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I am using an RS422/RS485 driver to send Arduino 5V logic data over longer distances. I use the SN75174 driver for this purpose. In order to avoid ringing, I am not sure if I have to connect a series resistor on the data input of the driver. In which cases is a series resistor for protection of ringing and current limiting necessary and how do I chose the resistor value?

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Only single-ended, unidirectional signals can be terminated using a series resistor at the driver.

RS485 is balanced so termination is different. The resistor goes between the balanced pair at the receiver for point-to-point. For multi-point, you daisy chain and terminate at each end of the daisy chain (not stubs) if the driver is not sitting there. The resistor is supposed to match the characteristic line impedance but not so high it forces the driver to source too much current. Usually 60-120 ohms.

For short distances (<10m), you don't need one at all for RS-422/485, especially at low data rates (<1MBPS).

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  • \$\begingroup\$ I mean the data input that goes from Arduino into the driver and is then converted into a differential signal. That input signal is unidirection and single-ended. Does that need a series resistor? \$\endgroup\$ – F. Heisenberg Apr 16 at 15:02
  • \$\begingroup\$ No. The trace length and data rates are nowhere near high enough. \$\endgroup\$ – DKNguyen Apr 16 at 15:04

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