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THVD4431: How to estimate cable reach without using actual cable?

Kazuhiko Hirano
Genius 15118 points
Part Number: THVD4431

Tool/software:

Hello,

My customer wants to do RS-422 communication over a 1.2km cable using THVD4431. Is there a way to determine if the RS-422 communication quality using a 1.2 km cable is satisfactory by monitoring the waveform at the transmit end of the THVD4431 without actually connecting a 1.2 km cable? 

Best regards,

K.Hirano

  • +1
    Guru 317280 points

    No; the signal at the receiver is affected by the characteristics of the cable, and nothing much is reflected back to the driver.

    The distance is limited by the voltage drop over the cable's resistance. (See section 5 of the RS-485 FAQ.) The 1200 m limit assumes AWG 18; you can improve the signal by using a cable with a lower resistance.

    (RS-422 uses only one termination resistor, so the signal at the receiver will be somewhat better than with RS-485.)

  • +1
    TI__Guru 73791 points

    Hi K. Hirano,

    Clemens is correct. You will need to test with a cable to verify your communication will work. 422 will give you a better chance of succeeding since the VoD will be larger with just one termination resistor present.

    It will also depend on the datarate that the customer is trying to support and how many nodes are on the bus.

    You ideally want to use a high quality cable with impedance control to lower the reflections on the bus.

    -Bobby