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Protocol & FramingModbus Term

Protocol Data Unit (PDU)

The function code plus data — the core payload shared across all Modbus variants.

Definition

The Modbus PDU (Protocol Data Unit) is the essential payload of every Modbus message. It contains the function code (1 byte) and the associated data (e.g., starting address and quantity). The PDU is variant-agnostic — it looks identical whether transmitted over RTU, ASCII, or TCP. Framing (CRC, MBAP header) is added by the outer layer.

Quick Facts

Max size253 bytes
StructureFunction code (1 B) + Data
Shared byRTU, ASCII, TCP

Related Terms

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ModPackQT Modbus Glossary · Updated 2026