V23 / Bell 202 Modem
CMX624
1.6.2 Ring Detector Interface
Figure 9 shows how the CMX624 may be used to detect the large amplitude Ringing signal received 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 9) 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 CMX624 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 V by discharging the external capacitor C22. The output of the Schmitt trigger 'B' will
SS
then go high, setting bit 6 (Ring Detect) of the FLAGS 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 R20-22 all 470kΩ as Figure 9, then setting R23 to 68kΩ will guarantee detection of ringing signals of
40Vrms and above for V
over the range 3.0 to 5.5V.
DD
R20, 21, 22
R23
R24
C20, 21
C22
D1-4
470kΩ
See text
470kΩ
0.1µF
0.33µF
1N4004
Resistors ±1%, capacitors ±20%
Figure 9 Ring Signal Detector Interface Circuit
2003 CML Microsystems Plc
17
D/624/7
CMLMICRO [ CML MICROCIRCUITS ]
