Option unit detecting apparatus for electronic equipment

Claim 1

An option unit detecting apparatus for detecting a connection of an option unit to an electronic equipment, the option unit being connectable to or disconnectable from the electronic equipment wherein when the option unit is connected thereto, a current is flowed in the optional unit from the electronic equipment and when the option unit is disconnected therefrom, the current flow is interrupted, the electronic equipment including an electronic circuit having an input/output port, the option unit having at least one semiconductor device, said apparatus comprising: a reference voltage source for supplying a reference voltage, said reference voltage source being coupled to said input/output port for applying a reference voltage thereto; a switching means having an input terminal and an output terminal coupled to said reference voltage source, said switching means being selectively enabled in response to an enabling signal applied to said input terminal or disabled in response to a disabling signal applied to said input terminal, wherein said reference voltage is applied to said input/output port when said switching means is disabled and application of said reference voltage to said input/output port is interrupted when said switching means is enabled; and a driving means coupled to said switching means for applying said enabling signal to said input terminal when said current flow is interrupted and said disabling signal to said input terminal when said current is flowed in said semiconductor device, whereby whether or not said option unit is connected to said electronic equipment can be detected depending upon whether or not said reference voltage is applied to said input/output port.