Modbus TCP Without the $250 Gateway
Every marina operator who wants to read solar charge state in their building management system faces the same answer from Victron: buy a Cerbo GX for $250 USD. Before you touch the controller.
What Modbus TCP Is and Why It Matters
Modbus TCP is the standard industrial monitoring protocol. Every professional building management system, every SCADA integrator, every marina control panel that talks to sensors and actuators speaks Modbus. It is the protocol that means "read this value from this device." It has been the standard for over forty years.
For a marina with solar-powered dock infrastructure — battery banks, lighting, security, bait tanks, pump houses — the ability to read battery state, charge current, and PV input from a central control panel is not a luxury feature. It is the difference between proactive maintenance and reactive callouts. Between knowing a battery bank is at 40% at 6pm on a Friday, and finding out it failed when the dock lighting went out.
The Victron Answer
Getting Modbus TCP data off a Victron SmartSolar MPPT controller requires the following:
- Purchase a Cerbo GX — approximately $250 USD
- Connect the Cerbo GX to the SmartSolar via a VE.Direct cable (additional cost)
- Connect the Cerbo GX to your network
- Configure the Cerbo GX network settings
- Configure the Modbus register map
- Maintain the Cerbo GX firmware separately from the MPPT controller firmware
Multiple users in the Victron Community forum and GitHub Home Assistant integration issues have documented this process as complex, the documentation as unclear, and the Modbus connection as unstable after software updates. One thread quote captures it precisely: "the Victron documentation seems a bit light and less than clear."
The Cerbo GX costs more than many of the controllers it is bridging. The configuration complexity means that any marina with multiple controllers needs either dedicated technical staff or an integrator on retainer to maintain it.
How It Should Work
Every other piece of professional monitoring equipment has Modbus TCP built into the device.
Power meters: Modbus TCP built in. One Ethernet cable. Shows up on the network. Read the registers.
Current transducers: Modbus TCP built in. Same.
Temperature sensors for HVAC: Modbus TCP built in. Same.
Flow meters: Modbus TCP built in. Same.
This is not a new idea. Modbus TCP was formalised as a standard in 1999. It runs over standard TCP/IP. Every device that needs to be part of a building management system includes it, because that is how you get into a building management system.
The solar charge controller should be no different. One Ethernet cable. It shows up on the network. You read the registers. There is no gateway required because the gateway is inside the device.
The Architecture
The MicroCore Solar Charger implements Modbus TCP natively. STM32F407 with an onboard LAN8742A Ethernet PHY and discrete magnetics (Bourns SM91072AL-E). M12 A-coded sealed Ethernet connector. The controller accepts a single Ethernet cable and presents a Modbus TCP server on port 502.
Register map includes: PV voltage and current, battery voltage and current, charge state, temperature (enclosure and battery), fault codes, historical data access, and configuration registers for charge profile parameters.
For a marina operator: the controller is visible on the LAN the moment the cable is connected. No additional hardware. No separate firmware update cycle. No configuration complexity beyond setting the IP address.
For a facilities manager integrating into a SCADA or BMS: standard Modbus TCP, documented register map, accessible from any client that speaks the protocol — which is every BMS and SCADA platform available.
Why This Matters at the Price Point
The MicroCore Solar Charger is priced at $250–$450 NZD. The Victron SmartSolar 100/30 with a Cerbo GX gateway for Modbus access is $350–$430 NZD for the controller plus $350–$400 NZD for the gateway — $700+ NZD before installation. The gateway adds ongoing maintenance overhead that a professional monitoring system shouldn't require.
Native Modbus TCP is not a premium feature. It is the correct architecture. The cost of including it — one Ethernet PHY, discrete magnetics, an M12 connector — is small. The cost of not including it — $250 for a gateway, configuration complexity, ongoing firmware dependency — is paid repeatedly.
MicroCore Systems is building an MPPT solar controller with native Ethernet Modbus TCP for marine and marina applications. Build log on Hackaday.io. Contact: jonathan@microcoresystems.co.nz