Parallel off-hook detection for both line-available and phone pick-up

Abstract

A secondary communications device, such as a modem, a facsimile machine, an automatic meter reader or an answering machine, shares a telephone line with a subscriber's telephone and must grant the telephone primary access to the line. The present invention prevents the secondary device from seizing the line when the telephone is already off-hook and will terminate a line seizure by the secondary device if the subscriber picks up the telephone handset during use of the line by the secondary device. An average interval for charging a capacitor to a threshold voltage in response to an on-hook voltage appearing across the telephone line is determined. When the secondary communications device is ready to use the line, circuitry measures an interval for charging the capacitor in response to the current voltage appearing across the telephone line. The secondary communication device seizes the line only if the measured interval does not exceed the average interval by more than a predetermined amount. After seizure by the secondary communication device, a steady-state condition is brought about between the secondary device, the line steady-state impedance, and the central office voltage source. If the line impedance drops from the steady-state value, the circuitry assumes that the subscriber has taken the telephone off-hook and terminates the line seizure by the secondary device.