There is a standard called RS232.  
http://www.camiresearch.com/Data_Com_Basics/RS232_standard.html

Google gives tons of information when you type "rs232 standard"

(-:

> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of kenasw@btinternet.com
> Sent: Friday, May 13, 2005 11:55 AM
> To: Microcontroller discussion list - Public.
> Subject: Re: [PIC] RS232, USART, and COM
> 
> 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?
> 


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