Ground wire for RS485 or not?

[be80be]

You not have a ground loop if it runs one way.

[lemming]

I'm not sure what you mean by that. Are you saying only connect shield to ground at one end?

[lemming]

As an aside, is Cat5 Ok for long runs of RS485?  Too many cores I know but its cheap and comes in UV resistant forms.

[be80be]

Cat5 work just fine as long as you use one set of pairs and ground one set of pairs a ground loop is just that a loop if you don't run back over your ground its not a loop

[jremington]

However then I run the risk of a ground loop which I'm not sure how to remove.

This is indeed a serious risk. Even in a single household there can be large ground loop currents, or at least significant potential differences between grounds, such as (in the U.S.) between different branches of the 120-0-120 incoming line.

Not too long ago I was running 240 VAC out to a well pump and got a shock off the protective ground lead. I measured 77 VAC from the protective ground to the well head. The power company investigated and discovered that a power pole close to the well, and carrying a transformer, had a faulty earth connection.

[Paul__B]

So really, RS-485 is not the right protocol for rural data transmission and maybe not even over significant distances within a building.

Consider telephone-style modems (these are always isolated) for low data rates, Ethernet for higher data rates.  Or do it properly; use optical fibre.

[be80be]

I've been wiring houses for 34 years and the only time I've seen ground problems is when the ground is cut hooked wrong in a outlet happens when you get people working on stuff they don't nothing about. Heck that's just 3 wires LOL think on that awhile. Or something with a bad motor. Now any of these need fixed if you like
your house and love yourself.
Next up any half way good cable installer knows that you don't run data cable with the power wires.

Then there this ground is at ground if one wire and grounded when you loop it it becomes a coil so like a coil
each tap can have a higher then ground level. That's why you use star grounds. And in this case one long ground with no loops 

[Watcher]

However then I run the risk of a ground loop which I'm not sure how to remove.

Ground loop current is limited by inserting an 100Ω  resistor in inline with the RS485 ground.

I highly recommend to also read this.

I  have done extensive testing with RS485 links both with and without ground wire connections. To summarize I 'd say that:

- Unconnected ground topology works as long as the potential difference between grounds is kept to a minimum (less than a few volts)

- Ground connection topology works better but then again if there is significant potential difference between grounds you risk a ground loop current. In this case opto-isolation is required.

I find that Maxims fail safe RS485 chips work best for higher distances eg MAX3085. There is also an opto isolated version.

Carefully choose the best chip for your particular application also taking into account required fanout etc.

[nickgammon]

If I was worried about this I would use opto-isolators (eg. like MIDI) where you specifically do not connect the grounds. However MIDI uses a shield which is connected at one end only and thus does not take part in the question about being a reference voltage.

[Paul__B]

Next up any half way good cable installer knows that you don't run data cable with the power wires.

Which is of coruse correct for safety and interference reasons.

However this bodes against using the power ground as the reference for both ends of the data link as by separating the two, you have clearly created a loop.

Then there this ground is at ground if one wire and grounded when you loop it it becomes a coil so like a coil
each tap can have a higher then ground level. That's why you use star grounds. And in this case one long ground with no loops  

But it is not the closure of the loop that creates a ground loop - it is the structure of the loop.  A loop is caused by a "return" being separate from the active (or in this case, actives).

Ground loop current is limited by inserting an 100Ω  resistor in inline with the RS485 ground.

Which indeed limits the current, but not the voltage, so it is of no use in removing a ground voltage mismatch.  It is in fact useless; just a half-hearted version of simply disconnecting the ground entirely or more accurately, providing some sort of second-rate ground if the other part of the loop is broken.

I highly recommend to also read this.

- Unconnected ground topology works as long as the potential difference between grounds is kept to a minimum (less than a few volts)

Less than one volt according to the specifications I previosuly cited.

- Ground connection topology works better but then again if there is significant potential difference between grounds you risk a ground loop current. In this case opto-isolation is required.

The current is not the point other than in terms of safety.  It is the voltage that matters.  Because the data ground will be the part of the cabling "loop" with the highest resistance, the voltage will still apaear between its ends, only marginally less thn if they were not conencted in the first place.

However MIDI uses a shield which is connected at one end only and thus does not take part in the question about being a reference voltage.

No, it does not take part in the question about being a reference voltage because the data path is opto-isolated and requres no reference voltage.  The shield ground is irrelevant.

[TomGeorge]

Hi,

I have been looking at wiring up a multidrop RS485 link over a distance up to a kilometr

Check distances and baud rates;

http://rs485.com/rs485spec.html

http://cds.linear.com/docs/en/product-selector-card/2PB_RS485fd.pdf

1km is under 4000ft (3280km) so should be ok.

Tom....

[MarkT]

If you arrange to use a balanced coding over RS485 (no DC component) you can stick it over
ethernet jacks with magnetics (transformers), giving isolation - however people normally stick
standard (DC biased) serial over it, so if you need isolation you need to add optoisolation yourself.

For runs of 1km or so you need isolation otherwise the next thunderstorm could blow up all your kit!
Fibre or wireless link makes a lot more sense for that sort of distance - at least add opto isolation and
protection circuitry at each end - you want to clamp the differential signal to a few volts and common-mode
to a few hundred (VDRs and TVSs?) to handle lightning-induced and other large pulses I think.

[dwightthinker]

It needs some common connection or there is nothing to
limit the common mode voltage. You are not isolated by a transformer!
Using Earth as that common mode level is fine as long as you
are not using it in an lightning prone area.
If so, one end or the other should be isolated with opto couplers
and an isolated supply( or even both ). The isolated side should be connected
to the shield as its ground.
I've also used such when one end had 5Hp motors that were turning on
and off with no loss of data.
You have to consider the noise environment.
Dwight

[ron_sutherland]

I have been using RS485 over CAT5 on my project(s), it is not a conventional use, e.g. it has full duplex RX and TX pairs, and additionally sends the DTR signal that resets an AVR device over a pair. I am working on setting up the DTR pair as a half-duplex bus management system (but I'm not there yet). Finally, one pair is used as a ground line. At present I can upload a sketch to a single powered up AVR board in my back yard with the Arduino IDE, I am hoping to daisy chain a number of boards while retaining the ability to upload firmware with Optiboot and the Arduino IDE.

I used a common ground, because it is my understanding that the differential pair signal must remain within the common mode range of all differential receivers (-7V thru +12V if memory serves), and I am not sure how to ensure that without a common ground. In addition to the CAT5 pair, a 14AWG ground wire was run to the ground system on my house. It is located near the meter and electrical panel. This heavy wire will survive a lightning strike (well that's the theory anyway), and may help limit the damage to the AVR board(s) and computer connected (though I'm expecting everything would get fried). There could be a ground loop in between the 14AWG wire and the CAT5, but I don't connect the 0V (CAT5) reference (e.g. battery negative) to the chassis only the 14AWG (the battery is also charged with an offline supply).

The board in the following link is still a work in progress but may help explain what I have been describing.

http://epccs.org/indexes/Board/RPUadpt/

[dwightthinker]

The system I did had a few repeaters and over 50 uP boards connected.
It was a single pair and everything was done with a simple hand shaking protocol.
It could detect within a second when any system was powered down
and did a periodic check of systems to determine power and run status .
I don't recall who's chips I used.
There was only one master and all waited until invited to send.
Like I says, there were a number of high power blowers.
It replaced a RS232 system that was so unreliable that it had to be
shut down when blower were turned on an off as well has a rats nest of wires
and drivers.
I still isolated at each machine ( with blower on each ).
Dwight