Re: Experience with Star Quad cable for CAN bus

On Wed, 07 Oct 2009 22:00:52 +0100, ChrisQ <meru@xxxxxxxxxxx> wrote:

Paul Keinanen wrote:

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.

By "floating" I refer to galvanic isolation.


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).

That's fine so long as the common mode voltage between the ends is
within the range of the receiver.

Typically 500-2500 V in case of galvanic isolation.

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.

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.

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...

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

Paul

.

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