MX860D1 データシートの表示（PDF） - MX-COM Inc

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MX860D1

Telephone Signaling Transceiver

MX-COM Inc  

Telephone Signaling Transceiver

9

CMX860 Advance Information

3.1 Ring Detector Interface

Figure 4 shows how the CMX860 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 4) 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 CMX860 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 DVSS 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 x [R20 + R22 + R23] / R23 ) x 0.707VRMS

where VtHI is the high-going threshold voltage of the Schmitt trigger A (see Section 6.1).

With R20-22 all 470kΩ as Figure 4, then setting R23 to 68kΩ will guarantee detection of ringing signals of

40Vrms and above for DVDD over the range 3V to 5V.

C20 R20

2-Wire

Telephone

Line

X R22

D1 - 4

RD

DVSS

R23

C21 R21

DVSS

CMX860

B

A

To Status

Register

RT

C22

R24

DVDD

Ring signal

Bridge rectifier output (X)

RT

Status Register bit 14

(Ring Detect)

VtHI DVSS

VtHI DVSS

Figure 4: Ring Signal Detector Interface Circuit

R20, 21, 22

470kΩ

C20, 21

R23

See text

C22

R24

470kΩ

D1-4

Resistors ±5%, capacitors ±20%, unless otherwise stated

0.1µF

0.33µF

1N4004

If the time constant of R24 and C22 is large enough then the voltage on RT will remain below the threshold of

the 'B' Schmitt trigger for the duration of a ring cycle.

The time for the voltage on RT to charge from DVSS towards DVDD can be derived from the formula:

VRT = DVDD x [1 - exp(-t/(R24 x C22)) ]

As the Schmitt trigger high-going input threshold voltage (VtHI) has a minimum value of 0.56 x DVDD, then the

Schmitt trigger B output will remain high for a time of at least 0.821 x R24 x C22 following a pulse at RD.

¤2000 MX-COM, Inc.

www.mxcom.com tel: 800 638 5577 336 744 5050 fax: 336 744 5054

Doc. # 20480222.001

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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