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APPENDIX A
VOLTAGE VS. CURRENT DETECTION OF ON AND OFF HOOK
The Whozz Calling? detects when the phone is on-hook and off-hook. This allows the unit to
determine the length of each phone call and report this detailed information to the software.
Telephone monitoring equipment can detect the occurrence of the On Hook and Off Hook
status of the phone line by two methods. It can either sense the voltage on the phone line itself
or sense current flow on a phone line passed through the equipment.
Voltages on incoming central office (CO) phone lines are consistent among all branches
throughout a physical location (building). If the voltage levels are in standard range,
monitoring equipment functioning in a voltage detection mode is able to sense On and Off
Hook on any branch in the building. The obvious advantage to this detection scheme is that
the monitoring equipment can be connected in parallel to the CO lines. Your unit comes from
the factory set to voltage detection mode because, in most locations, this detection scheme
works properly.
Current on phone lines only flows to the telephone device that is Off Hook. Monitoring
equipment that senses On and Off Hook by means of current flow must be connected between
the phone company’s incoming line and the device(s) that goes Off Hook. The current will
flow through the monitoring equipment to the Off Hook device. With this detection scheme,
any telephone device that makes or receives phone calls must be attached in series (i.e to the
output jacks) “behind” the monitoring equipment.
Voltage Detection Circuitry and Telephone Lines
In order to sense the voltage on the phone line, the unit’s circuitry must draw a small amount of
current from the phone line. The current draw is very small (70 mA, similar to a voltmeter),
and is drawn when the phone line is On Hook. The telephone company’s amplifier supplying
the power to your location may not be able to supply this current and still keep the On Hook
voltage level within the standard range. This occurs primarily in older residential and
commercial locations. An unsophisticated telephone company repair technician may diagnose
this small current draw by the unit as a “short” on the line. Therefore, the telephone company
will not address the shortcoming of their supply amplifier. If this situation occurs, the unit
cannot be used in the voltage detection mode, it must be reconfigured to the current detection
mode.
Summary of Contents for WHOZZ CALLING 2
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