On Mon, 5 Jan 1998 07:17:01 -0800 Andre Abelian <andre@compufire.com>
writes:

>1. since asynchronies method first sends start bit, data , parity then
>stop bit what if my data is four bit
>long how does it work since stop bit is 10th bit how does receiver
>figure out that data is less then 8 bit.

The receiver always expects 8 data bits, so to send 4 bits just  don't
use the other 4.  Sending a start bit, 4 data bits, and a stop bit will
not work because the receiver would still be collecting data bits when
the start bit for the next 4-bit group arrives.  So there is a lot of
wasted time sending 4 bits, unless you start packing two 4-bit data
groups into each 8-bit transmission.

Some UARTs can be set for fewer than 8 bits.  The 16X50 chip in the PC
can be set for 5, 6, 7, or 8 bits.  Modes other than 7 or 8 bits are very
rare.  I don't know of anything that uses 6 bits.  Five bits are used for
the Baudot code of old Teletypes, but there is little to no PC software
that supports it in this manner.

>
>2. since start bit always is logic 1 stop bit is logic 0  is there any
>rule for this  to be always 1 for
>start bit 0 for stop bit . what if some one modifies it just to be
>opposite.

Then it won't work.  Actually the start bit is a logic 0 and a stop bit
is a logic 1.  This is how the data comes out at TTL level from most UART
chips.  The signal is then inverted so than on the RS-232 line, a
positive voltage is a start bit or logic 0, and a negative voltage is a
stop bit or logic 1.  There is a rule for this, it's what makes the
RS-232 interface useful by making every device use the same format.