help needed for rs485 !!!!

Hi to newsgroup

I want to implement an RS485 interface to an microcontroller with the purpose of creating a network of 10 controllers located about 200 mt from a pc . Although everything is clear regarding the interface what is obscure is the protection needed. I've seen schemes using a 100 ohm resistor between grounds and a 2 zeners series connected clamping at 13 volts between each data line and ground . Other suggestions are zenistors and varistors , other suggest optical isolation . Does anybody have experience to suggest a simple solution (it is a cost sensitive application) to protect the devices and data lines from surges ??? . I know you can't protect from all surges but a usual protection is ok.

Reply to
michael nikolaou
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couple the device directly to the rs485 and isolate all the other I/O ?

Reply to
Jasen Betts

Two hundred milli-tesla??? That is a distance???

Reply to
Robert Baer

--
millitesla is abbreviated \'mT\'

According to: 
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Reply to
John Fields

--
Oops...

_200_mt or about 7054 oz, which is about 441 lb.
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Reply to
John Fields

At such long distances it is much better to use a small dc/dc converter and optocouplers to isolate the RS485 from your device. And use a good RS485 driver, there are several.

Whatever clamping you use, you will always have ground loops with problems if you don't.

P.

Reply to
P.

we use bi-directional TVS diodes across and to ground (3)

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Reply to
Jamie

Guys

mt stands for meters, thats 0.2 (km or kilometers)

John Fields wrote:

Reply to
michael nikolaou

The usual abbreviation is just "m".

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
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Reply to
Fred Abse

Small "m" so --- milli-ton???

Reply to
Robert Baer

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