----- Original Message ----- From: Subject: [PIC] RS232, USART, and COM > What would be the difference between rs232, usart, and pc com port. > I understand the the pc com port uses a usart, so why it is called com > or rs232 and not usart? Actually, the PC com port might not use a usart, more likely it is a uart. 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. '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 -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist