filler program because nobody else has any business changing it.
- Why it matters
- Keeping tags local is how two programs avoid stepping on each other, and it is how a program can be copied to another controller without dragging a hundred dependencies along. Global by default is convenient for a week and a mess for a decade.
- Hook
- Who is allowed to know.
Where it sits in the deck
Phase 4: PLC Memory, Tags, and Instructions
The data a program moves and the instructions that move it, from a single true or false value up to a structured array.
