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I am starting a project where I have two arduino-based devices positioned about 100 meters apart. The secondary device will be controlling a solar charging array and relaying stats to the main device. The main device will display stats to the user and send control commands to the secondary device.

I plan on using a RS485 shield on each device.

Because I will be burrying the data cable(s) underground I am wondering what I need to consider when choosing a cable type to run the distance in a cost effective way.

Can anyone recommend a wire to use?

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    \$\begingroup\$ What baud rate? What serial spec (RS232, RS485, ...)? \$\endgroup\$
    – HikeOnPast
    Commented Dec 1, 2012 at 1:11
  • \$\begingroup\$ Thats a good question, I am using a RS485, I guess 9600 baud, not sure if that is overkill for a status relay of a few analog inputs in one direction and a few control inputs in the other. \$\endgroup\$ Commented Dec 1, 2012 at 1:29
  • \$\begingroup\$ Hint: ensure your RS485 transceivers are the ESD hardened kind. otherwise lightning will induce ground potential differences and blow your transceivers. \$\endgroup\$ Commented Dec 1, 2012 at 4:16

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From TI's RS485 design guide, RS485 can support:

  • 10-Mbps maximum data rate (at 40 feet)
  • 4000-foot maximum cable length (at 100 kbps)

The following, from the same document, appears to conflict with the above:

enter image description here

Reading the fine print of (1), it looks like they dropped the somewhat important "M" from "Mbps" on the X-axis of the chart. Perhaps most useful for your application is the equation:

Line length(m) X data rate(bps) < 10^7

At 100m and 9800bps, it looks like you've got an order of magnitude margin on the guideline.

As for environmental protection, I'd recommend burial-grade PVC conduit rather than direct burial-rated cable. Laying the conduit might be a little more work, but you'll 1) be able to use much less expensive cable 2) be protected against errant shovels and 3) have the ability to pull additional conductors in the future if your application requires it.

At the length and baud rate you're contemplating, nearly any available twisted pair cable should be adequate.

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