Manchester encoding

communications, protocol A method of transmitting bits which

enables the receiver to easily synchronise with the sender.

A simple way of signalling bits might be to transmit a high

voltage for some period for a 1-bit and a low voltage for a 0

bit:

Bits Sent: 1 1 0 0

Signal: High ___________

Low |___________

Time: -@# . . . . .

However, when several identical bits are sent in succession,

this provides no information to the receiver about when each

bit starts and stops.

Manchester encoding splits each bit period into two, and

ensures that there is always a transition between the signal

levels in the middle of each bit. This allows the receiver to

synchronise with the sender.

In normal Manchester encoding, a 1-bit is transmitted with a

high voltage in the first period, and a low voltage in the

second, and vice verse for the 0 bit:

Bits Sent: 1 1 0 0

Signal: High __ __ __ __

Low |__| |_____| |__|

Time: -@# . ' . ' . ' . ' .

In Differential Manchester encoding, a 1-bit is indicated by

making the first half of the signal equal to the last half of

the previous bit's signal and a 0-bit is indicated by making

the first half of the signal opposite to the last half of the

previous bit's signal. That is, a zero bit is indicated by a

transition at the beginning of the bit.

Like normal Manchester encoding, there is always a transition

in the middle of the transmission of the bit.

Differential Manchester Encoding

Bits Sent: 1 1 0 0

Signal: High ____ __ __ __

Low |_____| |__| |__|

Time: -@# . ' . ' . ' . ' .

With each bit period half as long, twice as much bandwidth

is required when using either of the Manchester encoding

schemes.

(1995-11-23)