Re: RS485 is bidirectional does it mean it is fullduplex?

paul$@pcserviceselectronics.co.uk (Paul Carpenter) wrote:
>On Friday, in article <873brga5fa.fld@xxxxxxxxxx>
> floyd@xxxxxxxxxx "Floyd L. Davidson" wrote:

>>>Now consider the case where the left hand ground moves up to 80V away from
>>>the right hand ground, but that the impedance between the two ground
>>>connections remains less than 10 Ohms (not unreasonable in some very large
>>>buildings).
>>
>>Actually, I believe the specification *is* 10 Ohms, and the
>>target is 5 or lower. An 80 volt difference in ground potential
>>is... so unusual that we can ignore it. Lets assume it never
>
>In *your* experience, in two situations seen all over the UK, Railway Stations
>with multiple platforms can often see 10-20V from the main earthing point as
>they progress outwards across platforms. One near me saw a 60V in 100ft of
>cabling that went 60ft by line of sight. Certain Eurostar depot called North
>pole one contractor in its building phase was seeing all sorts of problems
>with ground potentials in excess of 70V.

We've already noted that yes you *can* find such situations, caused
by specific circumstances. They require special engineering to do
some things.

That doesn't make them common, or normal, nor should they really be
part of this discussion in the way you want them to be.

One thing they might be, is a very good reason not to use
RS-485, which can only handle a very limited common-mode voltage
range.

>>>Where you do need to connect the screen wire to both ends then you had
>>>better include some impedance in the connection that limits dangerous
>>>current flow and yet provides sufficient coupling to ground for the higher
>>>frequency signals. As I have stated before, all the circumstances have to
>>>be well understood to make the right choice.
>>
>>No. The trick is to ground the cable every 3 or 6 thousand feet, so
>>there is never get anything like an 80 volt difference.
>
>The outer shield that is for protective earthing to stop the metal
>outer casing becoming live by any means, including internal cable fault.
>It has nothing to do with the signal levels, and if it is used for that
>then you are asking for problems.

That is simply untrue.

>>Can't you come up with something less boorish than repeating the same
>>thing over and over?
>
>Learn the difference between signal reference and protective earthing.

Sure sonny. Your statement about cable shields above indicates you
have a shallow understanding of it.

>Keeping things floating is done for many reasons, knowing that in Medical
>Equipment in Europe you have to watch simple things like is the patient in
>an operating considered floating or earthed? Well it depends which country
>you are in, which effects signalling and protective earths how they are done
>and what is isolated from what.

You should be aware that medical equipment is an entirely different
subject than comm cables.

--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@xxxxxxxxxx
.

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