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This circuit describes a circuit for the improved detection of step by step selector switch cut-through in telephone offices by affording a more simple and less ambiguous detection than heretofore.
English (United States)
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Switch-Through Detector Circuit
This circuit describes a circuit for the improved detection of step by step
selector switch cut-through in telephone offices by affording a more simple and
less ambiguous detection than heretofore.
Optical coupler 1 is connected to the ring leads. The output of coupler 1 is
normally high while a call is being processed. The output goes low in response
to a ring lead current drop.
If the selector switch connected to the ring lead switches through, the current
in the ring lead drops for 4 to 30 milliseconds, causing the input voltage to driver
2 to change from ground to -V for the duration of the drop. Edge trigger circuit 3
receives this transition and triggers single-shot circuit 4 to produce a 50-
millisecond minimum duration pulse to appear at node OUT via OR gate 5. After
the switch-through, a valid stop dial condition may be present for several
seconds at the input node of the switch-through detector circuit. In this case, a
stop dial pulse overrides the single-shot pulse via the other input of OR gate 5 to
provide a pulse lasting for the duration of the stop dial pulse. The presence of a
pulse at the OUT node denotes a switch-through condition.
If the selector switch condition is a blocked call, no change in the ring lead
current will occur. A computer monitoring the output of this circuit can interpret
unambiguously the absence of a pulse to signify a blocked call and the presence
of a pulse to designate a switch-through.