Which TCP port should I use for an SB1000 Modbus connection?
Choose the service for the physical RS485 bus. The optional SB1000 management interface is a separate service.
Check the destination
- Identify whether the field device is wired to RS485 Port 1 or Port 2.
- Open that port's settings in the affected SB1000 web interface. For a BMS/PLC sending TCP requests to RTU devices, confirm TCP → RTU operation and record its current TCP port.
- Enter that port in the external client. The factory defaults are 4001 for Port 1 and 4002 for Port 2; changed settings take precedence.
- Enter the field device's RTU address as the request's Unit ID. The TCP port chooses the bus; the Unit ID chooses the device on it.
- Read one register defined in the field device's manual, using its function, address convention, and width.
TCP port 502 defaults to the optional Modbus Management Interface for SB1000 settings. It is not the default route to either RS485 bus. Factory initialization disables both serial port modes and MMI, so first confirm the service is enabled.
If the client only accepts 502, resolve any existing listener conflict before assigning that port to the required gateway. Record the change for other clients.
Expected result
A request through the configured port reaches the intended field device and returns its documented data. If TCP connects but the request times out, continue with “TCP connects, but Modbus reads time out”. Record the actual IP, port, bus, mode, Unit ID, and firmware if the destination remains unclear.