RS-232, 422 and RS-485 Communication


Thread Starter


Why is that we can drive data signals over RS-422 & RS-485 over longer distances as compared to RS-232 protocol???

Can any one explain or suggest suitable site where i can get more information on the above



Jose Calzadilla

Hi Ravi,

Here is an explanation: RS-232 uses a single-ended mode of operation, this means that the voltage level of the signals is a bipolar voltage with common ground. example:
To represent 1 could be 5 volt, to represent 0 could be 0 volt.

RS-422 and RS-485 uses a differential mode of operation this means that the voltage level of the signal is not a reference from ground, actuali is the difference between 2 lines with voltage, example:

Line A and Line B

voltage line A - voltage line B < -2 volt = 0
voltage line A - voltage line B > +2 volt = 1


the differential mode of operation can make a signal to reach longer distances and also provide an immunity for noise.

the single-ended mode cannot reach longer distances and doesn't have immunity for noise.

For more information visit the site you'll find very good information there.

Hope to be helpfull

Jose Calzadilla

Thomas Hergenhahn

RS422/RS485 use differential voltage for signal transmission. If noise happens to be added on the wire, it will most likely be added to both, positive and negative potential and thus effectively cancels out in the difference.
I believe that RS-232 is a single 12vdc signal but the RS-422 & RS-485 are +/- 12VDC pairs giving the equivelent of 4 times the signal.

one line is +12VDC the other is -12VDC then they switch.

I could be wrong though. it has been 20 Years since I saw the technical explanation and I am too lazy to look it up.

In a nutshell, because 422 and 485 are differential (versus 232 - single sided). In practice, 232 can run several hundred meters at modest Baud rate - original standard was very conservative.


RS-485 and 422 transmit signals as the difference between the two wires, with neither wire grounded - called "balanced differential". RS-232 transmits signals across one wire with a ground return path, which is called "single-ended". As with most communications, the problem of the receiver is to recover the original signal from the noise picked up during transmission. With a balanced differential signal such as RS-485/422, both of the wires pick up identical noise that the receiver can use in its noise canceling circuits leaving only the original signal. Noise cancellation is not possible in single ended circuits such as RS-232.

Dick Caro

Richard H. Caro, CEO
CMC Associates
2 Beth Circle, Acton, MA 01720
Tel: +1.978.635.9449 Mobile: +1.978.764.4728
Fax: +1.978.246.1270
E-mail: [email protected]
You are wrong on the numbers and polarity: EIA232F (that the right number - RS232C has been replaced a long time ago) is bipolar and single ended: Transmitter swings between -15V and +15V (max) but versus the GND signal Receiver hysteresis is expected to be in the band between -3V and +3V. In practice, the standard receiver chip swhitches around approx. +1V

EIA422/485 is bipolar and differential: Transmitter swings between +4V and -4V (max) differential voltage between the A and B leads. Receiver hysteresis is expected to be in the band between -0.2V and +0.2V.

EIA423 (not really used anymore) is bipolar and single ended. Transmitter as for 422/485 / receiver hysteresis between -0.4V and +0.4V

Meir Saggie.