Introduction to Data Communications
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9b. 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.

9c. Bipolar Line Encoding

Bipolar line encoding has 3 voltage levels, a low or 0 is represented by a 0 Volt level and a 1 is represented by alternating polarity pulses. By alternating the polarity of the pulses for 1s, the residual DC component cancels.

Bipolar Line Encoding

Synchronization of receive and transmit clocks is greatly improved except if there is a long string of 0s transmitted. Bipolar line encoding is also called Alternate Mark Inversion (AMI).

9d. Manchester Line Encoding

In the Manchester Line Encoding, there is a transition at the middle of each bit period. The mid-bit transition serves as a clocking mechanism and also as data: a low to high transition represents a 1 and a high to low transition represents a 0.

Manchester line encoding has no DC component and there is always a transition available for synchronizing receive and transmit clocks. Manchester line encoding is also called a self clocking line encoding. It has the added benefit of requiring the least amount of bandwidth compared to the other line encoding. Manchester line encoding requires 2 frequencies: the base carrier and 2 x the carrier frequency. All others require a range from 0 hertz to the maximum transfer rate frequency.

Manchester line encoding can detect errors during transmission. a transition is expected for during every bit period. The absence of a transition would indicate an error condition.

Introduction to Data Communications
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Copyright Eugene Blanchard Jan 1998, January 2007