CMX624 CML Microcircuits, CMX624 Datasheet - Page 17

• Manufacturer Part Number: CMX624
• Description: V.23-Bell 202 Modem
• Manufacturer: CML Microcircuits

V23 / Bell 202 Modem
1.6.2
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
6 (Ring Detect) of the FLAGS Register.
The minimum amplitude ringing signal that is certain to be detected is:
where Vthi is the high-going threshold voltage of the Schmitt trigger A (see Section 1.7.1).
With R20-22 all 470k
40Vrms and above for V
SS
2001 Consumer Microcircuits Limited
by discharging the external capacitor C22. The output of the Schmitt trigger 'B' will then go high, setting bit
Ring Detector Interface
R20, 21, 22
R23
R24
DD
as Figure 9, then setting R23 to 68k
( 0.7 + Vthi x [R20 + R22 + R23] / R23 ) x 0.707 Vrms
over the range 3.0 to 5.5V.
Figure 9 Ring Signal Detector Interface Circuit
470k
See text
470k
Resistors 1%, capacitors 20%
17
C20, 21
C22
D1-4
will guarantee detection of ringing signals of
0.1 F
0.33 F
1N4004
CMX624
D/624/6