Asus M3N WS - Manual (Page 56)

2-32 Chapter 2: Hardware information
7. Chassis intrusion connector (4-1 pin CHASSIS)
This connector is for a chassis-mounted intrusion detection sensor or switch.
Connect one end of the chassis intrusion sensor or switch cable to this
connector. The chassis intrusion sensor or switch sends a high-level signal to
this connector when a chassis component is removed or replaced. The signal
is then generated as a chassis intrusion event.
By default , the pin labeled “Chassis Signal” and “Ground” are shorted with
a jumper cap. Remove the jumper caps only when you intend to use the
chassis intrusion detection feature.
8. Optical drive audio connector (4-pin CD)
These connectors allow you to receive stereo audio input from sound sources
such as a CD-ROM, TV tuner, or MPEG card.