This schematic is used to interface any RS-232 device such as a computer to a TTL driven device. The most typical Ham Radio aspect would be to control a rig. The circuit is very simple to construct and requires just a few external components. One chip can interface between 2 sets of devices allowing you to control multiple rigs from multiple serial ports.
PLEASE NOTE: I MADE AN ERROR IN THE PICTURE. The Chip is by NATIONAL SEMICONDUCTOR not Dallas. It will be corrected the next time I can provide a revised schematic.