…here is one example: 3.50~24 Modification ModbusTCP - volatile register for GridSetpoint ... what about mqtt
Okay, I will email you.
No. Even settings on the GX device is debounced so that flash is not constantly written. We did however add a volatile modbus register for the grid setpoint recently, because there are people who use ESS mode 2 to write the setpoint several times a second, and that should probably be avoided even if it is debounced. But that’s a bit off topic here.
Some feedback from @hominidae 's system. I can confirm that Venus definitely only writes one register now: The register the SMA spec allows for writing. Venus 3.60~11 should be safe to use with SMA inverters.
I cannot confirm that the limiter actually works. On this system, it did not work. It accepted the modbus write, but didn’t react to it. In testing we have observed this to work with SMA inverters, so I assume this is a matter of setup.
Thank you @iburger for your efforts and investigation.
My SMA inverter has a hardware module for external control (from the DSO), so software limiting was never used. Another option, besides external control, with my DSO is limiting the inverter (hard-limit)/grid-feed-in (soft-limit) to 70%of the installed capacity…
Without ESS I opted for the external control, but now with ESS installed the 70%-soft limitation seems feasible and would allow me to return the external control box from the DSO and get rid of the rental-fee involved.
Edit: RE setup this feature in SMA after installation, I think one needs an “installer code” from SMA. I don’t know whether there is a fee involved with that.
I can confirm that you need a “Grid Guard Code” from SMA to change the parameter “Operating mode active power setting” in the SMA PV inverter.
Most of the parameters that change grid relevant behaviour are secured in this way.
If you enter the correct code in the user settings, you can change the parameter. It will then look like this:
Factory setting is “Off”.
You need to set this parameter to “External active power setpoint” if you want to use the PV inverter together with the SMA Home Manager 2.0 to control feed-in limiting like described here:
https://manuals.sma.de/HM-20/en-US/1071201163.html
Without the Grid Guard Code, the menu will look like this:
and you will not be able to activate (or in this case: deactivate) external power limiting.
To my knowledge, SMA will give the Grid Guard Code only to companies, and only after sending them some document to prove that you are really a company and not a private person.
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Still waiting for an answer from SMA support… 
Finally got some info regarding the 10120 error code in the SMA PV inverter.
It seems that, once this error has occured, you have to gain “credits” for further parameter changes.
For roughly every day that you leave the critical parameters alone (the ones marked with an exclamation mark in the parameter list), you earn the ability to perform one more change.
So, if you, for example, wait for 100 days without touching any of the critical parameters, you are allowed to perform 100 changes of them in a row. To do the 101st change, you will have to wait for another day.
I hope this info will help others who got into the same situation.
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