US Patent 7224084 Interface circuit

When an AC adaptor is removed and a power supply to a power node NP of a ROM writing apparatus is stopped, an analog switch which is controlled by a voltage at the power node NP is turned off. Thus, even if a power voltage VTG of a user board side is applied, a data signal DAT which is outputted from the user board is shut off by the analog switch and does not reach the power node NP via a diode. Therefore, a control terminal of a 3-state buffer is certainly set to “L”. There is not a risk such that a pass current flows in the 3-state buffer and a breakdown by heat is caused. When a power source is shut off, a breakdown of an interface circuit due to a wraparound of a power voltage from another apparatus is prevented.