Cash drawers are either serial port or receipt printer interfaces. Drawers with serial interfaces plug into a serial port (COM port) on a computer. Receipt printer interface drawers plug into a special connector on most receipt printers. There also is an external interface that plugs into a serial or parellel port that will allow you to connect up to two receipt printer cash drawers to one computer. Next, your point-of-sale software sends a special command to either the serial port, receipt printer or external interface, depending on your type of drawer, that acts as a switch that opens the drawer.