How to physically connect powermeter, inverter and usb-rs485 dongle for modbus rtu

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I have a Fronius Symo 8.2.3 Inverter and an Inepro Pro380-MOD Powermeter (actually a KDK COUNT3 PRO, but thats just a relabeled Inepro). A USB-rs485 dongle is in the mail.

What I want: With modbus rtu the inverter should be able to get all the readings from the powermeter. I want to use readings from both the inverter and the powermeter in openhab 2, via the usb-dongle.

My plan was to have a shielded pair of wires going from the usb-dongle to the powermeter, and from there to the inverter. The inverter already has a 120 Ohm resistor that can be applied via a switch, what I would do. OK so far? Then I could start to string the cables before all of the hardware is actually here.

I know next to nothing about modbus. Do I need to start reading, and if yes what would you recommend? Or does the modbus binding automagically configure everything for me and make it selfexplanatory? Then I could focus my resources on learning other stuff, openhab gives enough opportunities for that.

Okay.
Modbus is a master-slave protocol. One device (only one, no more no less) on a serial bus acts as master.
Since the inverter can read the meter, we can assume the inverter acts as a master on that interface.

Nope.
If the inverter is acting as modbus master on that serial bus, you cannot also add openHAB as modbus (which is always master) on the same wires.

Options for you -
Talk to the inverter on some other interface, and find out how to ask the inverter what readings it has taken from the meter. Has it got another RS-485 port? Or a TCP/IP interface?
or
connect a passive openHAB serial connection to the meter wires, so that it can “eavesdrop” on the inverter-meter conversation. Write lots of openHAB code to extract the data you want.

Nope. This is very manual and requires a degree of technical knowledge about your target devices. For example, you will need to understand the difference between 16-bit and 32-bit registers, etc.

You know there is a Fronius binding?

Ok, thank you for your answer. It seems I have to put more thought into what I actually need and want. I could use the Fronius binding, but that delivers only a fraction of the information that I could get from the powermeter. But would I need that info?
I am going to connect the meter and the inverter first and see if they can communicate ok, and then take it from there.