CMX624P4 View Datasheet(PDF) - MX-COM Inc
Part Name
Description
MFG CO.
'CMX624P4' PDF : 25 Pages View PDF
Bell 202 and V.23 Compatible Modem
17
CMX624 Preliminary Information
5.2 Ring Detector Interface
Figure 14 shows how the CMX624 may be used to detect large amplitude Ringing signals received at the
start of an incoming telephone call.
The ring signal is 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 in
a rectified and attenuated form. See point X in Figure 14.
The signal at point X is further attenuated by the R22 and R23 divider before being applied to the RD input. If
the amplitude of the signal appearing at RD is greater than the input threshold (VtHI) of Schmitt trigger 'A' then
the transistor (Q1) 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 found by the following calculation:
0.7 +
Vt HI
×
[R20 + R22
R23
+
R23]
×
0.707VRMS
= Min. Ring Signal VRMS
where VtHI is the high-going threshold voltage of the Schmitt trigger A. See Figure 15 and Section 6.1.3.
With R20-22 at 470kΩ as shown Figure 14, 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
CMX624
B
A
Q1
To FLAGS
register
RT
C22
R24
VDD
Ring signal
Bridge rectifier output (X)
RT
FLAGS register bit 6
(RING DETECT)
FLAGS register bit 5
(RING DETECT Change)
Figure 14: Ring Signal Detector Interface Circuit
VtHI VSS
VtHI
VSS
R20,21,22
R23
R24
Note 1
470kΩ
See text
470kΩ
±1%
±1%
±1%
C20,21
C22
D1-4
Note 1
0.1µF
0.33µF
1N4004
±20%
±20%
±20%
Tolerances for Resistors and Capacitors are as indicated unless otherwise stated.
Table 12: Ring Signal Detector Interface Circuit components
©1999 MX-COM, Inc.
www.mxcom.com Tel: 800 638 5577 336 744 5050 Fax: 336 744 5054
Doc. # 20480180.108
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
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