What is PLC Programming? Examples, Software, Languages, Work

A well-organized PLC program is easier to test, maintain, and scale. Most platforms follow a similar structure:

Tasks and programs:

• Cyclic (periodic) tasks run at a fixed rate (e.g., every 10 ms) for real-time control.
• Event tasks trigger on specific events (e.g., rising edge of an input).
• Background tasks run when the CPU is idle (e.g., logging).

Program blocks:

• Main program (e.g., OB1 in Siemens, Main in Rockwell) coordinates calls to subroutines/function blocks.
• Function blocks (FBs) encapsulate reusable logic with internal memory (e.g., a motor starter FB).
• Functions (FCs) return a value without internal memory (e.g., a math routine).

I/O abstraction:

Map physical I/O to internal tags once, then use tags throughout logic. This decouples code from wiring changes.

Naming and standards:

• Use clear tag names (Motor1_RunCmd, TankLevel_PV).
• Group by machine area: Safety, Interlocks, Sequences, Motion, Alarms, HMI.

Error handling:

Standard fault handler FBs, alarm buffers, and safe-state logic.

State management:

Use SFC or state machines for complex sequences (Idle → Start → Run → Fault → Recovery).