Three different networks, one cable shape

There are three separate conversations happening on a typical solar and storage install, and they are not related to one another:

• 01LAN — the equipment talking to the home network, and through it to the internet.
• 02CAN— the inverter talking to the battery's management system.
• 03Modbus over RS-485 — a controller talking to meters and other devices down a chain.

The trap is that the first two use the same patch lead. An RJ45 plug fitting a socket tells you nothing about whether it belongs there. A battery CAN port and a router LAN port accept exactly the same cable, and connecting one to the other does not blow anything up — it simply does nothing, silently, which is harder to find than a fault.

Label both ends of everything. It costs a minute and saves a return visit.

Local Area Network. This refers to the devices on the home network, whether they are hardwired over ethernet or connected wirelessly over Wi-Fi.

“Local” is the operative word. The LAN is the building's own network — everything on it can reach everything else on it, and reaching anything beyond the building is a separate matter, handled by the router.

When someone says a system is “on the network” but not “ online”, this is the distinction they are groping for: it is on the LAN, and the LAN is not getting it to the internet.

The router is the device at the centre of the local area network. In a domestic property it is usually combined with the modem that connects the local network to the internet and the world wide web.

Those last two are not synonyms, and the distinction is occasionally useful on site: the internet is the network of networks, and the world wide web is the front end of it — the part people actually look at. Monitoring platforms live on the web; the inverter itself just needs the internet.

One box, then, doing at least three jobs — modem, router, and usually wireless access point. Which is why “the router was replaced” is such a reliable cause of a system going dark: three things changed at once.

Controller Area Network. A system for connecting the inverter to the Battery Management System — the BMS. It is how the inverter finds out the state of charge, the cell temperatures, and what current the battery will actually accept right now.

It usually uses the same patch cables as LAN over ethernet. Same cable, same connector, entirely different network — this is the one from the first chapter, and it is worth saying twice because the mistake is so easy to make and so quiet when you do.

CAN comes from vehicles, which is a useful thing to know about its temperament: it was designed for short runs between devices that were built to work together, in an electrically filthy environment. It is robust over a couple of metres inside a cabinet. It is not a building network and should never be asked to be one.

Two names, because there are two separate things and the phrase runs them together.

Modbus is the protocol — the rules of the conversation. It lets one controller device address multiple devices on a daisy-chain network: A, then B, then C, each with its own address, each answering only when spoken to.

RS-485 is the signalling — the specific electrical standard the protocol is carried on, and what ensures the Modbus behaves as expected. It is a differential signal over a pair of wires, which is what lets it survive a long run past noisy things.

You will meet Modbus on meters, on export limitation schemes, and on almost anything that has to report a number to something else. When it does not work, the fault is usually in the signalling rather than the protocol — which is what the next chapter is about.

Both of these contradict the instincts you get from power cabling, which is exactly why they get broken.

1. Daisy-chain, not radial. Every device sits on one continuous run, passing through from one to the next. Not a star out of a convenient central point — that is how you wire lighting, and it is wrong here. A radial layout puts unterminated stubs on the line, and the reflections off them corrupt the signal in ways that look like an intermittent device fault.

2. Twisted pairs prioritise signal quality over voltage drop.The twisting is the point: two conductors sharing the same interference so the receiver can subtract it out. Reaching for heavier, untwisted cable because it “must be better” is a power-cabling instinct, and here it makes things worse — you have optimised the wrong property.

Use the cable the manufacturer specifies, in a chain, and it will behave.

Field notes, and a starting point for a conversation — not a design authority. The current edition of BS 7671, your DNO's own requirements and the equipment manufacturer's instructions all outrank anything on this page.