[SI-LIST] Re: Necessity of Ground signal for RS485 transmission at 500 ft cable length
- From: "Curt McNamara" <CurtM@xxxxxxxxxxx>
- To: "steve weir" <weirsi@xxxxxxxxxx>, <adeelmk@xxxxxxxxxxx>
- Date: Thu, 24 Feb 2005 19:54:09 -0600
Steve, I hope you are not saying you would connect remote grounds
without a fusible link! This would be a serious safety error as currents
through the cable could cause insulation failure and a fire hazard.
It would be good practice to use a fuse or other UL/CSA approved
safety device if it is possibile for remote grounds to be connected.
The resistor example is from the annals of history and was not a
recommendation on my part.
I see you also recommend the use of isolated transceivers
as best practice for communications links with remote grounds.
It should be noted that remote grounds might be the two sides of a
room! In many cases the AC on two separate circuits follow very
different paths back to the mains.
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: steve weir [mailto:weirsi@xxxxxxxxxx]
Sent: Thu 2/24/2005 4:54 PM
To: Curt McNamara; adeelmk@xxxxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: RE: [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
>intended for a specific individual and purpose, and is protected by law.
>If you are not the intended recipient, you should delete this message and
>are hereby notified that any disclosure, copying, or distribution of this
>message, or the taking of any action based on it, is strictly prohibited
>
>
>________________________________
>
>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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