mike7411@gmail.com wrote:
> 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.
Emoveray ethay Igpay Atinlay otay eplyray
Ron Kritzman (ron@dbOnayAmspaYmasters.com)