what does the common interface do ?

This from Wikipedia...

The Common Interface is an extensible digital interconnect found in the digital TV market.

All Common Interface equipment must comply with the EN 50221-1997 standard.

The interconnect is formed between a host and a module. Examples of a host are a digital television or digital set-top box. The EN 50221 specification allows many types of module but only the Conditional Access Module (CAM) has found popularity because of the Pay TV market.

Peccavi
March 2010