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I'm working on a project that will be using RS485 to communicate between a bunch of modules and a master controller. Low baud rate, 9600, but a decent distance, like a couple of hundred feet. I'm wondering if I can get away with reliably using Cat5e for this, and whether or not I need it to be shielded and connected to ground?

If that is the case, would I be able to use the remaining 3 pairs as the ground instead of getting cable with actual shielding?

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    \$\begingroup\$ probably. It depends on your environment \$\endgroup\$
    – BeB00
    Jun 4, 2020 at 6:36
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    \$\begingroup\$ Why not to use shield? It already is in the cable. Why not to connect it at one side? \$\endgroup\$
    – Chupacabras
    Jun 4, 2020 at 9:36
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    \$\begingroup\$ @Chupacabras Common UTP cable, aka Unshielded Twisted Pair, is much cheaper than STP, aka Shielded Twisted Pair. As the name implies, UTP cable does not have a shield at all; there is nothing to connect. \$\endgroup\$
    – Bob
    Jun 4, 2020 at 14:53
  • \$\begingroup\$ Dupe : Is CAT5 cable good enough for RS-485 vs “true” RS-485 cable \$\endgroup\$
    – J...
    Jun 4, 2020 at 16:02
  • \$\begingroup\$ DMX (used for professional lighting, like at concerts, etc.) is RS485 at 250kbaud. It has specific requirements for the cable - copied/pasted from RS485 - but also says that Cat-5 is acceptable for permanent installations. (What is not officially-acceptable, is the 3-pin microphone plug that everyone puts on their lighting gear. Officially, it should be 5-pin with 2 unused/reserved, but that's a different topic.) \$\endgroup\$
    – AaronD
    Jun 4, 2020 at 22:42

3 Answers 3

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Short answer: yes. That should work just fine. It is quite common to use CAT5 for RS485.

More detailed:

RS485 requires a common ground reference for all devices. Yes, you can use spare pairs in the cable as ground reference. Proper shielded cables provide better noise immunity. But it is not by any means necessary for RS485 communication to work. Depends on your environment. Lots of high power electrical motors and welders around? Then you may want to use shielded cables.

Depending on your exact cable length requirements, you may want to consider thicker gauge CAT5. Up to ~100m/300ft any CAT5 should do the job. Beyond that, the resistance of the copper conductor can start playing a role in attenuating your signal. Thinner conductor=higher attenuation. RS485 cables designed for 1000m/3000ft have really thick copper wires.

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    \$\begingroup\$ The cable's characteristic impedance will be a bit different from proper RS-485 cable (like Belden 9841, for example), so terminating resistors would need to be adjusted to match Cat5e impedance. The difference isn't large (100-ohm vs 120ohm), but depending on the cable length and noise environment it might be necessary to adjust the terminator to correctly match the cable. Noise rejection will also not be as good as true RS-485 cable - braid and foil make a difference. May or may not matter, again, depending on the noise environment. \$\endgroup\$
    – J...
    Jun 4, 2020 at 14:58
  • \$\begingroup\$ RS485 is used with several different impedance cables. There is really no single "proper" RS485 impedance. Yes, the termination resistors should obviously match the impedance of the cable. \$\endgroup\$
    – Timmy Brolin
    Jun 5, 2020 at 7:36
  • \$\begingroup\$ You're right, but there are very common cables that are frequently specified in the installation requirements for many industrial devices. You generally don't find CAT5 specified when a manufacturer outlines the connection requirements for their devices (I'm being charitable, here). Typically it's something like Belden 9841, etc. \$\endgroup\$
    – J...
    Jun 5, 2020 at 11:53
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    \$\begingroup\$ @EmanuelElliott RS485 requires a common ground reference for all devices. Yes, you can use spare pairs in the cable as ground reference. Proper shielded cables provide better noise immunity. But it is not by any means necessary for RS485 communication to work. Depends on your environment. Lots of high power electrical motors and welders around? Then you may want to use shielded cables. \$\endgroup\$
    – Timmy Brolin
    Jun 8, 2020 at 11:18
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    \$\begingroup\$ Depending on your exact cable length requirements, you may want to consider thicker gauge CAT5. Up to ~100m/300ft any CAT5 should do the job. Beyond that, the resistance of the copper conductor can start playing a role in attenuating your signal. Thinner conductor=higher attenuation. RS485 cables designed for 1000m/3000ft have really thick copper wires. \$\endgroup\$
    – Timmy Brolin
    Jun 8, 2020 at 11:26
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Definitely yes, the CAT5/5E or CAT6 cable is perfect for industrial use of RS485 network.

Pay attention of the shield connection and for long distance networks is recommended to use ISO tranciever like ADM2483 (Datasheet here).

If the shield is not connected (that is a common mistake) an isolated tranciever will almost solve communication issues (a traditional MAX485 or non isolated tranciever will probably not work and it will potentially be damaged due to compensation currents to earth)

In addition to that, the termination resistor on the farthest device can help to balance the entire bus.

Without termination resistor and the ADM2483 ISO tranciever i've reached 1350 meters with a CAT6 cable (shield unconnected), only A and B communication cables at 57600 baud.

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    \$\begingroup\$ Keep in mind that those are best-case numbers for the max 485 over CAT6 length. If it's an industrial environment with a lot of electrical noise then that number can get much smaller in a big hurry. \$\endgroup\$
    – J...
    Jun 4, 2020 at 20:26
  • \$\begingroup\$ I agree with @J..., I had never installed a CAT cable for RS485, always the correct one, but I have seen people doing this for money saving. Industry means production, possibly business damage for few bucks of correct cable. Further you are talking about ISO converters, while I have the opinion that OP doesn't have it. \$\endgroup\$
    – Marko Buršič
    Jun 4, 2020 at 21:34
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    \$\begingroup\$ "Pay attention of the shield connection" -- aren't the vast majority of Cat5/5e/6 cables unshielded? \$\endgroup\$
    – David
    Jun 5, 2020 at 0:17
  • \$\begingroup\$ I agree with all comments above, yes, many CAT5/5e/6 are unshielded, right: in my point of view is that the OP needs some additional informations and not only a shoot-and-forget answer (this is why i've talked about ISO trancievers). In industrial production i've seen in almost installations CAT5 or CAT6 shielded cables (and i haven't any issue), is not only a question of money saving, but simply convenience during cable laying \$\endgroup\$
    – VirtApp
    Jun 5, 2020 at 7:04
  • \$\begingroup\$ @VirtApp I'll just chime in with the opposite - I've never seen CAT5 used for RS-485, at least not for anything other than maybe building automation (ie: BACNet networks, etc). For industrial machines you'll almost never see field remotes wired with anything but the correct cabling. \$\endgroup\$
    – J...
    Jun 5, 2020 at 22:54
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Yes it could work, but you have mentioned something about the ground and shield. The RS485 forlong distance is better to be isolated one. The ground goes nowhere, since both devices are connected to the local earth (usually not directly, rather capacitive coupling from environment, the transceiver is floating).

The shield goes connected to the earth in a single point or it is divided into sections.

This situation may be different if you are using non-isolated RS485 transceivers, a third wire is needed - GND. If devices have GND shared with local earth, then large compensation currents will flow over this tinny wire.

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