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I have implemented a Modbus with the RS-485 bus. The length of the bus is about 80 meters and I have connected the master and 10 slaves. The system is battery-powered and will be outdoor.

My question is about the shielding of the bus cable. Is it sufficient to connect it to the negative terminal of the battery, or must it be physically connected to ground?

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The impedance of the shield connection must be as low as possible.

In general, the cable to the actual earth is so long that its inductance is too high, which results in high impedance at high frequencies. So it is likely to be a better idea to connect the shield to the power supply.

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  • \$\begingroup\$ Thanks. The problem is that the system can not use a power supply, it has to be powered by battery. In this situation, what is the best way to connect the bus cable shield? \$\endgroup\$ – FranMartin Oct 6 '17 at 8:04
  • \$\begingroup\$ The power supply is whatever supplies the power; in this case, the battery. \$\endgroup\$ – CL. Oct 6 '17 at 8:44
  • \$\begingroup\$ I have the bad habit of always differentiating between battery and power supply (connected to the mains). Thanks! \$\endgroup\$ – FranMartin Oct 6 '17 at 9:20
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I realize this question already has an accepted answer but I wanted to offer one of my own as well to clarify.

The shield of the RS-485 cable should be connected to a low-impedance return path. Generally the ground (negative side of the battery) is used to connect to the shield. However, it is important that only one side of the shield is connected to ground. Otherwise you will create ground loops which could cause strange behavior in your circuit.

In the RS-485 systems I have worked on they are designed as follows:

enter image description here

(Please pardon the crudity of the drawing. My MS Paint skills are lacking)

As you can see the shield is tied to the ground at the supply (battery negative terminal) but the other end is floating. On the first "drop" (first device on the RS-485 bus) the shield on the input side is left floating but the shield on the output side is connected to ground. This is repeated all down the string so that each shield is independent of each of the other shields. This prevents ground loops and preserves signal integrity.

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  • \$\begingroup\$ +1 That is a very good point, I learned this the hard way. \$\endgroup\$ – Hemal Chevli Oct 6 '17 at 12:01
  • \$\begingroup\$ Thank you very much for sharing your solution. I will connect the shield as I said. The power cord I am using also has shielding. Would it be correct to connect the shield in the same way? \$\endgroup\$ – FranMartin Oct 9 '17 at 6:00
  • \$\begingroup\$ Assuming your power cord is carrying DC, then yes, the shield should be connected to ground at the source \$\endgroup\$ – DerStrom8 Oct 9 '17 at 11:44
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Since the whole system is battery operated, functionally there is no reason to connect the shield to earth ground. I am of course assuming here the shielding is connected appropriately within the system.

However, there are two other reasons to attach your shield to earth ground.

  1. Static buildup. If your system contains moving parts or is subject to static build up due to say the wind blowing over the cables it is prudent to earth ground through a resistor to prevent said buildup.

  2. Lightning effects. Although there is pretty much nothing you can do to make your wires safe from a direct lightning strike, a proximity strike can cause the cable to suddenly have a rather large voltage on it. One that would be detrimental to anyone touching it at the time. As such an earth ground is again a good idea.

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    \$\begingroup\$ Thanks Trevor for taking your time and sharing your knowledge. You always learn from your answers. \$\endgroup\$ – FranMartin Oct 9 '17 at 6:08

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