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Why does ethernet not have common mode/noise problems?
3

Why does ethernet not have common mode/noise problems?

Why does ethernet not have common mode/noise problems?

(OP)
Why does ethernet appear to be such an easy plug & play imlementation that avoids the hassles of RS-485 with grounding, biasing, termination, common mode & noise?  

I've torn my hair out over the years with local RS-485 networks that aren't any longer than 100m in length, that have had all sorts of 'electrical' problems that just don't seem to appear with ethernet in the same environment.  The 485 work was done using good practice: twisted pair signal cable, not running cable in high voltage trays, limited T spur length, etc.  

What is it about ethernet that makes it work if the cable connector is wired OK?  

Dan

RE: Why does ethernet not have common mode/noise problems?

Ethernet is transformer-coupled, which is really good at rejecting common-mode noise. Also, since you use a hub/switch to connect nodes together, each segment is essentially a point-to-point connection and thus, you don't need to worry about the termination that you would on a multi-drop bus.  And on top of it all, you're usually running TCP/IP or some other protocol that has error-correction and retries built in.  It's all very robust, but each node is much more expensive and complex than an RS-485 connection.

I don't know if you remember when Ethernet was commonly connected with coax (10BASE-2).  That was probably 10 or 15 years ago.  At that time, you had to daisy-chain the coax between nodes and you had to put termination at the endpoints.  It was a huge pain to add more nodes. The current Ethernet implementations are so much easier to work with.

Glenn  

RE: Why does ethernet not have common mode/noise problems?

Yes 10Base2 was a huge pain in the butt.  I was in charge of the infrastructure at moderate sized engineering firm.  Something would go wrong and the whole network would go down.  Isolating bits and pieces was torturous.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Why does ethernet not have common mode/noise problems?

With RS-485 try using galvanically isolated nodes and run the twisted A and B pair plus a ground for each node. A lot of RS-485 devices use local ground and when they do this the A and B pair voltage shift is limited to about 7 or 8 volts. In an industrial environment it's fairly easy to get local ground shifts more than that.

FYI, in case you were not aware, RS-485 is not a differential pair.

 

RE: Why does ethernet not have common mode/noise problems?

All the specifications I have seen for RS-485 indicate that it is a differential transmission. The EIA specification defines ones and zeros in terms of differential voltages.
Charlie

RE: Why does ethernet not have common mode/noise problems?

Da! I was under that impression too Charlie.  


BTW: Welcome to Eng-Tips Charlie.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Why does ethernet not have common mode/noise problems?

I'm pretty sure RS-485 is differential or at least that was always my understanding.  RS-232 is single-ended referenced to local ground.  

 

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

RE: Why does ethernet not have common mode/noise problems?

Look up the electrical characteristic called "longitudinal balance".

To me, it is the single most important electrical characteristic that gives ethernet (and telephone lines) their noise immunity.. While transformers were mentioned as a common technique, the complete front end design along with the transformer itself, at both the driver and receiver ends must be designed with the degree of longitudinal balance needed in mind. The relatively short paper below discusses briefly.

http://www.waynekerrtest.com/appnotes/telecom%20transformer%20measurement%20requirements.pdf

Bell Labs and the telephony industry was well aware of how to make this stuff work before the computer industry in general adopted them.

The earlier days of my development engineering work in the computer industry was during the period when data transmission (for the computer industry) began moving away from RS-232, RS 485 and other standards prone to noise and crosstalk,  to balanced line designs with characteristics you quote regarding ethernet.

The thing that is always impressed me was that the telephony world (and you have to include T-1) was that you can hang 15 - 18 miles of unshielded pairs on telephone poles, generally 10 - 12 feet below high voltage AC power lines yet, have good signal to noise, with appropriate balance.





 

RE: Why does ethernet not have common mode/noise problems?

I posted that RS-485 is not a differential pair. The A and B wires are both positive conductors and both feed to the circuit common. The circuit relies on the common of the different transceivers being connected together. This connection can be via earth ground or via a third conductor. To me, using 3 conductors is not a differential pair.

