Asynchronous Serial Commuication (confused about start bit)

[eng_boody]

hi all I was reading on the internet that a start bit (transition from 1 to 0 ) indicate the start of a frame... but my question how do the receiver know that the transition between 1 and 0 is for start bit not a transition from 1 to 0 during data bits ...

another thing is there control bits for UART ?? I mean when I connect TX to receiver RX and a stream is already going from TX to RX do the receiver skips all the the bits before the next start bit ?? the question in another form when I connect TX to RX the first falling edge detected by the reciever ---> this falling edge is treated as transtion from idle to start or transition during data ??

 

[FvM]

how do the receiver know that the transition between 1 and 0 is for start bit not a transition from 1 to 0 during data bits ...

It can't be sure. If the receivers looses synchronization, it won't resync before a pause longer than character frame occurs.

 

[eng_boody]

pls explain to me in more details what do you mean by "a pause longer than character frame occurs"

also how it sync initially before a sync is lost

 

[ads-ee]

pls explain to me in more details what do you mean by "a pause longer than character frame occurs"

also how it sync initially before a sync is lost

What FvM means is a UART syncs to a start bit when there is a pause (RX input is 1) for longer than the time it takes to send: start bit, 8-bits, stop bit (e.g. 10 -bits).

This might help with your understanding. Looks like a reasonable explanation.
https://web.engr.oregonstate.edu/~traylor/ece473/lectures/uart.pdf