The Victron Cerbo GX is the central monitoring and control hub for Victron Energy systems. It connects to MultiPlus inverter/chargers, MPPT solar charge controllers, and BMS-compatible battery systems, then exposes all data via Modbus TCP, MQTT, and the VRM cloud portal. Bridging the Cerbo GX to KNX via Modbus TCP enables full solar self-consumption automation, battery SOC-based load control, and grid export optimisation from within an ETS6-programmed KNX installation.

Cerbo GX Hardware Overview

The Cerbo GX (part number BPP900450100) mounts on a standard DIN rail using a clip adaptor. Interfaces include: one HDMI output for the GX Touch 50 touchscreen display, four USB-A ports for dongles and USB-serial devices, two RJ45 Ethernet ports (one LAN, one for a second network segment), VE.Bus (RJ45, connects to MultiPlus/Quattro), VE.Direct (up to four MPPT controllers), VE.Can (CAN bus for Lynx BMS and large battery banks), Wi-Fi 802.11 b/g/n 2.4 GHz, and Bluetooth 4.x. Power input is 8-70 V DC from any Victron battery bus, or optionally USB powered.

Enabling Modbus TCP on Cerbo GX

On the Cerbo GX touchscreen or via the VRM remote console: navigate to Settings > Services > Modbus TCP. Set Enabled = On and confirm port = 502. The Cerbo GX Modbus TCP implementation uses Victron's published register list (available on the Victron Github repository under 'Victron-Modbus-TCP-register-list.xlsx'). Each physical device on the Victron system is assigned a Modbus unit ID, and registers within each unit ID address that device's parameters.

Modbus Unit IDs and Register Map

Unit ID 100 corresponds to the VEBus device (MultiPlus-II, Quattro). Unit ID 288 corresponds to the first SmartSolar MPPT charge controller. Unit ID 225 corresponds to the BMS or battery monitor (Lynx Shunt VE.Can or SmartShunt).

Critical registers for KNX integration: AC Load Power is at unit 100, register 817 (int16, watts, positive = load consuming). AC Grid Power is at unit 100, register 820 (int16, watts — negative values indicate export to grid, positive values indicate import from grid). Battery SOC is at unit 225, register 843 (uint16, percentage, scale factor 0.1 — divide raw value by 10 to get percent). MPPT PV Power is at unit 288, register 789 (uint16, watts, DC output power of the MPPT controller). MultiPlus State is at unit 100, register 844 (uint16, 0=off, 1=low power, 2=fault, 3=bulk, 4=absorption, 5=float, 6=storage, 7=equalize, 8=passthru, 9=inverting, 10=power assist, 11=power supply).

Intesis IN701KNX MAPS Configuration

In MAPS software, create one Modbus TCP connection to the Cerbo GX IP address. Add five signal objects with the parameters listed above. Object assignments: AC Load Power → GA 9/3/4 (DPT 14.056, scale 1.0), AC Grid Power → GA 9/3/1 (DPT 14.056, signed int16 — negative values are exported), Battery SOC → GA 9/3/2 (DPT 5.001, scale 0.1), MPPT PV Power → GA 9/3/0 (DPT 14.056, scale 1.0), MultiPlus State → GA 9/3/3 (DPT 5.010, scale 1.0).

Note that DPT 14.056 is a 4-byte IEEE 754 float used in ETS6 for power values in watts. If your KNX device uses DPT 9.001 (2-byte float), use that instead and accept a slight precision loss for values above 6500 W.

ETS6 Group Address Scheme

Recommended GA scheme for Victron data (main group 9 = energy): GA 9/3/0 = PV power (W), GA 9/3/1 = grid power signed (W, negative = export), GA 9/3/2 = battery SOC (%), GA 9/3/3 = MultiPlus state (0-11), GA 9/3/4 = AC house load (W). Status GAs are read-only from the Victron system; control GAs for setpoints and mode writes require additional IN701KNX objects with write direction enabled.

KNX Self-Consumption Automation Logic

The primary automation reads GA 9/3/1 (grid power) and GA 9/3/2 (battery SOC) to make load-shifting decisions. Export threshold logic: when grid power < -2000 W (exporting more than 2 kW) AND battery SOC > 50%, write SG Ready state 3 to GA 8/4/0 (heat pump). When grid power < -5000 W AND battery SOC > 80%, write SG Ready state 4 (maximum heat pump) AND set EV charger to maximum current (GA 9/5/0, DPT 5.001, value 32 for 32 A single-phase). Load shed protection: when battery SOC < 20%, write HVAC setback (GA 5/1/3, scene 3 = setback mode), write EV charger pause (GA 9/5/1, DPT 1.001, value 0), and write dishwasher delay (GA 10/2/0, DPT 1.001, value 0 = pause).

MultiPlus Fault Alert via ARISTO BewO

The MultiPlus state GA (9/3/3) feeds an ARISTO BewO alarm object. A KNX logic module generates a binary fault GA (GA 9/3/5, DPT 1.001): value 1 when GA 9/3/3 = 2 (fault state). ARISTO BewO pushes an SMS and email notification to the building manager when this GA transitions to 1, including timestamp and a deep link to the VRM portal dashboard.

Commissioning Steps

Step 1: confirm Modbus TCP is enabled on Cerbo GX and the Cerbo GX IP is reachable from the Intesis IN701KNX gateway on the same LAN segment. Step 2: in MAPS, use the 'Test' function to read each register and verify plausible values (SOC between 0 and 100, PV power positive during daylight). Step 3: verify GA values in ETS6 Group Monitor match MAPS readings. Step 4: temporarily disconnect the solar DC input (isolate MPPT) and verify GA 9/3/0 drops to 0 W within the Intesis polling interval (default 10 seconds).