> Actually, the PC com port might not use a usart, more likely it is a uart.

Sorry, I mean UART, not USART!!

>
> RS232 is basically an electrical interface.  It specifies the mark and
space
> voltages, and the meaning of a number of signals.  It can be used with
> synchronous ports, but on the PC, the port is an asynchronous port.

Why does pc com port use RS232, is there a standard that requires this.
On the other hand, why do the pics use uart without rs232,  is this
a pic problem which will make it more complex to connect to pc?

>
> 'com' refers to the DOS name for the device, com standing for
> communications.  It's easier to say COM port than "RS-232 asynchronous
> serial port".
>
> A usart is a universal synchronous/asynchronous device.  Typically, it's
> output levels are either TTL or CMOS levels.  These are quite a bit
> different than RS232 levels.  However, it's usart-ness isn't defined by
the
> voltages, but rather, by it's ability to take 8 bit data and send it one
bit
> at a time, or accept one bit at a time and stuff it into groups of 8 (or
> so).  And it gets the extra S that a UART doesn't have by being able to
send
> and receive the data without the help of start and stop bits.
>
> A usart, driven in asynchronous mode can communicate with a PC COM port
> through level converters that take the usart voltages and translate them
to
> RS232 voltages.  Maxxim's MAX232 is probably the stereotypical RS-232
level
> converter, but there are others, and transistors will do the job just
fine,
> too.
>
> --McD
>

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