Plug-in device discrimination circuit and method

A plug-in device discrimination circuit, comprising:

• a jack having at least three contacts and which is suitable for mating with a plug connected to a plug-in device which contains at least two circuits, one of said contacts being a common contact, each of said plug-in device circuits connected to one of said non-common contacts when said plug and jack are mated and having a respective electrical resistance between its non-common contact and said common contact,
• a comparison circuit connected to said jack contacts and arranged to compare the associated resistances of at least two of said plug-in device circuits, and to produce an output which varies with the results of said comparison,
• a switching network arranged to route signals between said comparison circuit and said jack contacts, and
• a controller arranged to control said switching network such that said comparison is enabled and said comparison circuit output is produced.

A method of discriminating between types of plug-in devices, each of which can be mated with a particular jack, comprising:

• detecting when a plug is inserted into a jack having a common contact and at least two non-common contacts,
• applying respective currents to each of said non-common contacts,
• comparing the respective voltages that appear at each of said non-common contacts in response to said applied currents, and
• producing an output which indicates the equality relationships between said respective voltages.