There are 2 major categories: return–to–zero (RZ)
and nonreturn–to–zero (NRZ). With RZ coding, the
waveform returns to a zero–volt level for a portion (usu-
ally one–half) of the bit interval.
• The waveforms for the line code may be further classified
according to the rule that is used to assign voltage levels
to represent the binary data. Some examples include:
– Unipolar Signalling: In positive–logic unipolar sig-
nalling, the binary 1 is represented by a high level
(+A volts) and a binary 0 by a zero level. This type
of signalling is also called on–off keying (OOK)
February
Polar Signalling: Binary 1’s and 0’s are represented
by equal positive and negative levels
– Bipolar (Pseudoternary) Signalling: Binary 1’s
are represented by alternating positive or negative val-
ues. The binary 0 is represented by a zero level. The
term pseudoternary refers to the use of 3 encoded
signal levels to represent two–level (binary) data. This
is also called alternate mark inversion (AMI) sig-
nalling
– Manchester Signalling: Each binary 1 is repre-
sented by a positive half–bit period pulse followed by
February 2005 Slide: 5