| 摘要 |
<p>PURPOSE: To prevent the port of a microcomputer on the side of a connection unit from being subjected to application of unjust voltage by turning off a serial communication circuit part when the power source of the connection unit is off. CONSTITUTION: A down signal from a portable telephone set 1 is received by pull-up by a resistor 35 on the side of the connection unit 2. Consequently, when the power source Vb of the unit 2 is in an On state and data is not transmitted from the side of the port 12, a transistor Tr21 is turned on and Tr30 is turned on. Then high frequency components are removed by a resistor 26 and the capacitor 28 so that the base of Tr27 becomes H when the power source Vb is on and L when Vb is off. When the base of Tr27 becomes H, Tr27 is turned on to turn on Tr30. As the result of this, a power source Va is applied to the resistor 32 of an up signal inputted to the port 5 ofμCPU 4, and to the base of Tr13 of a down signal outputted from the port 12 ofμCPU 4, H is inputted to enable serial communication operation.</p> |
