Why does reading getmode always return zero or timeout?

First confirm the DIP switch for Modbus communication is turned ON inside the outdoor unit. Also verify the slave address on the heat pump matches the unit ID configured in the portal. The LG Therma V Modbus interface requires the slave address to be set to a non-zero value, and the default may not be 1 on all firmware versions.

The gateway connects but the temp register shows no value. Why?

Register 5 is an input register read with FC4. Confirm your wiring uses the correct RS485 A/B polarity and that the serial parameters are exactly 9600 bps, 8N1. Some RS485 adapters swap A and B lines; swapping them on the heat pump terminal block usually resolves persistent silence.

FC3 and FC4 seem swapped compared to the printed manual. Is that normal?

Yes. A documented quirk of the LG Therma V Modbus implementation is that the manual labels holding registers as FC3 and input registers as FC4, but some firmware revisions invert this in practice. The register addresses in this template follow the corrected mapping as used by the OpenEMS implementation.

Is it safe to write any value to setmode without restriction?

Exercise care when writing to setmode (address 9, FC16). Writing an unsupported or out-of-range mode value may place the heat pump in an unexpected operating state. Always verify the target mode value against the LG installation manual, test with one step at a time, and confirm the readback via getmode before automating repeated writes.

Free Modbus template: LG Therma V R290 Monobloc (SG Ready)

The LG Therma V R290 Monobloc is an air-to-water heat pump using natural R290 refrigerant, designed for residential and light-commercial space heating, cooling, and domestic hot water production. This template connects via Modbus RTU over RS485 and targets the SG Ready interface: it reads back the current operating mode (getmode, address 9) and a temperature value (temp, address 5), and writes the desired SG Ready mode (setmode, address 9). The result is direct solar-surplus or grid-signal integration from the ModbusCloud portal without any additional controller hardware.

Name	Address	Function	Type	Access
setmode Operating mode setpoint: write to select SG Ready mode.	9	FC16	uint16	W
getmode Current operating mode readback from holding register 9.	9	FC03	uint16	R
temp Temperature reading from input register 5.	5	FC04	uint16	R