Re: Experience with Star Quad cable for CAN bus

On Wed, 07 Oct 2009 14:20:05 +0200, Oliver Betz <obetz@xxxxxxxxxxxxx>

Paul Keinanen wrote:

But IMO the star quad should give higher EMC immunity because there is
less common mode noise between data and power (GND) wires. With 2x2,
GND and data has a larger loop area and therefore is more susceptible.

If you use the ground wire at all. A correctly terminated line with
floating receivers and transmitters should work well even without the
Gnd connection.

Do you have any real world example of a "floating receiver" designed
to operate without GND connection?

The CANbus transceiver is quite similar to RS-422/485/Profibus-DP
transceivers and I have never have had any problems using these with
properly terminated lines without ground connection. With the
termination resistor present, this is a nice bipolar current loop
circuit and the floating receiver input transistors are biased to a
reasonable potential with the internal pull-up resistors (in the order
of tens of kohms).

Unless your isolation barrier between the transceiver and the "rest of
the world" has extremly low capacitance, common mode noise will also
appear at the transceiver terminals and cause problems.

The stray capacitance reactance and the receiver bias system
resistance is an issue.

One alternative would be to use a single shielded 2x twisted pair with
a proper termination for data transfer. The twisted pair will handle
the data communication without problems.

The cable shield could be connected at both ends with a 100-200 ohm
1-5 W resistor to local signal ground (possibly bypassed with small
capacitors). Those resistors will reduce the "ground loop" 50/150 Hz
current to a reasonable level and hence reduce the magnetically
induced noise voltage to a reasonable level.

Signal cable shields are really nasty in big industrial plants, since
quite a lot of the mains current neutral current will flow though
these cables, unless special precautions are used.