Like many before I have been challenged by installing an ET112 "grid meter" at mains AC entry point when my whole solar setup is in an outbuilding around 40m away. Hard wired communication back to a Multiplus 2 GX is just way too complicated, and would require laying underground conduit to hide cable runs across a large parking area. I did consider the Zigbee option, however, from my experience of trying to get a decent extended WiFi signal to my outbuilding I am not even sure the Zigbee would guarantee a solid connection (it is effectively a wireless mesh system operating on the same 2.4GHz as WiFi. At that frequency passing through 3x 25cm solid walls over the distance required is probably on the edge of operation?).
My thoughts turned to a RS485 radio link. At this stage I know I am subject to scrutiny and I make no guarantees that this would suit everybody's situation. If I could I would go for a hard-wired link... but I can't. I absolve myself from all liability if anyone decides to copy my project.
I have done some projects with LoRa in the UK so the available 868MHz band is well known to me. I stumbled across a RS485 transceiver board on ebay (....don't you just love it?), made by our friends in a different time zone. So, I purchased 2 of these boards to play around with. They use FSK modulation and make the board in 433/868 MHz versions (check your country rules!!). Ideally they run on 5v and the manufacturers claim a free air (line of sight) range of 500m. They appear to do what they claim.
The next part of the puzzle is the link to the GX device. Victron "recommend" FTDI UART chip based serial communication by implication (....that is what their standard connection cables seem to be based on?). There are 2 other UART chip devices in general circulation the CH340 and the CP2102(SiliconLabs). From what I have found the Victron equipment does not natively communicate with CH340 devices (you need the drivers installed). The CP2102 chip works with a Venus GX I have on a raspberry pi system and also with Multi 2 GX (which effectively has a Venus inside). I have not tested with Cerbo GX but there is no reason to assume it will be different since the software is all Linux based?
The USB to RS485 device I bought was the DSD Tech SH-U10. Which is a dongle that plugs into my raspberry pi. Be aware that this will not fit within the Multiplus 2 removeable protection panel over cable entry. You could just get a USB extender cable so that the dongle is outside or, as I am going to do, buy the DSD Tech SH-U10L which comes as a connection dongle on the end of a USB cable... much neater solution.
The transceiver uses less than 30mA at 5V so at the Victron end mine is powered off the DSD SH-U10. However, I have built small mains powered 5V DC power supplies into my devices so that I can run the radio link off a fused spur at my consumer unit end. In fact I used the opportunity at the grid entry point to install a Mains isolator between electricity board meter and house consumer unit in a small secondary consumer unit (designed for garages/outhouses etc). That also neatly houses the ET112 meter, since there was no space left in the main consumer unit.
So, that's it. I 3D printed some ABS boxes for neatness and to keep it housed safely. Although I only "need" the power supply at one end I have just printed the same box at both ends. I have played around with different cable options and have decided that standard internal BT telephone cable is a good option, it is just 2x twisted-pairs within the same cable. Of course this has not been tested to extreme but it has been soak tested for a week or so and appears to connect flawlessly?