question

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You definitely want to upgrade the firmware to 1910209 (latest Assistant Firmware). Somehow with firmware 1910159 (latest Non assistant) the Cerbo GX (which is what I am using, the Multiplus being connected through the original VE.Bus yellow cable - basically a standard ethernet cable for that matter) is unable to read values in realtime from the Multiplus (hangs the reading after ~3 reads). Works fine at that level with 1910209.

A few things I have found, additionally:

1. Upon activating DVCC, VRM constantly shows me this: "System overview - GX Error #48 DVCC with incompatible firmware: Warning". Nevertheless doing what I expect: read battery temperature using the original Multiplus sensor plugged to the Cerbo and sharing it with the SmartSolar charger (STS); reading current and volts directly from battery terminals using the SmartShunt and sharing them with the charger (SCS and SVS). I reckon it is not sharing these values withe the Multiplus, hence the warning.
2. An option exists in VRM to remotely download the settings file from the Multiplus, edit them in VEConfigure 3 and reupload them (which is convenient).
3. Notwithstanding, VRM offers an option to switch the Multiplus operation mode between Charger, Inverter, On and Off as well as to switch on and off Relay 2. I must say I haven't tested those.

My general impression is that it works fine, albeit the communication with the Cerbo being mostly unidirectional (read values only) and probably missing some functionality to be found on models capable of running more recent firmware.

You can always program Line (2) using the Multiplus standalone settings by adding an Assistant, I suppose (option 2, above).

A final word of caution: the CerboGX does not support upgrading the firmware on the old Multiplus. You will need a VE-Bus to USB adapter for that matter (I used an MK2).