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