rs485 and RTN

On Sun, 19 Feb 2012 15:06:53 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>>Actually, I had that in mind when I was responding to the post: the AC 
>>potential (and it was low impedance, too -- it wasn't just a "connect 'em 
>>together and be happy" thing) between grounds bolsters Jamie's position, 
>>and the system under test used a grounding system that included all the 
>>cases being electrically tied together, to bolster John's.  Hence -- the 
>>best answer depends on the circumstances.
>
>Sometimes you need a serious ground strap (think welding cable)
>between chassis to force the grounds closer together. The resistance
>of the strap has to be much less than the total resistance of the
>ground returns in the power system. 

By using such straps, you are messing around with the electric
distribution network. I hope you understand how the electric network
is constructed in a particular place and how that strap might effect
the protection system and what happens, if there is a ground fault on
one side.

>When things get extreme, big difference at low impedance, you need
>isolation in the signal circuits, or fiber.

If you are making some general purpose systems to me installed to
locations not yet known possibly by local people, you can avoid a lot
of trouble by using galvanic isolation in all cases, even if it is not
needed in some locations.

On 19 Feb., 11:39, ice <iceeye...@gmail.com> wrote:
> hi all
>
> system: 9600bps rs485 half-duplex with single master and multiple
> slaves; both master and slave connector have 3 pins (A,B,RTN)
> cable: about 500mt shielded cable with 2 twisted pairs inside, one
> pair used for A/B signals other pair used for RTN
>
> the system works well
>
> questions:
> 1. the shield of the cable is connected to earth ground (yellow-green
> wire) at master side only: is this right?
> 2. the function of RTN wire is keeping the master and slaves own logic-
> ground at the same potential and avoid dangerous voltage floating?
>
> thanks

Standard Modbus system has pullup to supply and pulldown to RTN to be
able to determine fault state (cable broken). So the RTN wont
nessesarily be floating around due to the construction of the supply
for the interfaces and the leakage currents.

Regards

Klaus