# Connecting USB-powered Beaglebone to car's CAN bus through OBD connector using common signal ground

I want to connect my Beaglebone Black, powered from USB port on my laptop, to OBD connector present in my Ford in order to see CAN traffic with can-utils.

Beaglebone operates on 3.3V, that's why I am using a logic level converter to interface to MCP2551 CAN transceiver operating on 5V.

I am using pins P9_26 and P9_24 from Beaglebone's header P9.

My DB9 to OBDII cable has the following pinout:

Questions:

1. Is the connection presented on the schematic correct?

2. Can I connect the Signal Ground from my Ford's OBDII connector with GND from Beaglebone (which I think equals GND from USB) safely?

3. Is connecting grounds of these two separate circuits (car <-> Beaglebone/laptop) necessary?

4. If the grounds are not connected - can it pose any danger to the car's transceiver/CAN computer or USB port in laptop?

• I have done the same. Used the MCP2551, but without any level conversion. I just put a 1k resistor before the BBB pin and it works like a charm. – ben Feb 24 '17 at 19:23

You would think because of the differential nature of CAN and the fact that you are not drawing power from the OBD port, it would all work fine without the common ground reference.

But, you are then at the mercy of your 2 floating grounds. The CAN transcivier will have an upper limit of the common mode voltage it can tolerate. If at some point the disparity between your floating grounds is large enough to exceed that, you can damage your transcievers. This is not something you can control: so yes use a common ground.

Edit 1:

If however you MUST use floating grounds then consider other (more expensive) transeviers with isolated inputs.

• Thank you. I have two additional questions: 1. Can the transceiver at the car's side be damaged as well? 2. Can the connection between USB ground and Signal ground damage the USB? – Peter Cerba Jan 22 '17 at 19:30
• Yes it can, because it will see the same, potentially harmful common mode voltage. – Adil Malik Jan 22 '17 at 19:33
• Is there any specific order in which car/Beaglebone should be powered after setting up a connection? – Peter Cerba Jan 22 '17 at 21:26
• No, but apply power to either only once you connect the grounds together – Adil Malik Jan 23 '17 at 8:54
1. Is the connection presented on the schematic correct?

Looks ok. Though to save cost, it might be a better idea to use a CAN transceiver with 3.3V supply. Example.

1. Can I connect the Signal Ground from my Ford's OBDII connector with GND from Beaglebone (which I think equals GND from USB) safely?

Yes, though be aware that when you initially do so, you might get an ESD discharge (like when you touch a car with your fingers). Once both grounds have the same potential, it is harmless.

1. Is connecting grounds of these two separate circuits (car <-> Beaglebone/laptop) necessary?

Yes. A car on rubber tires will have a completely different potential than your computer supply.

1. If the grounds are not connected - can it pose any danger to the car's transceiver/CAN computer or USB port in laptop?

The circuits are fairly rugged and since there are no high currents, this is unlikely. However, it may be impossible to do any form of data communication without a signal ground, as the voltage reference for the signal levels could be anything.

• Ad. 2 Is there any way to make sure that ESD discharge won't occur? How can I get rid of the accumulated charges on my circuit before I connect the DB9-OBDII cable? – Peter Cerba Jan 25 '17 at 21:14
• @PeterKowalski Not really, but as long as there are no sensitive circuits very close to the connector, it will be harmless. – Lundin Jan 26 '17 at 9:18
• Thank you. One more hing came to my mind: What if I need to connect Beaglebone to 5V DC power supply powered from mains. Does it introduce any problem with grounding? Can it cause any danger to the setup depicted in my post? – Peter Cerba Jan 28 '17 at 17:23
• @PeterKowalski In any form of data communication over wires, all parties involved must share the same ground, simple as that. This is why there is always a signal ground. – Lundin Jan 30 '17 at 7:49