Often, the common connection is done via earth ground. However, the transceivers can only handle a certain voltage between A or B and the common which I think is specified as -7V to +12V. In an industrial environment it is easy to have a higher voltage difference in earth ground between 2 points some distance away, especially during fault conditions. This ground shift can disrupt communications or destroy equipment.

It's a differential method of transmission in that the A>B = 1 and B>A = 0.
 

RE: Why does ethernet not have common mode/noise problems?

Lionel, I see the point of your distinction in that RS485 can only handle a relatively limited common-mode voltage, but I haven't seen that distinction commonly made in the definition of "differential pairs".  Is there a standard that you have seen that does this?

Glenn

RE: Why does ethernet not have common mode/noise problems?

I make the distinction about it not being a differential pair because it uses a pair of signal wires and a common. 3 wires is not a pair.
 

RE: Why does ethernet not have common mode/noise problems?

Interesting point.
A differential signal. Not a 'differential' line.

I'll agree to that.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Why does ethernet not have common mode/noise problems?

I think that you'll find that RS-422 actually does need the ground connection, as well, to prevent common-mode destruction, for the general case.

TTFN

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RE: Why does ethernet not have common mode/noise problems?

That's a very good point. It's hard to argue with the fact that 3 is not a pair. So, that would put RS485, RS422, and USB into this category that is not a differential pair.  So real differential pairs would include Ethernet (well, at least the common CATx cable implementation that we've been discussing) and what else?  There would be much fewer types of signaling that would fall into this category.

Glenn

RE: Why does ethernet not have common mode/noise problems?

Cat 6(usually) and Cat 7 are shielded, so they technically contain additional connections beyong the 4 twisted-pair signal wires.  Cat 7 is supposed to have a shield for each twisted-pair, so, technically, each physical comm link has 3 connections.

I personally don't have a problem with calling all of these twisted-pair, since that's a signalling concept, and not the actual hardware implementation, per se.

TTFN

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RE: Why does ethernet not have common mode/noise problems?

POTS would be differential too.. Right?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Why does ethernet not have common mode/noise problems?

Could be IR, I'm just asking.  

Keith Cress
kcress - http://www.flaminsystems.com

RE: Why does ethernet not have common mode/noise problems?

[bold]A differential signal. Not a 'differential' line.
[/bold]

I like it. That's a good way to sum it up.

IR - aren't those shields just connected to earth ground though, not part of the circuit path?

 

RE: Why does ethernet not have common mode/noise problems?

I found this interesting paper by Jan Axelson. It sheds some light on the common mode voltage limitation.
http://www.embeddedsys.com/subpages/resources/images/documents/microsys_art_RS485.pdf
BTW if you connect the RS-232 port of a laptop between Ground and B it will read the RS-485 traffic. By connecting Rx & Tx it's also possible to write. I find this very usefull when setting up a new connection.

RE: Why does ethernet not have common mode/noise problems?

re: shields: Eventually, perhaps, but, locally, they're connected to chassis ground, which is eventually tied to the power supply return, in the ideal case.

However, since the shields can potentially travel a great distance from the actual bulkhead connection, the signal shields are often terminated at each board, and a communal shield that's tied to chassis is often wrapped around all the signal cables going between boxes.  One reason for this is that the chassis grounds can often be quite noisy, since lots of stuff can get coupled into it, and further coupling of that noise to a twisted pair could introduce common mode noise that exceeds the input range of the receiver, so a local shield would prevent that.

TTFN

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RE: Why does ethernet not have common mode/noise problems?

Hey roydm, I understand the reading of RS-485 traffic with the setup that you mentioned, but I don't think I'm getting what you're doing when you say you it's possible to write by connecting RX and TX.  

Glenn

RE: Why does ethernet not have common mode/noise problems?

Sure, RS-232 marking state is -3V per the standard, so any signal capable of swinging through +3V to -3V will cause an RS-232 receiver to toggle.

Likewise, a fully compliant RS-232 driver should be able to toggle an RS-422 receiver.

TTFN

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RE: Why does ethernet not have common mode/noise problems?

Thanks Mr. Hutz.

geek; Most 485 is half-duplex, so, by tying the TX to the RX, as mentioned by Roy, it would let you talk to the 485 device too.  Of course you'll get the full echo to deal with.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Why does ethernet not have common mode/noise problems?

