USB-RS485 Isolation

I decided on RS485 to run data around the various modules inside my model helicopter and rather than having one RS-232 debug port for everything, I was going to have a PC be able to plug into the RS-485 network to configure and debug all the devices on the bus.

So I need a USB-RS485 converter. None of the devices inside the helicopter have isolated RS485 transceivers (too many extra isolated power supplies and wires running around just to power the bus-side of the transceivers. However, the power-side of the drive systems (main rotor motor, drive MOSFETs, and 60V drive battery) are galvanically isolated from everything else. Everything else includes the logic side of the motor driver, servos, radio, autopilot, etc. and run off of their own 7V battery.

Do I need a isolated USB-RS485 converter to protect my PC when connected to the system? I know plugging an unisolated RS-232 into a motor driver to your PC is a bad idea because a surge could make it's way into your PC and blow it. What about my described scenario? The biggest threat is already isolated, what about the "everything else"?

you should be fine. even without the galvanic on the power side it should be ok since it is still on a separate supply with no ground return.

the things that blow the PC are power surges (load dumps if you are used to auto terms), which can be on a shared ground but dumped to a separate supply, and more commonly ground loops that do not exist since you are talking to a model (the only ground connection is through the serial port).

in power systems it is common for the ground loop to blow a computer since there might be 50VAC between the neutrals.

Ubergeek63 said:

you should be fine. even without the galvanic on the power side it should be ok since it is still on a separate supply with no ground return.

the things that blow the PC are power surges (load dumps if you are used to auto terms), which can be on a shared ground but dumped to a separate supply, and more commonly ground loops that do not exist since you are talking to a model (the only ground connection is through the serial port).

in power systems it is common for the ground loop to blow a computer since there might be 50VAC between the neutrals.

Click to expand...

How does that not apply in my situation? The ground of the 7V is shared with the USB ground if an unisolated converter is used (A, B, and GND connections). "No such thing as 2-wire RS485".

You only have a common ground reference. The rest of your system, if understand you correctly, is "floating". ie the ground reference and the signals are the only connection.

The thing that causes trouble is when you have ground currents, an impossibility if it is running off of a battery.

I am not sure I follow your question. So are you wondering whether it better to have a device to convert USB to RS485 as a separate board, or just incorporate on board? From just a weight standpoint, I think you should leave off as much excess baggage as you can. A USB connector weighs a lot more than a simple RS485 conn. From that perspective I would say use separate board. From electrical side, I would be lying if I said I knew for sure, but I think the RS485 driver would take the brunt of any surge.