This is a port used to connect a keyboard or a mouse to a PC. Usually coloured purple for a keyboard or green for a mouse, these small, round 6-pin connectors are now considered legacy ports and have generally been replaced by USB connections. However, gaming keyboards are often provided in PS/2 versions to allow an unlimited number of simultaneous keypresses to be detected. The use of PS/2 peripherals also allows all USB ports to be disabled where required for security reasons.
