Application Note: AN-118 Detecting Line Polarity Reversal Associated with BT Calling Line Identification Protocol AN-118: December 22, 2012 www.ixysic.com 1 Application Note: AN-118 The following circuit can be used to detect line polarity reversal per the British Telecommunications (BT) Standard CDS™ for Calling Line Service. 15V sensitivity to the reversal event per the BT standard. C1 provides DC blocking and allows current w during the initial charge of the circuit that occurs when DC voltage is rst applied to the TIP/Ring lines or a polarity reversal occurs. This current s through the U1 LED causing U1 phototransistor to saturate which causes the -REVB line to pulse low until C1 is charged. The back to back LED’s in U1 enable the circuit to operate regardless of polarity. It’s important to note that this circuit will also generate pulses for the incoming ring signal, therefore, the CPE microprocessor should have the ability to discriminate between the two events. This is easily accomplished since the line polarity is a one-time event and the ring signal generates many pulses at the ring signal frequency (16Hz-60Hz). The circuit can also be used as a ring detect circuit and reverse polarity detect for most countries including CTR countires, North America and in Asia. Line Polarity Reversal indicates to the Customer Premise Equipment (CPE) that an incoming message is about to follow and the CPE should be ready to decode the calling line information. Referring to the timing diagram in Figure 1, the line reversal event indicates that an incoming call is imminent. The circuit in Figure 2 should be connected across the Tip and Ring connections of the incoming telephone line. The circuit operates in a very similar manner to a standard ring detector circuit. D1 and D2 are 10V zener diodes that prevent false triggering for voltages approximately < 12V (10V zener + Vf diode + Vf LED) while also providing the required LINE SIGNALING ≥ 100mS < 4.8 sec 80−262mS ≥ 45mS < 4.8 sec 88−110mS TONE ALERT LINE REVERSAL CHANNEL SEIZE 45−75mS ≤ 2.5 sec MARK MESSAGE > 200mS RINGING REVB Pulse width is approximately 2mS at TIP/RING voltage of 15V and Values shown. 16mS at TIP/RING voltage = 48V Figure 1 TIP TIP R1 6.2k 1/4W D1 10V D2 10V +5V TO TELEPHONE LINE R2 10k 0.1W -REVB TO uC IXYS INTEGRATED CIRCUITS DIVISION LDA100 OR EQUIVALENT U1 6 1 5 2 4 3 C1 0.47uF 200V TO CPE DAA LDA100SM RING RING Figure 2 AN-118 www.ixysic.com 2 Application Note: AN-118 For additional information please visit our website at: www.ixysic.com IXYS Integrated Circuits Division makes no representations or warranties with respect to the accuracy or completeness of the contents of this publication and reser t descriptions at any time without notice. Neither circuit patent licenses nor indemnity are expressed or implied. Except as set forth in IXYS Integrated Circuits Division’s Standard Terms and Conditions of Sale, IXYS Integrated Circuits Division assumes no liability whatsoever, and disclaims any express or implied warranty, relating to its products including, but not limited to, the implied warranty of merchantability or a particular purpose, or infringement of any intellectual property right. The products described in this document are not designed, intended, authorized or warranted for use as components in systems intended for surgical implant into the body, or in other applications intended to support or sustain life, or where malfunction of IXYS Integrated Circuits Division’s product may result in direct physical harm, injury, or death to a person or severe property or environmental damage. IXYS Integrated Circuits Division reserves the right to discontinue or make changes to its products at any time without notice. Specification: AN-118-December 22, 2012 ©Copyright 2012, IXYS Integrated Circuits Division All rights reserved. Printed in USA. 12/22/2012 AN-118 www.ixysic.com 3