USART confusion

[allanw]

I'm a little confused by how USART works. I'm trying to interface an ARM Cortex-M3 with an XPort Pro for serial->ethernet.

Do I only need to connect the RX and TX lines? There's also a clock line which I'm guessing is for synchronous mode. And CTS and RTS. What's the best way to hook up these two devices together? They're on the same board, not discretely wired.

I ask because the docs for the Xport Pro seem to imply that CTS/RTS/DTR are optional (and there is no clock signal)

http://www.lantronix.com/pdf/XPort-Pro_IG.pdf page 8

[EEVblog]

I ask because the docs for the Xport Pro seem to imply that CTS/RTS/DTR are optional (and there is no clock signal)

Correct. You only need TX, RX and GND to implement what's called a software handshake null modem interface. This is very common (the most common these days?)
The other lines are used for what's called hardware handshaking between the two devices.

Maybe some more info here:
http://www.beyondlogic.org/serial/serial.htm

Dave.