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I've only ever used RS-485 over short connections before. I may have a project coming up where the cables will run several 1000 ft. My questions are:

  1. Is the gnd wire needed?
  2. Should the shield in the cable be used as the gnd wire?
  3. When must terminator resistors be used?
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I would think it would be asking for trouble if you did not use termination and grounding. See Jan Axelson's write up. Also the further your cable extends, the lower the baud rate will be. At 4000' the maximum baud rate is 90kbps. I think using the shield as the ground is okay, but you need to isolate each node with 100 ohm resistors. I have used RS485 for short hops (~100' or so) without using termination or grounds, but I think for K's of feet you would be wise to stick to the rules.

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  • \$\begingroup\$ Thanks Steve, RE the 100 ohm resistors, where exactly in the circuit should they be placed? \$\endgroup\$ – fred basset Oct 3 '11 at 20:11
  • \$\begingroup\$ See Jan's document. It shows 100 ohm 1/2W resistors connected between the ground of each node and the common ground wire that connects all the nodes. \$\endgroup\$ – SteveR Oct 3 '11 at 20:17
  • \$\begingroup\$ Thanks, I see them. Is that just to limit the current that could flow in the shield wire between nodes? \$\endgroup\$ – fred basset Oct 3 '11 at 20:23
  • \$\begingroup\$ Because of circuit variations the line may be slightly imbalanced. The difference current is therefore returned through the ground. The 100 ohm resistor keeps multiple nodes from raising the ground level. Also note that the "ground" is a common reference, and does not necessarily need to be reference to earth ground. \$\endgroup\$ – SteveR Oct 3 '11 at 21:08
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For 3. you need termination resistors to avoid reflections, especially at distance or high speed. This is even more true for RS-485 where the signal can be driven from the middle (or at least not at the end).

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If dealing with 1000s of feet don't forget to keep an eye on cable resistance when speccing the cable. Yes termination will be necessary. In some circumstances you may be able to do without a gnd but this depends on the topology of the transceivers. You also definitely want one end to be galvanically isolated from local ground as you have no idea what potentials there might be between local grounds at each end.

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For the record, I have my own write-up here (with links to further reading at the end):

http://support.fccps.cz/download/adv/frr/rs485/rs485.html#standard

Suppose your baud rate is 9600 bps. That means 1 bit is about 100 us. Suppose that your cabling has maybe 66% velocity ratio. That's about 200 m per a microsecond. That's 1 km in 5 us, or a reflection round-trip of 10 us. So... the first reflection will be back in 10 us. Depending on where the "sampling point" is within a bit duration in your UART (some UART's have multiple sampling points per bit and make a "quorum consensus" of some sort) the reflection in 10 % of bit length may or may not be a problem. Also, the reflection tends to fade with distance... so does useful bandwidth :-)

Reflections are one thing, and a common gnd potential is another. If your ground potentials wobble about a lot, and your transceivers are not isolated, the RX part of the transceiver may see voltages outside of its working range (about -3V to +7V typical) and may garble random bits. Worse yet, if the transmission line goes through an outdoor environment, it may face atmospheric electricity. Providing a common reference ground (one more wire or shield in the TML) is always a good idea, but it may not be enough to "unify" local grounds at very distant locations. Consider using isolated transceivers (to make the normal ground wobble irrelevant) and consider adding surge arrestors on top of that. Arrestors combined with transceiver isolation are as good as it gets. Except that a good arrestor (dual-stage cascaded, with spark chambers and transils) can be more expensive than an SFP transceiver, and an optical cable is not significantly more expensive compared to decent twisted-pair copper. Yeah right - SFP's are no good for RS485. I'm mentioning this just in case Ethernet was an option.

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