Patent ID: 7003612

Claim:
A PC switching device, installed between a keyboard without a power control key and a plurality of personal computers, intended for keyboards with power control keys, that are connected to the keyboard without a power control key, comprising: a plurality of power control switches corresponding respectively to the plurality of personal computers; recognizing means for recognizing that some of the plurality of personal computers, which correspond to at least one of the plurality of power control switches firstly pressed, are in a power-on state; selective inputting means for selectively inputting commands from the keyboard without a power control key to one of the plurality of personal computers; code transmitting means for transmitting codes assigned to the power control switches to certain ones of the personal computers and powering off the same when the certain ones of the personal computers in the power-on state are selected by the selective inputting means and the power control switches that correspond to the certain ones of the personal computers in the power-on state are pressed again; transistors controlling connecting states between power supply terminals of the plurality of personal computers for powering the keyboard and a power receiving terminal of the keyboard; comparators comparing a first voltage at each of the power supply terminals with a second voltage of the power receiving terminal, and turning on some of the transistors when the former is higher than the latter, but turning off other transistors when the former is lower than the latter; first voltage dividers dividing a voltage at each of the power supply terminals; and second voltage dividers dividing a voltage at the power receiving terminal by a ratio equal to that of a corresponding one of the first voltage dividers, wherein the first voltage is a voltage divided by each of the first voltage dividers, and the second voltage is a voltage divided by each of the second voltage dividers.