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Ground with RS488 standar

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julian403

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One different between RS232 and RS485 is the signal. RS232 is a digital signal referenced to ground and in RS485 there is a diferential voltage between two wires. This reduce noise's effect and with it the wire can be longer.



i was watching documentation and RS485 connector has a ground wire. Why? This ground it's for the control signal and not for the data signal?
 

On a RS-422 and RS-485 system the ground is expected to
provide the low impedance shield for the shielded twisted pair.
It cuts outgoing EMI and incoming common mode EMI. There
is a limit to the maximum common mode separation between
Tx and Rx ground that has to be enforced somehow for the
receiver to work properly, too. As a rule the TX control and
Tx driver ground are the same point. The Rx may be allowed
to deviate, often you don't want the cable to be tying the
grounds together by itself since it then becomes part of a
ground loop or perhaps a fusible link. But something needs
to enforce a sensible gnd_Tx:gnd_Rx relation.
 
Hi,

Non functional: shielding. It protects the signals from beeing influenced.

Functional.
There are (active) receivers with limited common mode voltage.
Some receiver ICs maybe have a common mode input voltage range of +/- 7V. To ensure this you need GND as absolute reference.
GND is not used do decide whether the signal is high or low.

Other devices - like transformers or optocouplers - don't have this limitation. They may work up to kilovolts common mode.

Klaus
 
Some interface standards based on RS-485 like PROFIBUS are specifying galvanic isolation and don't need a ground wire. They still have a shield, but it's not connected through a signal pin. Non-isolated RS-485 needs a ground wire for reliable operation.
 
So, It's not necesary have a ground's wire.

this it's because I'm try to do a communication between two terminal which are separated 50 meters in a building and what I want to do is a communication with just two wires which has a differential voltage. As there is just two wires, control bits are redundant data, sending the start bits and a CRC-8.

What do you think? it's necesary ground in this case. I think no, because is not an industrial places so there is not a lot of noise.
 

You need to review the specification of the RS-485 interfaces. If they are isolated, they can usually work without a ground wire. Remember that proper termination may be required if not already implemented in the interfaces.
 

You need to review the specification of the RS-485 interfaces. If they are isolated, they can usually work without a ground wire. Remember that proper termination may be required if not already implemented in the interfaces.
Click to expand...
Do you means that's necesary an opto-isolator because the potential in the wires can be bigger on the Rx that the Tx?
 

Hi,

It´s not clear to me:
Do you have two existing devices, or do you want to build one or two of them?

***
If the devices exist, then you need to read their datasheets.

If you want to build at least one of them on your own, then I recommend to use optical isolation for extended common mode range.

Klaus
 

Do you means that's necesary an opto-isolator because the potential in the wires can be bigger on the Rx that the Tx?
Click to expand...
RS-485 Isolation doesn't work with a simple opto-isolator. It also requires isolated DC/DC converter for the transceiver.

Voltage levels shouldn't be a problem, just common mode voltage. If you are not sure about interface isolation, better use a ground wire.
 

Well for now I'm going to do a simple RS232 but I still want to know how the difference potential on RS485 works

RS-485 Isolation doesn't work with a simple opto-isolator. It also requires isolated DC/DC converter for the transceiver.
Click to expand...

Why it's necesary an isolated on the terminals with a differential voltage signal?
 

Isolated means both signal and power (incl ground) isolation.
Anything less will fail to, you know, isolate. Think outside the
normal signal range, this is abnormal-conditions, safety and
fault protection / management type stuff. Like if somebody
upstairs does a screwdriver short from AC line hot side to the
user equipment, it's be nice if only that box, and not the other
end and/or the cabling in the walls got roasted along with it.
Of course you can't prevent everything, but some things are
credible threats with available solutions (and may be dictated
from above).
 

When do you means there must be ground to insolated, you means that the communication device's ground must be grounded to earth (house's electrical ground)? So in that way there can not be a difference potential between the device's wires and the earth to protect the system and people. Like for example the telephone's twisted pair which the ground is bonding to the 110VAC house's ground.

As this video show where all system ground are together https://www.youtube.com/watch?v=pzHQPgoXm30
 

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