| 摘要 |
In an environment where a contact of a mother board is connected to a logic HIGH level voltage through a strong pull-up resistor, and a corresponding contact of an add-in board is connected to an open-drain driver, such that a control line driven by the open-drain driver is provided through the two contacts in normal mode operation when the two contacts make electrical connection, a circuit to sense whether or not the contact of the add-in board is making electrical connection with the contact of the mother board includes a weak pull-down resistor connected at one end to a node connecting the output of the open-drain driver to the contact of the add-in board, and connected at the other end to a ground reference. Accordingly, a voltage detected on the node is pulled up close to the logic HIGH level voltage if the contact of the add-in board is making electrical connection with the contact of the mother board and the open-drain driver is turned off, and is pulled down close to ground if the contact of the add-in board is not making electrical connection with the contact of the mother board and the open-drain driver is turned off. The voltage thus detected is provided to other circuitry on the add-in board, and/or back to a host computer on the mother board.
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