Splitting CAN & RS485 Port from LuxPower/EG4 Inverter to Battery using Ethernet Breakout Board, how to daisy chain RS485 Modbus with multiple devices

Right, been down this rabbit hole with my static caravan setup and it's properly mental trying to get everything chatting nicely.

The Ethernet breakout board approach is solid if you've got the patience for crimping RJ45 connectors in a moving vehicle at 3am, which I definitely don't recommend. I've got my LuxPower daisy-chained with a Victron MPPT and Fogstar battery monitor using a proper RS485 bus — the key is getting your termination resistors spot on or the whole thing throws a wobbler.

One thing nobody tells you: the CAN side of things is often more robust than RS485 for multiple devices, but you'll need compatible hardware. If you're using the EG4 specifically, make sure you're not mixing communication protocols or your inverter gets confused faster than I do at IKEA.

The daisy chain worked fine for me until I added a fourth device, then I lost comms to the furthest one. Turns out cable length matters more than the manual suggests — kept mine under 30 metres with decent gauge shielded twisted pair.

Has anyone else gotten more than three devices stable on RS485 Modbus without going full industrial control setup? Reckon I'm missing something obvious but can't find decent UK-focused documentation that doesn't assume you've got an electrical engineering degree.

What's your device list looking like? Might help troubleshoot if I know what you're trying to network together.

Right, so the thing that's caught me out here is the impedance matching on longer RS485 runs. If you're daisy-chaining multiple devices through that breakout board, you need proper termination resistors at both ends — 120 ohm between the twisted pair.

I've seen people bodge it with the Modbus and wonder why comms drop out under load. The CAN side is usually more forgiving, but RS485 will bite you if the cable run gets messy.

What length are we talking for your runs? And have you sorted termination yet? Also worth checking if your LuxPower firmware supports the daisy chain properly — some firmware versions are finicky about multiple slaves on one port.

Might be worth a dedicated RS485 hub if it's getting complicated, honestly. Saves a lot of troubleshooting headaches down the line.

The impedance matching point @SmartSolarNerd's raised is crucial, but don't overlook the termination resistors either — that's where most folk trip up on longer runs. I've got a similar setup in my Vivaro with an EG4 and multiple BMSs daisy-chained, and I learned the hard way that improper termination causes sporadic comms dropouts that'll drive you mental debugging.

Key thing: only terminate at the ends of your RS485 bus, not at intermediate nodes. I'm running about 15m total cable and added 120Ω resistors at each end. Cable quality matters too — twisted pair shielded, preferably CAT6 or equivalent, and keep it away from your high-current DC runs.

Have you confirmed your Modbus addresses aren't conflicting? That's another common culprit with daisy-chains. What's your total cable length looking like?

Cheers for bringing this up. I'm wrestling with similar issues in my garden office setup — got a LuxPower inverter trying to talk to a Victron MPPT and a couple of Fogstar monitoring devices over RS485.

What I've found is the cable quality matters more than people think. I switched to proper shielded twisted pair (CAT5e minimum) rather than whatever came bundled, and termination resistors at both ends sorted out the noise issues I was getting.

One thing @SmartSolarNerd and @VivaroWanderer haven't mentioned yet — are you running the RS485 and CAN separately or trying to multiplex them? I initially tried combining them and it was a nightmare. Splitting them cleanly through the breakout board made everything way more stable, even if it meant running extra cabling.

What distance are we talking for your runs?

The termination question is critical here. Worth noting that most people get the location of those 120Ω resistors wrong — they go at the physical ends of the RS485 bus, not near the inverter itself. If you're breaking out via Ethernet, you're essentially extending the bus, so you need to reconsider where those endpoints actually are now.

I've had decent results with shielded twisted pair CAT6a and keeping runs under 50 metres without active repeaters. Beyond that, a Modbus gateway (Victron's MK3-24 can handle this) becomes genuinely cheaper than chasing signal integrity issues.

One thing @SmartSolarNerd and @VivaroWanderer haven't touched on — what's your actual device count and refresh rate? If you're only polling 3–4 devices every 10 seconds, you can often get away with looser cable specs. If you're hammering it every second with eight devices, you need to architect this properly from the start.

What's your current topology looking like?

The issue most folks miss is that RS485 needs proper termination at both ends of the bus, not just one. If you're daisy-chaining multiple devices, you'll want 120Ω terminators at the inverter and the last device on the run — nowhere in between.

I've had grief with this on my setup. Initially tried terminating at every junction and got complete noise on the line. Sorted once I realised the LuxPower's comms port already has one built in, so I only needed the resistor at the far end.

Also, cable length matters more than people think. Keep your RS485 runs under 1200m if possible, use proper twisted-pair shielded cable, and don't run it alongside heavy power lines. Victron gear handles this better than most, but the LuxPower's implementation can be fussy.

What's your actual cable run looking like? And are you using a dedicated comms isolator?

@ExBrickie nailed the termination bit, so I'll add the bit everyone forgets — RS485 is a bus, not a star topology, so if you've branched your Ethernet breakout like a Christmas tree to three devices you've already lost the plot before you even think about resistors.

In my shepherd's hut I ran a single daisy-chained trunk: inverter → Fogstar battery → shunt — each device tapped off the same pair, stub lengths kept brutally short (under 100mm ideally).

Quick pinout reminder for LuxPower via RJ45:

Set your Modbus addresses sequentially (001, 002, 003...) and for the love of all that's holy, don't let two devices share the same address or you'll spend a weekend debugging what looks like a haunted inverter.

Great points from @ExBrickie and @CamperCarl on termination and topology. One thing nobody's mentioned yet — baud rate consistency will kill your comms silently if you're mixing devices. LuxPower typically runs at 9600 or 19200 depending on firmware version, and if your Victron Cerbo or BMS disagrees by even one setting, you'll get garbage data with no obvious error.

Also worth checking: some EG4/LuxPower units have a 120Ω termination resistor already fitted internally at the inverter end. Measure across A/B before adding your own, or you'll end up with 60Ω and wonder why everything's flaky.