Experience with Star Quad cable for CAN bus

is

Star Quad cable has higher capacitance than the standard twisted-pair cable, so one might expect reduced signal rise times and maximum cable length. I don't know if CAN has any specs regarding cable capacitance.

-a

...compared to...

Source? Values?

The rise time does _not_ depend on capacitance (per length) if the cable is correctly terminated.

But the propagation speed is affected:

v = Z / L' = 1 / (C' * Z)

IOW: With constant Z, the higher the capacitance or inductance per unit length, the slower the signal propagation.

Who cares?

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.

Oliver

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.

My understanding is that a 2x pair has a better balance than between two randomly selected wires in a 4x cable, thus having better differential mode noise rejection.

Paul

The cable I originally selected had 40nF/km, the offered star quad

What you can compare is the 4 conductor star quad with it's 2 conductor companion in the same datasheet (which is ofcourse a normal twisted pair), they are both 58nF/km.

That is something to consider as maximum bitrate in CAN depends upon propagation time. But as we are not using maximum cable length and not maximum speed, I suspect it's not that critical here.

Better immunity is always welcome in our environment.

But things get a little weirder now. The cable assembly manufacturer sent us a cable datasheet which says the cable is 4x1x0.22mm^2 in "sternvierer" (german for star quad). Today I searched that cable manufacturer's website and found only one cable meant for CAN. This cable has the same ordering code as the one I was offered earlier, but the datasheet seems to have been updated:

And now they call it 2x2x0.22mm^2, but still "sternvierer"??? Also the impedance has changed from 120 +/- 10% ohm to 100 - 120 ohm.

If I got this datasheet to begin with, I don't think I would have wondered about the usability of that cable. :-)

The CAN module manufacturer advices cables with GND connection.

When using a star quad, it is not advisable to randomly select wires. You still need to pick 2 oposite wires for the "pairs". At least that's what I found in my web searching last night. ;-)

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

Although I would be able to design such a circuit, I would not consider it if I had a chance to get a GND wire.

The available transceiver ICs I know are not designed to work without a ground wire (or a special balancing circuit).

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.

As Stef wrote, you use opposite conductors in a star quad. The quad is also twisted like a pair.

Oliver

Stef scrobe on the papyrus:

. .

The photograph on that data sheet does not show a star-quad cable as I would understand it. It looks like a twin pair single screened cable, although there may be internal foil screens which have been removed. A true star-quad cable has all four conductors twisted in a common spiral and opposing conductors are commoned to provide a high noise immunity twisted pair.

HTH

John

My practical experience with 4-wire RS-422 (2x2), the installers seems to be quite capable of making split pair errors :-(.

I am not so sure, if they could pick the correct wires from a 4x cable.

Paul

Yes, that was my impression as well and they also call it 2x2x0.22, not

But for more weirdness, it seems the datasheet on the dutch helukabel site is out of date and is actually the older one. The german site only has the newer version (in english this time):

Well I've had enough of cable datasheets for now. Seems the star quad (or "sternvierer" or "stergroep" or "quarte en étoile" or "cuadrete en estrella" :-) ), is at least a decent alternative for the dual twisted pair. Thanks all for responding.

But that also goes for dual twisted pair cables, easy to pick the wrong wires and it really happens. Fortunately they are color coded, so just specify which color to use for what. I doubt an installer would even spot the difference between the 1x4 and 2x2 cable, both have 4 wires, they can have the same color coding and the wires are a little curly in both.

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).

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

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.

Paul

You're not hitting either of the absolute limits, true.

But the actual, relevant limit is ~50 Mbit*m/s for the product of bus length and baud rate, and you're within roughly a factor of two from the ceiling on that: at 500 kbit/s the theoretical maximum CAN bus length is ~100 meters.

So you're not exactly in the critical regime yet, but neither are you comfortably far away from it. Any single design choice would have to be pretty bad at this stage to break the design all on its own --- but several mediocre choices could break it, too. So you'll have to take care.

That's fine so long as the common mode voltage between the ends is within the range of the receiver. Usually, each end is grounded, so you effectively get the third gnd wire anyway, but if the ends are separated by a large distance, or on different line phases with gnd significantly above zero, you can worst case burn out the line receiver, or at best get lost data.

That's why most comms equipment has both a frame gnd, going to power earth, and logic gnd, with the two separated.

In the classic case of 10BT ethernet, you always have a transformer to provide the gnd isolation...

Regards,

Chris

Thanks for that warning. This actually confirms my "need" for a good cable. I know there are at least a few mediocre (high speed signal wise) connectors in the bus that can not be easily avoided. Using a good cable at least reduces the number of probable problem spots.

The idea for now is to do some measurements on the completed system to check how the CAN signals behave. We'll probably use a CAN monitor, but that will only give us a hit/miss condition for the bits, not how close or far they are from the limit. So in addition we want to see the signals on a scope to get an indication of the signal integrity. Any ideas on suitable methods?

It's RS485, not CAN bus, but here is one example of a transceiver designed to work without a ground connection that allows kV of common mode signal:

I'm glad that parts like this exist now; I had to roll my own back in the day.

Regards, Allan

Having only 1x2 plus an optional shield will reduce the wiring errors considerably.

Paul

By "floating" I refer to galvanic isolation.

Typically 500-2500 V in case of galvanic isolation.

I use the following rule of thumb when specifying communication systems for large industrial plants:

• RS-232 is OK within the same equipment rack
• non-isolated RS-422/485 is OK within the same equipment room
• isolated RS-422/485 is OK within the same building
• fiber optic connections are required between buildings with separate ground electrodes due to the ground bounce created by a lightning stroke into the lightning rod of one building.

The only thing I like about ethernet to serial RS-232/422/485 converters is that you get the galvanic isolation for "free".

Paul

(floating receivers and transmitters without GND connection)

The product(s) passed all the EMC immunity tests, especially ESD, EFT and BCI?

A (multidrop) current loop is something completely different.

Do the math:

If the internal resistors - both in parallel - are 40kOhms and the capacitance of the isolation barrier is 50pF, 1kV/us will result in

And you will get much more than 1kV/us in ESD and EFT tests and in real world operation - guess why good couplers specify a common-mode transient immunity of >10 kV/us.

Or consider the BCI test (EN61000-4-6). At 1MHz, 50pF is i*3kOhms, so you will see the full common mode voltage even at this low frequency.

we are talking about _isolated_ transceivers. 50Hz and 50pF give tens of Megahoms impedance.

Therefore you use isolated transceivers.

But with most isolated transceivers, you need a GND wire.

As I wrote - it is _possible_ to make a true two wire isolated transceiver, but I'm not aware of real world products.

Oliver

(floating receivers and transmitters without GND connection)

I can't read anywhere in the data sheet that it is designed to work without GND reference. Where is this stated?

Anyhow, the data sheet is rather poor. They neither state a coupling capacitance nor a maximum spec for common mode dV/dT. The receiver common mode range (referred to GND2) is also missing.

And see figure 12 - they _do_ connect GND2 to the cable shield. Well, they have a huge 10nF capacitor between the grounds in this example so there definitely need a GND connection there.

I'm curious whether anybody successfully made a two wire (without GND) floating RS485 interface with this part and surviced EFT and ESD tests or even current injection.

Oliver