Abstract: 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.

Abstract: 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.

Abstract: A data transmission system includes a first device and a second device communicating with each other by electromagnetic induction. The first device includes a preparing circuit that prepares data having a voltage defined related to a first threshold, and a transmitting circuit that transmits the data. The second device includes a receiving circuit that receives the data from the transmitting circuit, a bias circuit that raises the voltage of the received data by a predetermined voltage to produce a sum voltage, and a judging circuit that judges whether the voltage of the data indicates high or low by comparing the sum voltage with a second threshold that is equal to or larger than the first threshold. The second device may also include a power supply that provides electric power for the first device using a magnetic field.