I am trying to interface an RS422 driver and a receiver.

What are the important voltage parameters I need to understand when selecting the relevant chips. From what I understand the most important is the differential voltage parameters. Now, for eg - I have a driver(RS422) that has a Differential output voltage of 1.8V(min) to 3.3V(max)(Vcc).

Now, I have a receiver of input differential voltage of -400mV to +400mV. Can I interface them 2. The Vcc of the receiver is 3.3V as well.

New Voltage ratings image


You will need to look at the maximum voltage that the inputs of the receiver can withstand. In a differential receiver it is normal to see a maximum voltage spec for the common mode voltage range that the receiver can withstand. The maximum voltage of the outputs of the driver plus the maximum voltage difference between the GND at the driver and the GND at the receiver needs to be within the common mode voltage range of the receiver.

It is typical for the receiver to specify the minimum differential sensitivity voltage level at which it will reliably produce the correct 0 or 1 output depending upon the differential input voltage polarity. This may be the +/-400mV rating that you noted for the part you looked at. It is possible to find parts sensitive down to +/-250mV.

The receiver responds to a much lower level of differential swing than that produced by the driver so that it will still detect the signal properly even though it has been attenuated a huge amount after being sent down extremely long cables. The termination resistors used on the signal lines to prevent reflections and ringing of the signal also contribute to the overall signal attenuation from driver to receiver.

  • \$\begingroup\$ thank you. Now, the values you mention are the the Vih and ViL,right. I am attaching a new image with those details in my question please. Kindly refer the image added. \$\endgroup\$ – Board-Man Oct 30 '15 at 5:57
  • \$\begingroup\$ So, i understand that RS422, can supply a max of 6V differential. Now, supposing, my receiver can tolerate only Vcc(3.3V) voltage levels and one of the drivers give it 6V,it would be bad I assume. So, I need to step it down via a simple voltage divider. As rs422 is voltage level based, a simple potential divider would suffice,right. \$\endgroup\$ – Board-Man Oct 30 '15 at 6:01
  • \$\begingroup\$ @VUK - Rather than guessing look at the device data sheets as I described. A driver that operates off 3.3V will NOT place 6V across its too outputs. Only up to +3.3V in one state and -3.3V in the other state. If your receiver was designed for RS422 it likely has a common mode voltage range in the neighborhood of -7V to +10V. Maybe even more. As such you simply connect the driver to the receiver and move on. No divider resistors needed. Then when the connection becomes 4000 feet and the resistance of the wires reduces the voltage seen at the receiver you know (continued) \$\endgroup\$ – Michael Karas Oct 30 '15 at 12:55
  • \$\begingroup\$ (continued from above) it will still work as long as the seen level are above 400mV and -400mV between the two wires. \$\endgroup\$ – Michael Karas Oct 30 '15 at 12:55

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