Patent Publication Number: US-2007121923-A1

Title: Telephone line communication interface

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      The application relates to and claims priority from provisional patent application Ser. No. 60/717,299, titled “Telephone Line Communication Interface,” filed Sep. 15, 2005, the complete subject matter of which is hereby incorporated by reference in its entirety. The application also relates to patent application Ser. No. 11/321,262, titled “Direct Access Arrangement Device,” filed Dec. 29, 2005, the complete subject matter of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION  
      This invention relates generally to alarm systems, and more generally to, a communication interface between an alarm system and a telephone network.  
      Security alarm systems are utilized in a variety of applications in both residential and commercial environments. Security alarms monitor one or more remote components and, based upon feedback from the remote components, carry out various security and emergency related functions. Security alarm systems typically communicate with one or more remote terminals, such as at a host or central operations terminal, over conventional phone lines maintained within the phone network.  
      Security alarm systems generally include a security panel joined to a modem that provides bidirectional communication over the phone network. The modem conveys security and emergency related data at various connection speeds (e.g. 2400 bps) between the phone network and the security panel.  
      A telephone line communication interface may be placed between the modem and/or the security panel, and the incoming phone line(s). The communication interface works to transfer control from the house phone to the security panel when the security panel requests to transfer data to a monitoring station over the phone network. Each phone network operates with a standardized profile of parameters such as line input and output levels, signal attenuation, line impedance and the like. One example of an average US line profile is a line impedance of 600 ohms, a line output level of approximately −23.5 dBm, a line input level of −10 dBm, and a line attenuation of 13.5 dBm. The communication interface provides the interface to the phone network by matching impedance levels, ring levels, and the like. Different countries and geographic regions have different line requirements which, in the past, have typically required many different build configurations of the communication interface which increases the cost.  
      The phone line typically has two wires interfacing with the communication interface which are referred to herein as TIP and RING. When the alarm system goes off-hook requesting a phone line, there is a level of voltage across TIP and RING. The tip-to-ring voltage may change based on the length of the phone line, wherein a longer phone line results in a lower tip-to-ring voltage as a longer phone line represents a higher resistance in the phone wire. Typically, communication interfaces have used a transmit opto-coupler and a receive opto-coupler connected in series with one another. A minimum amount of off-hook voltage is required for the opto-couplers to operate properly, thus limiting the operable length of the phone line.  
      Therefore, a need remains for a communication interface which meets the requirements of different countries with a minimal number of build configurations and which requires less tip-to-ring voltage when in an off-hook condition to enable operation over longer phone lines.  
     BRIEF DESCRIPTION OF THE INVENTION  
      In one embodiment, a telephone line communication interface (TLCI) module is configured to interface a security panel with a phone line of a phone network and comprises receive, transmit and hook control opto-couplers. The receive opto-coupler has a receive input side and a receive output side. The receive input side includes a receive input line configured to receive a signal on the phone line from the phone network and the receive output side includes a receive output line configured to convey the signal to the security panel. The transmit opto-coupler has a transmit input side and a transmit output side. The transmit input side has a transmit input line configured to receive transmit signals from the security panel and the transmit output side has a transmit output line configured to convey the transmit signal to the phone line of the phone network. The transmit output side is joined in parallel with the receive input side of the receive opto-coupler. The hook control opto-coupler has a hook input side and a hook output side. The hook input side has a hook control line configured to receive a hook signal from the security panel. The hook output side has a hook input line and a hook output line. The hook output line activating a hook switch to convey the hook signal to the phone line of the phone network. The hook output side is joined serially with the receive input side of the receive opto-coupler.  
      In another embodiment, a security system comprises a security panel for performing control operations associated with at least one of security and emergency functions. A TLCI module is configured to interface the security panel with a phone network and comprises receive, transmit and hook control opto-couplers. The receive opto-coupler has a receive input side and a receive output side. The receive input side includes a receive input line configured to receive a signal on the phone line from the phone network and the receive output side includes a receive output line configured to convey the signal to the security panel. The transmit opto-coupler has a transmit input side and a transmit output side. The transmit input side has a transmit input line configured to receive transmit signals from the security panel and the transmit output side has a transmit output line configured to convey the transmit signal to the phone line of the phone network. The transmit output side is joined in parallel with the receive input side of the receive opto-coupler. The hook control opto-coupler has a hook input side and a hook output side. The hook input side has a hook control line configured to receive a hook signal from the security panel. The hook output side has a hook input line and a hook output line. The hook output line activates a hook switch conveying the hook signal to the phone line of the phone network. The hook output side is joined serially with the receive input side of the receive opto-coupler.  
