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As many know, one can implement a simple node to node rs485 communication by using only two wires, A and B. Well, the standard specify to connect the ground of the two node together.

from Wikipedia:

In addition to the A and B connections, the EIA standard also specifies a third interconnection point called C, which is the common signal reference ground.

I stumbled upon tens of articles that speak about this third connection but still couldn't understand the concept.

  1. Why can't the receiver just act as a simple voltmeter? measuring the voltage between A and B?
  2. If both nodes are battery operated(different battery for each node) does the ground connection make any difference?
  3. Why is it better for (outdoor) nodes to be earthed when cables are long?
  4. How is this ground connection good for protection from transient?
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  • \$\begingroup\$ I don't get it. There are two perfect answers and open bounty? WTF? \$\endgroup\$ – Gregory Kornblum May 14 '16 at 17:14
  • \$\begingroup\$ By the way. If your system only works with two wires, that only means you never tested it under all possible conditions. \$\endgroup\$ – Gregory Kornblum May 14 '16 at 17:14
  • \$\begingroup\$ @GregoryKornblum lol those "perfect answers" didn't explain a lot. it's like asking about a TVS and how a design actually works after de-soldering them.You get an answer that "things could go wrong and then this diode will protect you". It is a correct answer but it's not a "perfect answer" \$\endgroup\$ – fhlb May 23 '16 at 11:22
  • \$\begingroup\$ Wrong. There was a perfect explanation about common mode voltage. If there still is something not clear, just ask the specific questions. \$\endgroup\$ – Gregory Kornblum May 23 '16 at 11:23
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Note: I don't claim to have the most scientific answer but i will try to explain things the way i understand them.

  1. I don't know if The Voltmeter argument is 100% valid , since it is a low speed device , a more appropriate example would be the Battery operated oscilloscope , or a differential oscilloscope probe.

  2. Remember the definition of Voltage which is a potential difference between 2 points. The Signals A and B are differentiated by the electrical components inside the Amplifier (mainly transistors) which all have absolute maximum ratings between their base and collector/emitter .. This is referred to as differential amplifier maximum Common Mode Input Voltage with respect to its own ground. Hence the A and B voltages are meaningless without specifying what they are referenced to. For example if the difference between A and B is 2.5v but this voltage is 20v shifted above the receivers amplifier supply , would would the amplifier see (2.5 or 22.5) ?

  3. 4: The long distance outdoor cables are more prone to noise or ESD or whatever source of charge or current that may enter the bus ( that has specific impedance and DC resistance) so if amount of charge/current is high enough multiplied by the higher resistance (of long cable ) would cause a larger voltage spike at the receiver that may cause damage. Earthing in this situation can be used to provide path for the spikes hitting the shield , and may be used as a stable ground reference.

Whether the RS485 receiver circuit can be changed to be floating and fully capable of acting like the voltmeter/Oscilloscope might be entirely possible by adding extra components, isolation circuits, etc.. but with added cost, complexity and size which is all beyond the capability of a small IC such as the MAX485.

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It is a myth that you can make RS485 interfaces work without the Common (C) ground wire connected up between the various devices on the bus. The receiver is only capable of measuring the relative potential between the A and B signals when the common mode voltage of the A and B inputs is kept within -7V to +12V of the GND reference of the receiver.

The idea that both sides of the interface being operated on batteries would somehow make a difference is also a myth. It all comes down to what the common mode voltage is between the transmitter GND and the receiver GND. The third wire connection keeps the common mode voltage under control. Without it any undue influence to either unit or to the bus between the two can lead to the common mode voltage going outside the -7V to +12V range. This influence could be due to coupling into other systems through EMI. It can also be common to see this show up as AC variation that follows the mains line frequency.

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  • \$\begingroup\$ I tested it and it works. using a PTZ controller operated by an isolated wall adaptor to a dome cam operating from a battery with no earthing. \$\endgroup\$ – fhlb May 8 '16 at 12:46
  • \$\begingroup\$ maybe large pull-up/pull-down resistance at the receiver end gives (floating) A and B a valid voltage level as referenced to receiver's ground. That is if Vab=5V, A is connected to a pull down, then Va=0, Vb=-5V \$\endgroup\$ – fhlb May 8 '16 at 12:49
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    \$\begingroup\$ @Fhib, it only woks because of current flowing in the ESD protection diosed of the RS485 transceivers allows the two nodes to a achieve a somewhat matched common voltage. \$\endgroup\$ – Jasen May 8 '16 at 12:53
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    \$\begingroup\$ @fhlb - Just because you tested it and it appeared to work for you does not mean that you can then simply eliminate the C wire connection. You got lucky in that one instance that there was nothing in the vicinity of the driver and/or receiver that placed undue influence on one or the other device. \$\endgroup\$ – Michael Karas May 8 '16 at 12:57
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    \$\begingroup\$ Because of Ohm's law. A common mode voltage will cause a current through some part, be it something functional or something protective. Once this voltage and/or current becomes too large, the part fails. \$\endgroup\$ – sekdiy May 11 '16 at 22:04
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You are right in that a pure receiver could just measure the difference between the two signal lines. However, any means to do that will have some common mode range that the individual signals must stay within. The spec gives the common mode range that nodes must be able to tolerate.

Without a third reference wire, there is no way to define this common mode voltage, and then there'd be no way to make a receiver that is guaranteed to be compliant.

Even if your receiver was set up so that the data lines drove opto-isolators, for example, you still have a common mode voltage limitation. It might be a few thousand volts instead of a few volts, but there will always be some common mode voltage beyond which the receiver won't work anymore.

So far that was just about receiving the RS-485 signal. Driving the RS-485 signals is much more limiting. The data signals are specified as being 0-5 V nominal with respect to the ground wire. Without a ground wire, you have no way to ensure that. The circuit that drives the two signals will be referenced to something. That something needs to be connected to the other transmitters and receivers on the bus.

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  • \$\begingroup\$ The voltmeter surely also has a common mode voltage range. But since it is battery operated, this spec is not even mentioned. The COM pin is probably directly connected to the battery's ground. Why can't an RS485 receiver act just like a voltmeter? Is this related to the assumption that an RS485 device is earthed !? \$\endgroup\$ – fhlb May 8 '16 at 13:05
  • \$\begingroup\$ @fhl: Voltmeters have a common mode operating range, but it is dependent on the environment around the voltmeter, so can't be specified by the manufacturer. With a high enough common mode voltage on its leads, some insulation somewhere is going to break down. However, all this is really missing the point. \$\endgroup\$ – Olin Lathrop May 8 '16 at 13:12
  • \$\begingroup\$ Hmmm... In case of sending data A:0V, B:5V at sender. What will be the common mode voltage at the receiver knowing that the common ground is unconnected? \$\endgroup\$ – fhlb May 8 '16 at 13:44
  • \$\begingroup\$ @fhl: The point is you don't when. When the common mode ground is unconnected, two nodes can float arbitrarily. That's just another way of saying the common mode voltage is undefined. \$\endgroup\$ – Olin Lathrop May 8 '16 at 14:28
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Point C is a return path for the current on A and B. This allows current to return back to the source to complete the circuit.

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