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

[danw2]

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

 

[roydm]

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.

 

[IRstuff]

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

FAQ731-376

 

[geekEE]

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

 

[IRstuff]

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

FAQ731-376

 

[itsmoked]

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

 

[roydm]

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

 

[geekEE]

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

 

[danw2]

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

 

[itsmoked]

Thanks Dan.

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

Keith Cress
kcress -

http://www.flaminsystems.com

 

[roydm]

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.

 

[felixc]

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.

 

[roydm]

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