When a Modbus TCP-to-RTU gateway bridges an RTU segment with several slaves, the Modbus TCP client uses the Unit ID in the MBAP header to target the specific RTU slave behind the gateway.
True
A very common pattern is to place a single Modbus TCP gateway at the front of 10-30 RTU slaves on an RS-485 trunk. The TCP client connects to the gateway IP and sets Unit ID = N to address the RTU slave with address N; the gateway translates and forwards the request on the RS-485 side. By convention, Unit ID = 0xFF (255) or 0x00 points to the gateway itself (for internal statistics). This simplifies BMS integration: a single IP address on the TCP side, yet N addressable RTU slaves below.
Document the Unit ID -> RTU slave mapping in the project file: it is the first thing a successor will need to debug a missing point through the gateway.
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