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Protocol & FramingModbus Term

Unit ID (Slave ID / Slave Address)

The 1-byte device address identifying which device a request is intended for.

Definition

The Unit ID (also called Slave ID or Slave Address) is the 1-byte identifier that tells the bus which device should respond to a request. In Modbus RTU, only the slave with the matching address responds; all others ignore the frame. Valid addresses are 1–247 for slaves; 0 is reserved for broadcast. In Modbus TCP, the Unit ID is carried in the MBAP header and is typically used when a TCP gateway bridges to an RTU bus.

Quick Facts

Valid slave range1–247
Broadcast address0 (no response expected)
Gateway passthrough248–255 reserved

Related Terms

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ModPackQT Modbus Glossary · Updated 2026