Re: Experience with Star Quad cable for CAN bus



Paul Keinanen wrote:

(floating receivers and transmitters without 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

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

termination resistor present, this is a nice bipolar current loop

A (multidrop) current loop is something completely different.

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).

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
20V common mode voltage on your receiver - likely too much.

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.

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.

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

Signal cable shields are really nasty in big industrial plants, since

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
--
Oliver Betz, Munich
despammed.com might be broken, use Reply-To:
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