Re: Question about serial interface

Ed Casas (edc@ece.ubc.ca) Thu, 13 Apr 2000 11:54:42 -0700 

Date: Thu, 13 Apr 2000 11:54:42 -0700
From: Ed Casas <edc@ece.ubc.ca>
Subject: Re: Question about serial interface



On Thu, Apr 13, 2000 at 12:19:53PM -0700, FENN CORWIN HO MING wrote:

> 1) How can we distinguish a start bit and a stop bit
>    from the bits of the data being transfer?

>    From the lecture note, it says that the start bit
>    and stop bit are longer (1.5 bit) than the data bits.
>    It that right?

No, they are the same duration!  Where does it say that they are
different?

>  If so, what is the answer for question
>    #5 c) in the past final exam (98/99 winter term2) which
>    says:
> 
>    (c) Which of the following changes to the transmission
>        format of ASCII characters over an RS-232 serial
>        interface would not change the characters received?
> 
>         1. Using two start bits?
>         2. Using two stop bits?    

The answer is "2."  The stop bit voltage level is an idle (mark)
level.  A second stop bit would be indistinguishable from a
1-bit-long pause between characters.

An extra start bit would insert a '0' bit into the LS bit
position and shift the remaining bits up (left) by 1 bit
position.

-- 
Ed Casas   edc@ece.ubc.ca   http://casas.ece.ubc.ca   +1 604 822-2592