B. Serial Transmission format
1 1. Asynchronous
Data Transfer
·
It is “Character” oriented data
transfer.
·
One data bit is transferred at a time.
· When data is not transferred the output
line of transmitter is High i.e. Marking State.
· The start of data is indicated by a low
start bit, which is used to synchronized transmitter and receiver. After the
start bit, data bits are transfer serially.
· The data bits are followed by a one or
more high stop bits. After the stop bits same logical level is maintained on
output lines.
· The receiver will check for marking
state. When it detects low on data line then it is treated as valid start bit.
· After start bit receiver will start
accepting data bits D0 to D7. At the end it will check stop bits and go back to
initial marking state.
·
The asynchronous data transfer format
consists of a start bit, data bits and stop bits called as Frame.
· The clock to both transmitter and
receiver are separate. But operating frequency for both is maintained same.
·
It is “Block of data” oriented data
transfer.
·
The block of data bytes transferred
serially one after the other.
· Before sending data, special characters
are transferred by transmitter to receiver to achieve synchronization between
transmitter and receiver. This special character called as Sync Character.
· To start transmission, the Sync
Character bits are sent by transmitter followed by data bits.
· When data is not transferred the output
line of transmitter is High i.e. Marking State.
· The receiver will go on checking input
lines; and comparing whatever data received with Sync Character.
· When match occurs then it is consider as
the receiver is in synchronization with transmitter.
· When the receiver is synchronized, then
it will go on accepting data bits in the same sequence.
· The clock of both transmitter and
receiver is same to avoid wrong data bits.
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