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Protocol & FramingModbus Term

Application Data Unit (ADU)

The complete Modbus frame including addressing and error-checking fields.

Definition

The Modbus ADU (Application Data Unit) is the full on-wire message. It wraps the PDU with variant-specific addressing and error-checking fields. For RTU this means prepending the slave address byte and appending the CRC-16 (total overhead: 3 bytes). For TCP it means prepending the 6-byte MBAP header (no CRC needed). The ADU is what actually travels over the wire.

Quick Facts

RTU ADUSlave Addr + PDU + CRC
TCP ADUMBAP Header + PDU
Max RTU ADU256 bytes

Related Terms

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ModPackQT Modbus Glossary · Updated 2026