As Keith says, you get the echo but its handy to send short commands. I usually start with the baud rate low e.g 1200 until I get the handshakes working correctly.
Using the laptop lets you se straight away if the communication is OK otherwise you get a bunch of garbage characters.
Must connect between Ground and the negative line (B I think)
Cheers
Roy

RE: Why does ethernet not have common mode/noise problems?

This RS-485 trick sounds interesting.  Let me see if I understand.

- You're connecting the TX and RX of the RS-232 port to the negative line of the RS-485 pair.
- When driven by an RS-485 transmitter, the negative line swings between about 0 and 5V. The RS-232 receiver will have to respond to what is essentially TTL-logic levels. That's fine, I haven't seen any RS-232 receivers that this trick won't work on since the early 80's.
-The RS-485 lines will have pull-ups to 5V or thereabouts. The RS-232 TX transmitter will have highs above 5V and lows below 5V, thus the RS-485 receiver will receive it like it was a normal RS-485 signal.

Let me know if I did not understand something. If I understood it, then here's what I see as the issues:
- The RS-232 transmitter will never float its output, so no RS-485 transceivers will be able to transmit on the lines.  If the communication is only one direction (like RS-422), and the PC is the transmitter, then there's probably no need to connect the RX line since you know what you're transmitting.
- When the RS-232 transmitter is putting out a low (mark) voltage, can't the voltage exceed the recommended limits of the RS-485 transceiver that it is connected to?  For example, a MAX232 RS-232 transceiver will have an output that swings to about -8V or -9V, whereas an SN75176A RS-485 transceiver data sheet says the voltage at any pin should be above -7V.  Is the voltage swing less for a typical laptop RS-232 port (if any laptops still have RS-232 ports), or are you relying on the currents being low enough not to damage the chip?

Sorry to hijack the thread!

Glenn

RE: Why does ethernet not have common mode/noise problems?

(OP)
I'm the OP, so I can say, "hijack away".  I'm happy, I learned that ethernet is transformer coupled and I lucked out having missed all that fun with 10base2 coax/terminators.

Who knows what we'll all learn about futzing with serial 232/485 with all this chatter?   Carry on . . .

Dan

RE: Why does ethernet not have common mode/noise problems?

Thanks Dan.

geek; Laptops often don't swing much over +/-5V some not even that.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Why does ethernet not have common mode/noise problems?

Good point Geek, you are probably correct,  the 232 output may disable the 485 outputs in some cases (depending on the output impedance). I normally just use the Rx to monitor the traffic between two or more 485 devices that I am trying to get talking.
I don't advocate using this method for a permanent connection but it is OK as a troubleshooting tool. Obviously a 232 - 485 converter would be a better solution but who has one of those handy?
As Keith pointed out most laptop outputs are quite low.
If the 232 Tx voltage is too high it would be easy enough to add a couple of  resistors to reduce it somewhat.

RE: Why does ethernet not have common mode/noise problems?

Dual-shielded Ethernet cables will diminish crosstalk and allow for higher speeds, but will they bring back the ground path problems that the xx Base-T so conveniently got us rid of?

I've seen many computer ports and adapters mentioning RS485 where in fact they were RS422.  RS485 being the multidrop (single-pair bidir mode) version of RS422.  Strictly a hardware standard, a RS-485 application will involve additional protocol for direction control, often the RTS signal is used, and for source/destination ID.  This protocol is not part of the specification.  In a dual-pair RS-422 configuration, no protocol is required and a direct RS232-to-RS422 hardware conversion works readily.  Of course if the RS422 originator works at RS232 speeds.

Early RS232 standards were defining the voltage swing range between plus or minus 3 to 18 volts.  A direct connection with RS422/485 has a potential for problems.

 

RE: Why does ethernet not have common mode/noise problems?

FelixC,
            As I said, its only for testing. I usually have my multimeter handy to identify which wire is Tx so I see the voltage then.
I have never seen any more than +/-15V, I doubt you will find a laptop any more than +/- 5V.
Do you have a better way of monitoring the traffic on a new connection?
Roy

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