PLC Memory Guide: User-Defined Types (UDTs) vs Arrays

Key Takeaways:

• Arrays provide rapid, index-based access for uniform data types but lack contextual readability.
• PLC User-Defined Types (UDTs) group heterogeneous data logically, mirroring real-world assets like motors and valves.
• Proper memory structuring is critical for maintainability and reducing downtime on the plant floor.

The choice between arrays and PLC User-Defined Types (UDTs) is a constant architectural debate in industrial automation. Engineers frequently encounter spaghetti code inherited from older PLCs where 100-element integer arrays act as cryptic catch-alls for machine state. Structuring memory correctly from day one prevents this technical debt.

flowchart LR
    classDef arrayStyle fill:#2563eb,color:#ffffff,stroke:#1e40af,stroke-width:2px
    classDef udtStyle fill:#16a34a,color:#ffffff,stroke:#166534,stroke-width:2px

    subgraph memory_options ["Memory Options"]
        A["PLC Arrays"]:::arrayStyle -->|Index Based| B["Uniform Data"]
        C["PLC UDTs"]:::udtStyle -->|Tag Based| D["Heterogeneous Data"]
    end

Understanding PLC Arrays

Arrays are continuous blocks of memory reserved for a single data type, such as INT, REAL, or BOOL. They are addressed using an index (e.g., TemperatureSensor[0] through TemperatureSensor[99]). Arrays shine when processing large batches of identical data through loops in Structured Text (ST).

What Are PLC User-Defined Types (UDTs)?

PLC User-Defined Types (UDTs) allow engineers to define custom data structures containing multiple native types. For example, a Motor_UDT might contain a BOOL for Run Command, a REAL for Speed Feedback, and an INT for Fault Code. When instantiated, the UDT creates a cohesive tag structure.

flowchart TD
    classDef udtStyle fill:#dc2626,color:#ffffff,stroke:#991b1b,stroke-width:2px
    classDef varStyle fill:#2563eb,color:#ffffff,stroke:#1e40af,stroke-width:2px

    subgraph motor_udt ["Motor UDT Structure"]
        A["Motor_1 (UDT)"]:::udtStyle
        A --> B["RunCommand (BOOL)"]:::varStyle
        A --> C["SpeedFeedback (REAL)"]:::varStyle
        A --> D["FaultCode (INT)"]:::varStyle
    end

The Real-World Nuance: Legacy Integration

On the plant floor, adopting PLC User-Defined Types (UDTs) often clashes with legacy SCADA systems or HMI faceplates expecting contiguous array blocks for optimized polling. A common paradox is mapping a beautifully structured UDT back into a flat array via block transfers just to appease an older OPC DA server. Engineers must balance modern structural elegance with the harsh reality of existing infrastructure.

Arrays vs. UDTs: Side-by-Side Comparison

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Frequently Asked Questions

Can I put an Array inside a UDT?

Yes, most modern platforms (like Allen-Bradley ControlLogix or Siemens TIA Portal) allow arrays within a UDT, offering powerful ways to structure alarm histories or step sequences.

Do UDTs consume more memory than Arrays?

Yes, padding and alignment rules in PLCs often mean a UDT consumes slightly more memory than a tightly packed array, though this is rarely an issue in modern high-capacity controllers.