Line current protection circuit for use with a PCMCIA- architecture modem card

摘要

A novel line-current protection circuit that is useable with a PCMCIA modem card is disclosed. The protection circuit is adapted to detect when an excessive voltage is present across the tip and the ring leads of a telephone subscriber loop by sensing the amount of line- current being supplied to line interface circuitry disposed on the PCMCIA modem card. The protection circuit includes a current sensing circuit portion that is placed in series with the tip lead and which detects when the line current exceeds a predetermined maximum amount. In the event the line current exceeds the maximum amount, the current sensing circuit asserts an excessive line current signal which is being monitored by a digital processor. The digital processor responds by causing a relay circuit to be opened, thereby causing the PCMCIA modem to be in an "on- hook" state. When the magnitude of the line-current does not exceed the predetermined maximum amount, the excessive line current signal is not asserted and the digital processor responds by closing the relay circuitry, thereby causing the PCMCIA modem to be in a "off-hook" state. The protection circuit also includes a backup fuse, which will trip if the primary components of the protection circuit fail.