The CNV3020 line extender (and the USB version called CNV3023) is able to remote an RS232 type serial connection to over 500 meters away.
The peculiarity of these extenders is the ability to extend all the control signals (RTS, CTS, etc.) as well as the normal Rx and Tx lines.
The connection between the two units is made by a simple twisted pair, furthermore the local serial port and the remoted one are electrically isolated from each other.