question

I'm pretty sure the Victron VE.Direct USB adapter has an isolated VE.Direct port. The one you made probably isn't isolated. So ... you created a ground loop upsetting the BMV's idea of ground.

Hi Kevin - I think you are right.. as Further testing shows that even touching the USB plug onto the USB port sheild drops the voltage when the ground wire is connected at the BMV end... It works fine with just the RX and TX connected.. and reads correctly, but i'm concerned that i'm missing something by only having the 2 wires connected.. i'll probably stump up for the official cable in this instance as its such a critical part of the system.. but would be interested in peoples thoughts on just the 2 wire issues.. From what I can gather this issue only affects the BMV... not any other VE direct devices..

Steve

Hahahahahaha, indeed it made me laugh! And feel sorry a bit for the time spent (wasted? ;o) by others and yourselves.

Note that by leaving the V+ out of the connector, you're cable still isn't isolated. Which might work ok, or might not work ok; depending on a whole load of things; which is why the standard is to isolate such connections.

Have a good Sunday! Thanks for sharing, and enjoy your system!

Ps. for anyone else looking to self make cables, see here my full opinion on it. The conclusion is, please please don't; and if you do, never ask for help if things don't work.

https://community.victronenergy.com/questions/48562/vedirect-diy-cable.html

Hi.

You noted this is not an issue with other ve. direct devices.....

The BMV uses a different data line voltage to other ve. direct parts like the MPPT solar charge controller's for example. This may have been part of your problem.

However as noted by many other contributors your most likely issue was related to a ground loop. When using TTL and UART devices ground loops can be a real pain in the backside. A good example of this is factory automation and PLC controlers. All PLC controlers have isolated inputs via optocouplers to avoid these issues.

Standards such as RS232 and RS485 almost allways specify iether a common single end ground point and/or isolated inputs. When controling machines in different buildings for example you can even have dangerous voltages on data lines and shielded cable terminations.

It really is a minefield. Aside from the increase in speed ground loop problems were the reason that optical transmission was adopted as there is no electrical connection or interference issues with fibre optic cables. Issues like the above are great learning experiences. I learnt the hard way to at university when I recieved a rather nasty belt from a coaxial cable coming from another building.

I personally think its time victron adopted a standard communication protocol via optical link instead of a mixture of RS485, RS232 and CAN as they do now. The cost of the transceivers will be more than covered by the savings in manufacturing, design and programming.

Yes Victron parts are expensive and sometimes overpriced but that's the price you pay for some of the most reliable and best quality equipment in this field. One of my university lecturers used to say "buy cheap many times or premium once". That's an ethos that has allways stuck with me. A good job to as I spent a great deal of my career in the Navy as a submariner. A place where the cost is virtually irrelevant, its the ruggedness and reliability that matters.

Thanks for listening to my extended rambling everyone :D