Maybe its something to do with the weather in Scotland as my car gave me some EMI grief yesterday as well.

Here's what happened. I had removed the controller to fit a bigger heat sink, well actually to fit a heat sink.

This involved removing the Zivan charger to get to the controller. After putting it all back together and a round trip home and back to work the next day, I put it on charge. In the afternoon, went to drive the car and discovered it hadn't charged. So put it back on charge and started getting serial comms errors which caused the bms to switch off the charger, as it should. A bit of investigation and I tracked the problem down to the control wires from the solid state charger relay being to close to the mains lead, the errors where happening while the car was plugged in, even with the relay and charger switched off.

Moving the wires further away helped, but I had also modified the master board when I was trying out the current loop idea, basically it was still connected to the master with Peter's isolated DC-DC trick as well. The current loop was getting it's supply from the same 12 volt supply as the charger relay circuit, so the noise must have been feeding back through this. I have now removed the current loop circuit from the master and it charges fine. Will give the car a run later, as I had to leave it at work last night, and check it's all o.k.
I don't know if any of that helps, but try changing one thing at a time while looking at the noise on the scope. Shielding seems to be a bit of a black art, I have found my comms better without the shielding on my data cable earthed than with one end of them earthed.

I am investigating a different idea to improve serial comms between the slaves, its called the
EVILbus. It was originally implemented at 1200 baud, but I have been corresponding with Lee Hart, one of the designers about changes to improve the speed to 9600 baud and reduce some of the component count. I need to breadboard it to see if it works and will post results when I do.
This would also require a new master as well and this project has caught my eye. The
Maximite is a Pic32 chip with a basic interpreter programmed in. Output is composite video (or VGA) and input is a ps2 keyboard or usb. It also has a SD card slot for loading programs and storing data. What he has done is recreate something like the old TRS80 computer on a Pic32, including a BASIC interpreter that has line numbers

, how retro can you get, but with the added benefit of 20 I/O pins from the pic being available. Yes the interpreted basic won't run as fast as a complied program, but this is a 32 bit 80mhz processor. And being able to program your master software in a free version of basic is nice, but with this you don't even need a computer to do the editing, just plug in a keyboard and away you go. At the moment though it has one glaring omission, no serial comms.

But he has said this will be added in a firmware update, so I will keep an eye on this project.