Calling Line Identifier with VMWI
CMX612
1.5.9 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 8 illustrates their use in a typical application.
Figure 8 : Ring or Line Polarity Reversal operation
When no signal is present on the telephone line, RD will be at VSS and RT pulled to VDD by R5 so the
output of the 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 8) in a rectified and attenuated form.
The signal at point X will be further attenuated by the potential divider formed by R1 and R2 before being
applied to the CMX612 input RD . 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 C5. The output of the Schmitt trigger 'B' will
then go high, activating the DET and/or IRQN outputs depending on the states of the MODE 1 and
MODE 2 inputs.
The minimum amplitude ringing signal that is certain to be detected is:
ã 2002 Consumer Microcircuits Limited
12
D/612/3
CMLMICRO [ CML MICROCIRCUITS ]
