[SI-LIST] Re: Necessity of Ground signal for RS485 transmission at 500 ft cable length

Curt, I have never been a real big fan of the resistor as it can become its 
own fire hazard under a number of conditions.  But then, I just basically 
loathe box to box interconnects that are not isolated.  If DC offsets don't 
bite, EFTs often do.

Regards,

Steve.
At 04:37 PM 2/24/2005 -0600, Curt McNamara wrote:
>In two widely separated systems earth ground (or neutral) often
>varies by many volts, effectively becoming a common mode offset.
>
>You can connect a ground between the systems, however this
>wire can definitely conduct large amounts of current! In RS-232
>days this connection was often made with a small series resistor
>which became a "fusible link" if the grounds were too far apart.
>
>For distant RS-485 transmission try an isolated transceiver ...
>
>                                        Curt
>
>
>Curt McNamara. P.E.
>Senior Electrical Engineer
>Logic Product Development
>411 Washington Ave. N Suite 101
>Minneapolis, MN 55401
>Tel 612-436-5178
>Fax 612-672-0443
>curtm@xxxxxxxxxxx
>www.logicpd.com
>This message (including any attachments) contains confidential information 
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>________________________________
>
>From: si-list-bounce@xxxxxxxxxxxxx on behalf of Steve Weir
>Sent: Thu 2/24/2005 8:09 AM
>To: adeelmk@xxxxxxxxxxx
>Cc: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Necessity of Ground signal for RS485 transmission 
>at 500 ft cable length
>
>
>
>Adeel there is no such thing as the "receiver not supporting the
>common".  The receiver most definitely has its common mode referenced to
>the negative supply rail.  This is clear in any '485 receiver
>specification.   You can answer your question by evaluating the loss and
>the allowable ground noise between the two points.  Your limit will be set
>by when the signal either falls below the nominal 50mV receiver threshold,
>or the CM noise take you out of the CM range.
>
>Steve.
>At 07:02 PM 2/24/2005 +0500, Adeel Malik wrote:
> >Steve,
> >         Using the ground signal wire keeps the common voltage range
> >within -7V to +12 V, that of course improves noise immunity as well as
> >reliability. But the point is that if the slave device doesn't support
> >the ground signal connection, then up to how much distance the Master
> >can transmit the data reliably at 57600 baud rate over RS485 Network
> >without that ground wire.
> >
> >Adeel
> >
> >-----Original Message-----
> >From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> >On Behalf Of steve weir
> >Sent: Thursday, February 24, 2005 6:40 PM
> >To: adeelmk@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> >Subject: [SI-LIST] Re: Necessity of Ground signal for RS485 transmission
> >at 500 ft cable length
> >
> >Adeel, RS485 is not galvanically isolated.  Think receiver common mode
> >range.
> >
> >Steve.
> >At 06:24 PM 2/24/2005 +0500, Adeel Malik wrote:
> > >Hi All,
> > >             Can someone comment on the need of separate Ground Signal
> > >Wire in addition to the standard 2-wire RS485 Connections between the
> > >Master and Slave device. I have come to know that separate Ground wire
> > >b/w the two devices communication via RS485 at higher baud rates and
> > >increased cable length improves signal transmission.
> > >Regards,
> > >Adeel Malik
> > >
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