The port can be selected to operate as either a RS232 port or a RS485 port. ( See FAQ for more info on the difference )

Being bi-directional it allows you to both Transmit to and Receive from the connected device. Normally used when status information is required. With some video projectors you need to continually transmit the required command until the projector acknowledges it has received the signal, this is only possible with a bi-directional port. It also allows the gathering of status information.