On May 13, 2005, at 4:24 AM, Olin Lathrop wrote:

>> What would be the difference between rs232, usart, and pc com port.

COM is a logical device name, more or less specific to windows/dos.
(it would be /dev/tty under most unix systems, for instance.)

Normally, COMn connects to to a particular type of asynchronous serial
communications port.  "async serial communications" implies a standard
for bit ordering, timing, synchronization, and error conditions that
allow for sending computer data over a single wire (in each direction.)
(X.20 describes it, I think, although a uart manual probably does 
better.)
A UART is the common name for the device that interfaces to a computer 
on
one side and the serial port on the other.  However, a uart is digital
logic device, so the serial output is still at digital logic levels.

To move serial data across significant distances, digital logic levels
are not ideal, so there are additional standards that describe how to
shift those levels for more reliable long-distance use.  rs232 is one
of these standards.  It's a rather complete standard defining how two
devices talk to each other with serial ports.  It is the most widely
violated standard in the world, and includes lots of bits and functions
that no one uses any more (it specifies a 25 pin connector, and has a
use for nearly all of the 25 pins.)  When most people say rs232, they're
talking about the most basic pieces - the voltage levels representing
a zero or one on each signal line.  These voltages are "inconvenient" 
for
digital logic to produce, and so there's usually a separate chip to
do the conversion.

The three are completely orthogonal; you can have com ports that are
are actually network connections, serial ports that aren't "async",
serial ports that aren't rs232, and rs232 ports that aren't async 
serial.

BillW
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