Specifications

Manchester Encoding
4-2 Ethernet Data Frames
In Ethernet, the digital D.C. signal is transformed into discrete time
segments called bits by a method called Manchester Encoding. With
Manchester Encoding, the incoming digital signal is checked at specific
time intervals for its change of state. In other words, the signal is checked
to see if it is changing from 0 volts to -1.2 volts or from -1.2 volts to 0 volts
during a certain time period. Depending on its change of state in this
specific time interval, or bit time, the signal is assigned a logic “1” or a
logic “0” for that time interval. The resulting signal is a steady stream of
bit times (or bits) with values of either logic “1” corresponding to a change
from -1.2 v to 0 v or a logic “0” corresponding to a change from 0 v to
-1.2 v. If there is no change of state during a certain bit time, that bit time is
assigned a value corresponding to the value of the bit time preceding it.
Manchester Encoding also provides the digital signal with a method of
alignment by ensuring that the transitions to the logic levels happen only
during the center of the bit time. Figure 4-1 shows how Manchester
Encoding works on a random digital signal.
Figure 4-1. Manchester Encoding
Bit Time
.1 µs
Logic "0"
Logic "1"
Changes to logic levels take place
at the center of each bit time.
00 110
0 volts D.C.
-1.2 volts D.C.
no change: value
stays the same as
preceding bit
1913-05