CMX868P4 View Datasheet(PDF) - MX-COM Inc
Part Name
Description
MFG CO.
'CMX868P4' PDF : 43 Pages View PDF
Low Power V.22bis Modem
6
CMX868 Advance Information
3.1 Ring Detector Interface
Figure 3 shows how the CMX868 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 either C20 and R20 or C21 and R21 to appear at the top end of
R22 (point X in Figure 3) 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 CMX868 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 14 (Ring Detect) of the Status Register.
The minimum amplitude ringing signal that is certain to be detected is:
¨§ 0.7
©
VtHI R20
R22
R23
R23 ¸· 0.707VRMS
¹
where VtHI is the high-going threshold voltage of the Schmitt trigger A (see Section 6.1).
With R20-22 all 470k: as Figure 3, then setting R23 to 68k: will guarantee detection of ringing signals of
40VRMS and above for VDD over the range 3 to 5V.
C20 R20
2-Wire
Telephone
Line
D1 - 4
C21 R21
X
R22 RD
R23
CMX868
B
A
To Status
Register
RT
C22
VDD
R24
Ring signal
Bridge rectifier
output (X)
RT
Status Register bit 14
(Ring Detect)
Figure 3: Ring Signal Interface Circuit
VtHI VSS
VtHI
VSS
R20, 21, 22 470k:
C20, 21 0.1PF
R23
See text
C22
0.33PF
R24
470k:
D1-4 1N4004
Resistors r5%, capacitors r20%
Table 3: Ring Signal Detector Interface Circuit External Components
¤2001 MX-COM, Inc.
www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054
Doc. # 20480205.007
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
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