[SI-LIST] Re: Necessity of Ground signal for RS485 transmission at 500 ft cable length
- From: "Curt McNamara" <CurtM@xxxxxxxxxxx>
- To: <weirsi@xxxxxxxxxx>, <adeelmk@xxxxxxxxxxx>
- Date: Thu, 24 Feb 2005 16:37:22 -0600
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
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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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