Electronic – Implications of using a 145 ohm cable for RS-485

dcrs485

My current workplace has made some custom cables that have conductors for DC power and conductors for for RS-485 communications in the same bundle. My predecessor apparently had the cables measured to have a 145 ohm characteristic impedance. I believe most RS-485 drivers specify 120 ohm cable, so what will the implications be for using this slightly mismatched cable? Data rate is 57 kbit/s.

Also, the DC power wires that are in the cable are used to power brushless DC motors at 36 V and about 8 A. Should I be concerned about induced noise on the data cables? The data cables are not separately shielded and are not twisted pairs either.

Best Answer

What's the data rate of your RS-485? Since your cable isn't twisted pair you may have to limit the speed to 100kbps and use short cables.

But IMO the mismatched impedance isn't the real problem.

"The characteristic impedance of an RS-485 cable is 120 ohm according to the standard, but cables designed for high frequency operation usually has an impedance between 135 and 165 ohm, which limits the capacitance to approximately 30 pF/m." (source).

You don't mention voltage or current for the motor's power, but at 200W both may be too high to avoid trouble. A high current will inductively couple with the data wires, a high voltage will do the same capacitively. I expect you may see serious disturbances.

The cable just doesn't seem to be fit for the application. It should have been a double twisted pair at least, preferably with different twist rates.

edit
All will depend on how the motors behave and what your data look like. Have you had problems yet with the deployed cables? Eight noisy amperes coupled over a long stretch doesn't sound good. Bad looking waveforms in itself are not necessarily a problem if the receiver can properly detect edges. What are you sending, UART format? If you have continuously glitches sooner or later they might be seen as a start bit, and you'll have a ghost byte in between your actual data. Not only the data format is important, also what happens at the higher protocol levels. Is there a checksum or CRC? Acknowledge? Can faulty received data be retransmitted? If you would encounter communication problems I think this is the level where you can attack them.
It's also possible that for the installed cables no problems are reported at all. The 57kbps may help here. And maybe there is an error detection/correction mechanism in the protocol.

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