Circuit for sensing whether or not an add-in board is inserted into a bus connector of a mother board

Claim 1

A sensing circuit for sensing whether or not a first contact of a first electronic module unit is electrically connected to a second contact of a second electronic module unit, wherein said first electronic module unit includes an open-drain driver connected to said first contact, and said second electronic module unit includes a pull-up resistor connected between said second contact and a logic level voltage, such that when said first and second contacts are electrically connected, a voltage on the connection of said first and second contacts is pulled up close to said logic level voltage when said open-drain driver is turned off, and pulled down to a ground reference voltage when said open-drain driver is turned on, said sensing circuit comprising: a pull-down resistor connected between said first contact and a ground reference voltage, wherein said pull-down resistor has a resistance value greater than a resistance value of said pull-up resistor of said second electronic module, such that, when said first and second contacts are electrically connected and said open-drain driver is turned off, a voltage on a node defined at the connection between said pull-down resistor and said first contact is substantially pulled up to said logic level voltage, and when said first and second contacts are not electrically connected and said open-drain driver is turned off, said voltage on said node is substantially pulled down to said ground reference voltage; and means for communicating a logic level indicative of said voltage on said node defined at the connection between said pull-down resistor and said first contact, said communicating means including an addressable tri-state driver having an input connected to said node defined at the connection between said pull-down resistor and said first contact, and an output indicative of a voltage on said node defined at the connection between said pull-up resistor and said first contact, wherein said first electronic module unit has a third contact connected to said output of said addressable tri-state driver, and said second electronic module unit has a fourth contact electrically connected to said third contact of said first electronic module unit, such that a host computer on said second electronic module unit enables said addressable tri-state driver after addressing said addressable tri-state driver.