| 摘要 |
An internet protocol telephone includes a substrate having an input and an output that are capable of being connected to the internet protocol (IP) network. A relay is disposed on the substrate and is connected between the input and the output of the substrate. The relay includes first and second native FETs that have a threshold voltage of approximately zero volts. Therefore, the relay is nominally turned-on, even when little or no voltage (or power) is applied to the IP telephone substrate, as during the discovery mode of IP telephone operation. During discovery mode, The IP phone is configured to be responsive to extended link pulses and block data packets that are associated with legacy devices. Data packets have a higher signal duration and are more continuous than extended link pulses. The IP phone includes a switchable ground that is connected to the gates of the native devices, and is controlled by a rectifier and filter circuit that are connected to the substrate input. If the IP phone receives legacy data packets during discovery mode, then the high signal duration and continuous nature of the data packets are sufficient to cause the rectifier to generate a rectified signal having sufficient amplitude to activate the switchable ground, so as to ground the gates of the native devices and therefore turn-off the native devices. Therefore, the data packets are rejected and are not passed back to the switch. Extended link pulses have a frequency that is too low to generate a rectified signal that is sufficient to activate the switchable ground, and therefore the native devices remain turned-on. Accordingly, the extended link pulses are passed back to the switch.
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