CMX644AD2 View Datasheet(PDF) - MX-COM Inc
Part Name
Description
MFG CO.
'CMX644AD2' PDF : 31 Pages View PDF
Bell 212A / V.22 Modem with Call Progress and DTMF
22
CMX644A Preliminary Information
5.2 Ring Detector Interface
Figure 12 shows how the CMX644A 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 12) 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 CMX644A 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 TONES DETECT register.
The minimum amplitude ringing signal that is certain to be detected is
0.7 +
Vt HI
×
[R20 +
R23
R22
+
R23]
×
0.707VRMS
=
Min. Ring Signal
VRMS
Where VtHI is the high-going threshold voltage of the Schmitt trigger A
With R20 - 22 all 470kΩ as Figure 12, 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.
2-Wire
Telephone
Line
C20 R20
X
D1 - 4
R22 RD
R23
C21 R21
CMX644A
B
A
Q1
To TONES DETECT
Register
RT
C22
R24
VDD
Ring signal
Bridge rectifier output (X)
RT
TONES DETECT register bit 6
(RING DETECT)
FLAGS register bit 5
(RING DETECT Change)
Figure 12: Ring Signal Detector Interface Circuit
VtHI VSS
VtHI
VSS
R20,21,22
R23
R24
470kΩ
See text
470kΩ
C20,21
C22
D1 - 4
0.1µF
0.33µF
1N4004
Resistors ±1%, capacitors ±20%
Table 27: Ring Signal Detector Interface Circuit
2000 MX-COM, Inc.
www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054
Doc. # 20480197.006
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA All trademarks and service marks are held by their respective companies.
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