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

 

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

 

I read in sci.electronics.design that Roger Lascelles

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.

 

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

http://www.bb-elec.com/

They also have some tech notes.

I have no affiliation with these people. Good luck.

John

 

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

 

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

 

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

/Anders