Dell Studio 1558 (Early 2010) - Manual (Page 320)

320
Ports
Keyboard and Mouse Port (PS2)
The PS/2 standard, introduced by IBM in 1987, stands for Personal System/2.
PS/2 port is commonly used to plug in a keyboard or mouse.
Pin Signal
1 Keyboard/mouse data
2 (Reserved)
3 Ground
4 Keyboard/mouse power
(+5 V)
5 Keyboard/mouse clock
6 (Reserved)