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I'm planning a system that has two sub systems that need to communicate with the PLC via RS485. The baud rates are 9600 and 19200 bps.

The cables are static (i.e. not moving) and placed inside of a electric cabin.

In the cabin area there are a few more electric appliances, such as PLCs and stepper motors. Those are placed in other electric cabins or behind metal separation. The cabin is not tangent to the motors or PLCs but in a few meters distance from the motors, with a metal separation and the cabin between them.

At first, I thought about using CAT5e FTP cables due to low cost and because I have read that this application is possible. Unfortunately, my lack of understanding if that is considered a 'harsh environment' and if the Foiled Twisted Pair will be good enough for running the applications properly.

  • Do you think using the CAT5e cables (with a proper termination resistors) will do a good job or should I consider other cable?
  • If one of my sub-systems is working with EIA-RS485, will that shifting away from the standard can cause issues?

Thank you.

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For RS485, CAT5 should be just as good as any other shielded twisted pair cable. The impedance should be within tolerance for RS485. What matters most is how you connect the common ground between devices, and how you connect the cable shield, and where you connect the terminators, but that does not depend on if the cable is CAT5 or something else.

The RS485 is defined by TIA/EIA so it means the same TIA RS485 standard.

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  • \$\begingroup\$ Thank you very much for your response. I need to adjust the termination resistors for the CAT 5e cable correct. If the impedance of the cable is 100Ohm, I need to use 100 Ohm resistors? \$\endgroup\$
    – GabriLev
    Commented Nov 8, 2020 at 11:08
  • \$\begingroup\$ In theory, yes. But in practice, it depends on cable length, and transmitter slew rate too. In some RS485 systems it is said to use 120 ohm termination like normally in a system, even if the signal is wired via CAT5. If there is an impedance mismatch, it is slightly better to have the termination impedance higher than the transmission line impedance. The good thing also is you can just swap the cable to 120 ohms without changing the termination. \$\endgroup\$
    – Justme
    Commented Nov 8, 2020 at 11:32
  • \$\begingroup\$ So basically, placing 120 Ohm resistors is going on the safe side. \$\endgroup\$
    – GabriLev
    Commented Nov 8, 2020 at 12:02
  • \$\begingroup\$ do not use wire-wound resistors ... use resistors that have low inductance \$\endgroup\$
    – jsotola
    Commented Nov 8, 2020 at 18:06

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