Re: On-Hook Signal

Anyone know exactly what the on-hook signal for a plain old telephone

> looks like in terms of voltages? The information on the web is real > contradictory.

The info may appear contradictory because there are no standards in terms of voltage. On and off hook states on a POTS line are a function of loop current. Battery (power) is provided by the central office, and an on-hook telephone is an open circuit. To go off hook you close the circuit and current flows.

What you would see with a voltmeter across tip and ring (the two wires of a basic phone pair) is the voltage drop across the middle resistor of three in series. The middle one is the phone itself and the other two are the resistance of the cable back to the C.O. When the phone is on-hook the resistance is infinite, so the voltage will be essentially whatever the CO is providing. For a residential POTS line this is typically in the range of 48-52 volts. When the phone is off hook, the drop across the resistor is typically 6-12 volts. You need to know that this is not reliable at all. The phone may not be connected to a C.O. It may be wired to a PBX, intercom, or a VOIP adapter providing

24v, 12v or who knows what. You may be far from the CO and the loop resistance may be very high. The voltages can and will be all over the place, and reading of zero volts could just as easily be a short as an open.

The end result: The voltage across the line can give you a quick and dirty guess as to whether the phone is on or off hook. If you want to know for sure, look at loop current.

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Reply to
Ron Kritzman
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