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RS-485

Hi,

I need to transport an RS-485 signal over a "long" cable (100 meter). The cable will receive lots of noise (large motors, ...) I'm planning to use the SN75176 as a driver and receiver.

Is it ok to use UTP cable, or should i use STP? What kind of grounding topology i have to use in the transmitter and in the receiver?

Can somebody help me with this, or give some URL where i can find more info?

Thanks, Joble

Reply to
joble
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I read in sci.electronics.design that Roger Lascelles wrote (in ) about 'RS-485', on Wed, 8 Dec 2004:

I agree: this will cost more initially but it will work first time and continue to work reliably. Copper without isolation over that distance and in that environment is seriously bad news. You might even consider a complete fibre-optic connection.

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Regards, John Woodgate, OOO - Own Opinions Only. 
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Reply to
John Woodgate

Maxim has RS485 chips that include opto-isolation. You can also roll your own with opto or some other kind of isolation, but you also need power on the other side of the isolation, so an isolated DC-DC converter or perhaps a spare winding on the PS transformer could be used.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

Here is a link to a supplier of optically coupled RS485 devices...

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They also have some tech notes.

I have no affiliation with these people. Good luck.

John

Reply to
John Smith

As mentioned : have the GND and Vcc on the cable too and use it to optoisolate the other side. A shielded cable reduced e-filed coupling. And the twisted pair reduces the magnetic coupling. Yes, take STP.

Rene

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Reply to
Rene Tschaggelar

Maxims devices are very nice. You pay for the ease of implementation - but they're nice =)

/Anders

Reply to
Anders F

The question you have to ask, is "what happens if I get this wrong"? If your neck is on the line, or comms failure cannot be tolerated, do it right first time.

With big current around and motors starting, ground movement and common mode pickup is going to be pushing the common mode limits of the receivers - and you can get damaged receiver ICs too. Also if you connect the shield to ground at both ends, current is going to go up and down the shield.

I recommend opto coupling at least one end or even better both ends. That way, no common mode voltage worries and no shield current.

The $$ for shielded vs unshielded is probably insignificant compared with the investment elswhere in the plant.

Roger

Reply to
Roger Lascelles

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