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Connected receiver inputs on SN75C1167

Sepeedah Zadeh
Intellectual 2850 points
Other Parts Discussed in Thread: SN75C1167

Hello,

I was wondering is there a reason to connect the differential receiver inputs on SN75C1167 (1A and 1B for example) via a small resistance (~100)? If so can you explain why. If not, then please verify that this is not an appropriate connection.

Thank you!

  • 0
    TI__Guru*** 140026 points
    Sep,

    The rise and fall time of RS-422 signals is fast enough that the RS-422 cable becomes a transmission line. All transmission lines have a characteristic impedance. The driver see this impedance and the signal power propagates down the cable just fine. At the receiver end of the cable, one of two things can happen. If the receiver has a resistor whose value matches the cable impedance then the power from the cable is absorbed and nothing else happen (except resistor heat). If there is no resistor, the power from the cable has nowhere to go; it will reflect back down the cable back to the driver. Unless specifically designed, the driver won't match the cable impedance so some of the power heads back to the receiver again. All these reflection greatly degrade the signal quality.