Abstract: Over-voltage protection circuitry for use with a PCMCIA modem card or a mobile modem is disclosed. The protection circuitry detects excessive voltage between the tip and ring lines of a telephone subscriber loop when the modem is “off-hook”, and immediately places the modem “on-hook”, protecting the line interface circuitry, when the over-voltage condition is detected. The over-voltage protection circuitry of the present invention may share common components and signal lines with the modem's ring-detection circuitry, and may be implemented using a minimal number of additional components.

Abstract: Protection circuitry for use with a modem is provided that detects excessive voltage between the tip and ring lines of a telephone subscriber loop when the modem is off-hook. When the over-voltage condition is detected, a switch is active, creating a circuit path that directs current away from the DC-holding circuit of the modem, thereby protecting the modem. The protection circuit of the present invention may be implemented using a minimal number of simple and inexpensive components.