question

Hi Udo, i have also about 43 years of serial comms on my back .. :)
As far as i know the VE.Bus stuff uses 485 .. and if you 'read' the signals with that idea and a typcial diffential receiver the bus is clean.

A usual 485 receiver (75176) will have a few kohms input impedance, but 76 ohms sound that there might be a line end termination resistor installed (which would make sense, the speed on your screenshots points to 250kbps .. there you would terminate. Maybe the devices have a programmable resistor, but maybe the receiver is just fried ..
Would help to take a pic at the schematic and what receiver chip is used.
Do you have a Mk3 ? I have some simple lua code to read and dump ve.bus messages, not that i understand everything, but at least one can see cerbo/venus comms with the phase master as well as messages between the devices and i am able to get setpoints, actual values of all devices etc etc. Could help to run this to see if checksum errors happen, and if, if they are happening just with one device.

Johannes

The 76 Ohms occur at only ONE of the three Multis and only on one Pin. All other five Pins (the four on the other two Multis and the second pin of the suspicious Multi) show the 14 kOhms.

Also there is no Termination or something else plugged on the VE.Bus connectors. I haven't found a "Termination on"switch or something like that in the specification or software.

So I can't explain the measured resistance nor the low Amplitude on the third Multis. I haven't found any specification of the Amplitude or Signalforms of the VE.Bus yet.

It's frustrating, after some hours the Multis stop working after an VE.Bus Error occurs. Earlier this month there were different Error, now it's "Incompatible Firmware", which is ridicoulous, because for some hours its working and I had crosschecked the firmware of all multis, Octo and mppts, it's always the newest one.