Specifications

157
Dual Dial Click Option (for 702-9117)
Dual Dial Click Option (for 702-9117)
In some rural areas, DTMF (tone) dialing is not normally available or used. This makes it
difficult to use the Model 2200 on end-to-end telephone lines and require the caller to dial
in the pager number. All that comes through from a rotary dial telephone is audible clicks
from the telephone's contacts opening and closing as the dial turns. This type of telephone
can be supported on the Terminal by adding the Dial Click Detector option board and
companion software.
For use with rotary dial telephones, the telephone system must pass residual audio clicks
of the dial turning, without disconnecting the caller. The optional “dial click” detector,
with special software converts these clicks into a usable pager number. This conversion
process generally works, but each installation should be tested before having callers rely
upon overdial click interpretation. Note that most electronic “Universal Dial” phones with
FET loop current interrupters do not provide clicks that are sharp enough to be passed
through phone equipment.
The dial click detector processes the incoming audio clicks from the telephone line and
converts them into high/low digital pulses that are read by software. Since dials turn at
different and varying speeds, and sometimes produce extra clicks during their rotation, the
software is designed to be extra smart in interpreting these clicks and converting them
back into the 0-9 digits dialed by the caller. The process is not 100% perfect and it is
strongly advised that dialing via several different phone company central offices should be
attempted before assuring subscribers that their paging will always work correctly. If dial
click detection proves unsatisfactory, a DID (direct inward dial) line will be needed to
page reliably from rotary telephones.
The Dual Dial Click card performs the detection function on two separate audio channels
for Dual Trunk Card applications. The Dual Dial Click card processes two audio channels
and produces a separate logic 1 output for each click of audio from the corresponding
telephone input.
Trunk A audio comes in on J5 pin 7 and the resulting digital signal is output to J5 pin 8.
Trunk B audio comes in on J5 pin 9 and the resulting digital signal is output to J5 pin 10.
Each channel of audio passes through its own band-pass filter to remove any speech
component and properly shape the pulses. The filter for trunk A is built around half of U2
and the filter for trunk B is built around half of U5. Variable gain amplifiers (Trunk A -
half of U1, Trunk B - half of U4) allow for field adjustment of the filter output to a
constant peak level.
The output of these amplifiers is full-wave rectified by the other halves of U1 for Trunk A
and U4 for Trunk B. The resulting waveforms are used to drive the comparators
comprised of half of U2 for Trunk A and half of U5 for Trunk B. The outputs of the
comparators are inverted via transistors Q1 for Trunk A and Q2 for Trunk B. The
inversion results in a digital logic 1 being present on the corresponding output pin
whenever a dial click is detected. The comparator outputs also drive detector lamps DS1
(Trunk A) and DS2 (Trunk B).