Low Power V.32 bis Modem
CMX869
4.2
Ring Detector
R20
R21
R22
C20
C21
D1, D2
D3
10kΩ, 0.5W
470kΩ
100Ω
0.33µF
18V zener
Opto isolator (NEC PS2701-1)
1N4004
D4
0.33µF, 250V
Resistors ±5%, capacitors ±20%. Typical Values only
Figure 3 Ring Signal Detector Interface Circuit
Figure 3 shows how the CMX869 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 C20, R20, D1 and D2, and appear at the terminals of D3. When the signal reaches the zeners’
(D1, D2) turn-on voltage, current will flow into the opto-isolator diode, turning on its output transistor and
discharging capacitor C21. Resistor R22 limits the current drawn by the opto-isolator output to ~30mA
peak. Whilst the ring tone is active and exceeds the zener voltage, the RDN node will be taken low and the
output of the Schmitt trigger will go high. The state of bit 14 (Ring Detect) of the Status Register directly
corresponds to the state of the Ring Detect Schmitt trigger output. If the corresponding interrupt mask bit
is set to 1, a C-BUS interrupt will be initiated (see the Status Register description in section 6.8). Note that
the optocoupler diode (D3) must be protected by a reverse voltage diode. If this is not available in a single
package, then an external IN4148 diode (D5) must be fitted, as shown.
The minimum amplitude ringing signal that is certain to be detected is: Vzener + Vdiode + Vopto + 2V.
This requirement is met with ample margin by ringing signals of 40Vrms or above, for DV
range 3.0V to 3.6V.
over the
DD
© 2004 CML Microsystems Plc
8
D/869/4
CMLMICRO [ CML MICROCIRCUITS ]
