CMX624 Просмотр технического описания (PDF) - CML Microsystems Plc

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CMX624

V.23 / Bell 202 Modem

CML Microsystems Plc 

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 VSS by discharging the external capacitor C22. The output of the Schmitt trigger 'B' will

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 VDD over the range 3.0 to 5.5V.

R20, 21, 22

R23

R24

470kΩ

C20, 21

See text

C22

470kΩ

D1-4

Resistors ±1%, capacitors ±20%

0.1µF

0.33µF

1N4004

Figure 9 Ring Signal Detector Interface Circuit

 2003 CML Microsystems Plc

17

D/624/7

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