Nrzl

Manchester

Differential manchester

Fig. 5.6. Digital signal encoding format.

NRZ-L is used generate or interpret digital data by terminals and other devices. In NRZ-L, '0' represents high level and '1' represents low level. In the Manchester code, 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'. In differential Manchester. The mid transition is used only to provide clocking. The encoding of a '0' is represented by the presence of a transition at the beginning of a bit period and a 1 is represented by the absence of a transition at the beginning of a bit period.

5.4.2. Asynchronous Transmission

Asynchronous transmission involves separate transmission of groups of bits or characters. In each group, a specific predefined time interval is used for each discrete signal. The transmission times of the group are independent of each other. Hence a sample clock is reestablished for reception of each group.

To alert the receiver for the arrival of a new group, a start bit usually a '0' is added to the beginning of each byte. To indicate the completion of the group, a stop bit usually a '1' is appended to the end of the byte. Thus the size of the group is atleast 10 bits. That is 8 bit contains information, one for start bit and one for stop bit. The transition of each byte is separated by a gap of varying duration. The gap is usually identified by an idle channel or by a stream of additional stop bits. The Fig. 5.7 shows the typical character format.

Idle or next start bit

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