Dell Latitude C540 - Manual (Page 15)

Docking Connector
NOTE: Docking devices may not be available in all countries.
Connects the optional docking device. A docking device allows you to easily use
external devices with your computer, such as an external keyboard, mouse, and
monitor.
See the documentation that came with your docking device for additional information.
PS/2 Connector
NOTE: You can use the integrated keyboard and an external keyboard at the same time. When you
attach a keyboard or numeric keypad, the keypad is disabled.
Connects PS/2-compatible devices, such as a mouse, keyboard, or external numeric
keypad.
Shut down the computer before attaching or removing a PS/2-compatible device. If
the device does not work, install the device drivers from the floppy disk or CD that
came with the device, and restart the computer.
USB Connector
Connects USB 1.1-compliant devices, such as a mouse, keyboard, or printer.