Baxi Platinum+ inverter heat pump (5-16 kW, R32) provides Modbus RTU on a RS485 service port for setpoint control and telemetry. It also supports SG Ready 2-contact dry-contact interface for tariff-coordinated operation. Here is the complete KNX integration approach.

Modbus RTU Configuration

RS485 port location: RJ45 connector on the Baxi Platinum+ control board (marked 'COM' or 'SERVICE'). Pin assignment: pin 3 = B+ (RS485 positive), pin 6 = B- (RS485 negative), pin 5 = GND. Baud rate 19200, parity even, stop bits 1, data bits 8 (19200/8E1).

Unit ID: 1 (default — confirm in Baxi control panel menu under Communication > Modbus Address).

Connect to MDT SCN-MOD.02 RS485 port or Weinzierl KNX Modbus RTU gateway RS485 terminals. Maximum cable length RS485: 1200m at 19200 baud with twisted pair, 120 ohm termination at both ends.

Register Map

Holding registers (FC3): - Register 0: System operating mode uint16 — 0=off, 1=heating, 2=cooling, 3=DHW (domestic hot water), 4=auto - Register 1: Setpoint temperature int16, degrees Cx10 (e.g. 350 = 35.0 degrees C flow setpoint) - Register 10: Flow temperature (supply) int16, degrees Cx10 - Register 11: Return temperature int16, degrees Cx10 - Register 12: Outdoor ambient temperature int16, degrees Cx10 - Register 20: Compressor status uint16 — 0=off, 1=running - Register 21: Defrost active uint16 — 0=normal, 1=defrost cycle active - Register 30-31: Energy counter uint32, kWh (heat pump electrical energy consumed)

Read flow temperature (reg 10) and outdoor temperature (reg 12) every 60s for KNX weather compensation visualisation. Write setpoint (reg 1) from KNX DPT 9.001 thermostat GA — multiply KNX value by 10 before writing (KNX sends 35.0, write 350).

MDT Modbus Gateway Setup

MDT SCN-MOD.02: RS485 mode, 19200/8E1. Read task A: registers 0-2 (mode and setpoint, 60s). Read task B: registers 10-12 (temperatures, 30s). Read task C: registers 20-21 (compressor + defrost, 60s).

KNX objects: flow temp -> DPT 9.001 GA 6/5/10. Outdoor temp -> DPT 9.001 GA 6/5/11. Compressor running -> DPT 1.001 GA 6/5/12. Write object: GA 6/5/20 (DPT 9.001 setpoint) -> register 1 with scale factor x10.

SG Ready Integration

Baxi Platinum+ has SG Ready 2-contact interface on terminals SG1 and SG2 (24VAC or potential-free). Four states: - SG1 open, SG2 open: normal operation (state 1) - SG1 closed, SG2 open: reduced/blocked operation — utility request to reduce load (state 2) - SG1 open, SG2 closed: increased operation — excess grid power, run at boosted setpoint (state 3) - SG1 closed, SG2 closed: maximum operation — force heat pump to run at maximum power (state 4)

KNX implementation: MDT AKK two relay outputs -> SG1 and SG2 terminals. KNX logic: when Tibber spot price < 5 cents/kWh: GA 1/7/0=0, GA 1/7/1=1 (state 3, boosted). When price > 30 cents/kWh: GA 1/7/0=1, GA 1/7=10=0 (state 2, blocked).

OpenTherm Alternative (Baxi Gas Boilers)

Baxi Duo-tec and Platinum gas combi boilers have OpenTherm 1.0 interface (2-wire, polarity-insensitive). Use Remeha OpenTherm-to-KNX bridge or Teletask TDS12051 OT module. KNX sends modulating setpoint (DPT 9.001) to boiler. OT bridge enables 20-100% modulation — far more efficient than on/off thermostat control.