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Old October 9th, 2015, 11:04 AM   #1
arbj
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Advice for connecting ground of RS 485 in PLC

Hi,

I am using a Kinco PLC (type: KT506-24DT) the PLC has a terminal for A & B connections of the RS 485 communication.

All these time I have been using a third ground line along with the A & B lines in my earlier circuits. The ground line would be connected to ground via 120 Ohm resistor on both ends.

My question is should I run a ground line along with the other two lines A & B of the RS 485 communication ? If so should this be connected straight to ground or via a 120 Ohm resistor as mentioned above ??

At present I have left the ground line open, and PLC is communicating very well with an Anybus gateway in a lab environment with a short RS 485 line, but I am not sure how its going to perform in an high noise environment.

Please advise on the best way of connecting the ground line ??


thanks
a
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Old October 9th, 2015, 11:51 AM   #2
nonuke
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Quote:
Originally Posted by arbj View Post
All these time I have been using a third ground line along with the A & B lines in my earlier circuits. The ground line would be connected to ground via 120 Ohm resistor on both ends.

My question is should I run a ground line along with the other two lines A & B of the RS 485 communication ? If so should this be connected straight to ground or via a 120 Ohm resistor as mentioned above ??

thanks
a
You don't need a ground line. You should use a crossed pair cable with screen. The 120 Ohm termination resistors should be connected between Data+ and Data- in the first and last RS485 module.
Search this forum for RS485 and you will find lots of information.
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Old October 9th, 2015, 08:12 PM   #3
gclshortt
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Shielded Cable with Drain Wire
The drain wire is connected to the housing of the communication cable end connected to the PLC side of the cable. This will filter the ambient noise to one end. Connecting both ends will produce more noise on the cable.


Just some additional information. Hope it helps.
Garry
http://www.accautomation.ca
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Old October 10th, 2015, 10:57 AM   #4
danw
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Quote:
Originally Posted by arbj View Post
My question is should I run a ground line along with the other two lines A & B of the RS 485 communication?
RS-485 should have a signal ground wire, say all the people who deal with it:



The problem is that many vendors use the case/housing ground as the signal reference and over long distances, ground potential differences show up as a common mode voltage.



Those are the situations where common mode can become a problem and the only solution I know is to use an RS-485 isolator module.

I've seen 100 ohms used in a signal ground connection to earth ground, like in this Maxim App note 1137 (below), but that resistor should not be confused with the termination resistor needed to consume the energy of reflected waves at the transmission line end points.

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Old October 10th, 2015, 12:52 PM   #5
arbj
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Quote:
RS-485 should have a signal ground wire, say all the people who deal with it:
Thanks for that information
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Old October 10th, 2015, 12:55 PM   #6
Mickey
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Some say yes some say no..see link

https://www.google.com/search?q=RS-4...m=122&ie=UTF-8
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Old October 10th, 2015, 02:30 PM   #7
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RS-485 works as long as the common mode stays within limits, as B&B points out:

"While a differential signal does not require a signal ground to communicate, the ground wire serves an important purpose. Over a distance of hundreds or thousands of feet there can be very significant differences in the voltage level of "ground."

"RS-485 networks can typically maintain correct data with a difference of -7 to +12 Volts. If the grounds differ more than that amount, data will be lost and often the port itself will be damaged. The function of the signal ground wire is to tie the signal ground of each of the nodes to one common ground. However, if the differences in signal grounds is too great, further attention is necessary. Optical isolation is the cure for this problem."

My experience mirrors that assessment. It always works the bench, it's when it gets distributed in the field that the problems start.
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