Telephone Signalling Transceiver / Least Cost Router
CMX860
1.4.1
Ring Detector Interface
Figure 3 shows how the CMX860 may be used to detect the large amplitude ringing signal voltage
present on the 2-wire line at the start of an incoming telephone call.
The ring signal is usually applied at the subscriber's exchange as an ac voltage inserted in series with
one of the telephone wires and will pass through either C20 and R20 or C21 and R21 to appear at the top
end of R22 (point X in Figure 3) in a rectified and attenuated form.
The signal at point X is further attenuated by the potential divider formed by R22 and R23 before being
applied to the CMX860 RD input. If the amplitude of the signal appearing at RD is greater than the input
threshold (Vthi) of Schmitt trigger 'A' then the N transistor connected to RT will be turned on, pulling the
voltage at RT to DV
SS
by discharging the external capacitor C22. The output of the Schmitt trigger 'B'
will then go high, setting bit 14 (Ring Detect) of the Status Register.
The minimum amplitude ringing signal that is certain to be detected is:
( 0.7 + Vthi x [R20 + R22 + R23] / R23 ) x 0.707 Vrms
where Vthi is the high-going threshold voltage of the Schmitt trigger A (see section 1.7.1).
With R20 - R22 all 470kΩ as Figure 3, then setting R23 to 68kΩ will guarantee detection of ringing
signals of 40Vrms and above for DV
DD
over the range 3V to 5V.
R20, 21, 22
R23
R24
470kΩ
See text
470kΩ
C20, 21
C22
D1-4
0.1µF
0.33µF
1N4004
Resistors
±5%,
capacitors
±20%
Figure 3 Ring Signal Detector Interface Circuit
©
2002 Consumer Microcircuits Limited
10
D/860/5
CMLMICRO [ CML MICROCIRCUITS ]
