USB-8473 BusMonitor Error Frame

[Doc_Holiday1865]

It is good practice to connect both the shield and the ground connection.  But you have to be aware that connecting the ground can also cause issues due to ground loops.  Shielded CAN in vehicle is not common place because of cost and it shouldn't be necessary.  One also has to be aware that connecting the shield to ground can cause a ground loop as well, which is why in the CAN spec for the physical layer, there is only one solid ground connection for the shield.  The rest of the nodes have a ground connection through a 1 ohm resistor coupled in series with a 0.68 uF capacitor, thus blocking any DC loop currents. 

The point is, you need to be thoughtful when making connections.  I have seen CAN trasceivers fail because of ground loops many times.  This is why there is so many offerings for optical isolators for CAN.

[don_m]

Thanks for the information.

It is greatly appreciated.

Don

[JDW1]

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Doc_Holiday1865 wrote:

It is good practice to connect both the shield and the ground connection.  But you have to be aware that connecting the ground can also cause issues due to ground loops.  Shielded CAN in vehicle is not common place because of cost and it shouldn't be necessary.

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Doc, your first sentence appears to contradict the latter two.  For if there is potential for ground loops and if shielded CAN in vehicles "shouldn't be necessary," then can one say it is really "good practice"?

In my case, I honestly did not know what to do based on the limited info the NI documentation gave me.  I simply thought about it logically.  Logically, the 9-pin D-SUB cable I used has a shield (i.e, foil around the cable that covers the other wires), and since the documentation mentions that the 8473 has a "Shield" connection, it made sense to connect that pin to my cable's shield in order to "improve signal integrity" from the 8473 module to the OBD2 connector (which in my case is 99cm).  I did not subsequently connect the Shield to Ground (which is the Ground coming from the OBD2 connector).

So unless someone can show otherwise, it seems to be easiest and less troublesome (no potential for Ground Loops) to connect the Shield only, and NOT connect the Shield also to Ground.