Introduction to Data Communications
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21b. Polar Encoding

When the digital encoding is symmetrical around 0 Volts, it is called a Polar Code. The RS-232D interface uses Polar line encoding. The signal does not return to zero, it is either a +ve voltage or a -ve voltage. Polar line encoding is also called None Return To Zero (NRZ). Polar line encoding is the simplest pattern that eliminates most of the residual DC problem.

There is still a small residual DC problem but Polar line encoding is a great improvement over Unipolar line encoding. Polar encoding has an added benefit in that it reduces the power required to transmit the signal by one-half compared with unipolar.

RS-232D TXD

Polar line encoding has the same synchronization problem as Unipolar line encoding. If there is a long string of logical 1s or 0s, the receive oscillator may drift and become unsynchronized.

Introduction to Data Communications
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