Calling Line Identifier
MX602 - PRELIMINARY INFORMATION
10
4.8 Ring or Line Polarity Reversal Detector
These circuits are used to detect the Line Polarity Reversal and Ringing signals associated with the Calling Line
Identification protocol. Figure 10 illustrates their use in a typical application.
DET
From Tone Alert,
Energy Detector and
Data Retiming blocks
C3
C4
R3
R4
IRQ
D1-D4
A
Line
B
Line
Protection
Network
ZP
R1
B
A
RD
MODE
R2
RT
C5
R5
VDD
Ring Signal
VtHI
VSS
RD
RT
VtHI
VSS
IRQ (ZPLO and/or MODELO
)
DET (ZPHI)
Figure 10: Ring or Line Polarity Reversal Operation
When no signal is present on the telephone line, RD will be at V and RT pulled to V by R5 so the output of the
SS
DD
Schmitt trigger 'B' will be low.
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 C3 and R3 or C4 and R4 to appear at the top end of R1 (point X in Figure 10)
in a rectified and attenuated form.
When the amplitude of the signal appearing at RD is greater than the input threshold (Vt ) of Schmitt trigger 'A' then the N
HI
transistor connected to RT will be turned on, pulling the voltage at RT to V by discharging the external capacitor C5.
SS
The output of the Schmitt trigger 'B' will then go high, activating the DET and/or IRQ outputs depending on the states of
the MODE and ZP inputs.
The minimum amplitude ringing signal that is certain to be detected is :
R1+R2 +R3
(
)
0.7 + Vt
0.707V
(
RMS
)
HI
R2
Where Vt is the high-going threshold voltage of the Schmitt trigger A (see section 6.1).
HI
With R1, R3 and R4 all 470kΩ as indicated in Figure 2, then setting R2 to 68kΩ will guarantee detection of ringing signals
of 40V
and above, for V , over the range 3.0 to 5.5V.
RMS
DD
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