| 摘要 |
A first electronic device, such as a computer, is connected to a second electronic device, such as a printer, over a communication bus. The second electronic device has device specific integrated circuitry for performing the functions of the device, and also has pad driver circuitry. The pad driver circuitry is connected between the communication bus and the device specific integrated circuitry. The first electronic device is powered by a first voltage source, such as AC power through an on/off switch. The second electronic device is powered by a second voltage source, such as AC power through a different on/off switch. In addition to being powered by the second voltage source, the pad driver circuitry is also powered by a third voltage source, isolated from the second voltage source. When the pad driver circuitry in the second device detects that the first device is powered-on and the second device is powered-off, it assures that the device specific circuitry in the second device will not become inadvertently powered-on by the first device through the communications bus. Pad driver circuitry also assures that power from the first device through a data line on the communications bus is not inadvertently fed back to the first device through a different data line on the communications bus. Pad driver circuitry also assures that current from the first device is not sunk (i.e. finds a path to ground) through the pad driver circuitry.
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