Warning metering

External meter not communicating (SDM120)

Symptom: The inverter reports no meter / meter offline, even though the SDM120 display is lit.

Work through these in order — the first two fix most cases.

1. Wiring

Confirm RS485 A→A, B→B between meter and inverter. If unsure, swap the two data wires — it won’t damage anything.
Make sure the meter is on the inverter’s meter / CT RS485 port, not the BMS port.

2. Meter settings

Set the meter to 9600 baud and the Modbus address the inverter expects (commonly 1).

3. Locked-ID variant

Some SDM120 units ship with the address effectively locked — the menu won’t save a new ID, so the meter answers on the wrong address and the inverter sees nothing.

Fix: fit the meter variant with a user-changeable ID (the SDM120CT-MID style), set its address to match the inverter, 9600 baud. Full background in SDM120 CT meter ID fix.

Carry one known-good user-settable meter in the van — swapping is faster on site than fighting a locked unit.