      In another embodiment, a TLCI module is configured to interface a panel with a phone network and comprises receive, transmit and hook control opto-couplers, and means for matching impedance of the phone line of the phone network. The impedance of the phone line at least one of set at predetermined resistance and capacitance values and at least one range of resistance and capacitance values corresponding to at least one predetermined range of frequencies. The receive opto-coupler has a receive input side and a receive output side. The receive input side includes a receive input line configured to receive a signal on the phone line from the phone network and the receive output side includes a receive output line configured to convey the signal to the security panel. The transmit opto-coupler has a transmit input side and a transmit output side. The transmit input side has a transmit input line configured to receive transmit signals from the security panel and the transmit output side has a transmit output line configured to convey the transmit signal to the phone line of the phone network. The transmit output side is joined in parallel with the receive input side of the receive opto-coupler. The hook control opto-coupler has a hook input side and a hook output side. The hook input side has a hook control line configured to receive a hook signal from the security panel. The hook output side has a hook input line and a hook output line. The hook output line is configured to convey the hook signal to the phone line of the phone network. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates a block diagram of a security alarm system that is formed in accordance with an embodiment of the present invention.  
       FIG. 2  illustrates a block diagram of a portion of the TLCI module formed in accordance with an embodiment of the present invention.  
       FIG. 3  illustrates a block diagram of the TLCI module interconnected with the internal phone lines, external phone lines, and the security panel in accordance with an embodiment of the present invention.  
       FIG. 4  illustrates a schematic diagram of the TLCI module in accordance with an embodiment of the present invention.  
       FIG. 5  illustrates a chart of country/regions, corresponding telephone line impedance and tolerance requirements, and return loss requirements over the associated frequency range in accordance with an embodiment of the present invention.  
       FIG. 6  illustrates a chart of build options, associated country/regions, and examples of component values which may be used for the impedance matching network in accordance with an embodiment of the present invention.  
      The foregoing summary, as well as the following detailed description of certain embodiments of the present invention, will be better understood when read in conjunction with the appended drawings. To the extent that the figures illustrate diagrams of the functional blocks of various embodiments, the functional blocks are not necessarily indicative of the division between hardware circuitry. Thus, for example, one or more of the functional blocks (e.g., processors or memories) may be implemented in a single piece of hardware (e.g., a general purpose signal processor or a block or random access memory, hard disk, or the like). Similarly, the programs may be stand alone programs, may be incorporated as subroutines in an operating system, may be functions in an installed software package, and the like. It should be understood that the various embodiments are not limited to the arrangements and instrumentality shown in the drawings. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  illustrates a block diagram of a security alarm system  10  that is formed in accordance with an embodiment of the present invention. The system  10  includes a security panel  12  configured to perform various security and emergency related functions. The security panel  12  may include, among other things, a processor module  14 , memory  16 , and modem  42 . A telephone line communication interface (TLCI) module  44  may interface with the modem  42  and/or the security panel  12 , and may be integrated with, or separate from, the security panel  12 . The TLCI module  44  is interconnected with external telephone (phone) lines  46  of a phone network central office (CO)  48  as well as internal phone line(s)  28  connected to house phone(s)  34 . The security panel  12  may connect to and receive communications from a monitoring station  30  via the TLCI module  44 , phone lines  46  and phone network CO  48 . Voice and data are conveyed through the modem  42 , TLCI module  44  and phone lines  46  bi-directionally.  
      The phone lines  46  have a line impedance  47  which is determined by the country or geographic area or region. Different line impedances  47  exist, and the TLCI module  44  has an impedance matching circuit  78  having components chosen to accommodate multiple countries and areas. Therefore, instead of each country requiring a different build of the TLCI module  44 , a minimal number of build options, such as one, two or three build options, may be provided. Therefore, each build option is configured to meet the requirements of multiple countries or areas.  
      The security panel  12  communicates over a single, common communications bus  18  with various components, such as keypad  20 , exterior audio station  22 , interior audio station  24 , GSM cellular communicator  26 , video verification module  32 , cameras  36  and the like. As shown in  FIG. 1 , the modem  42  is a separate component from the processor module  14 . Optionally, the modem  42  may be part of the processor module  14 . As a further option, the modem  42  may communicate with the processor module  14  over the bus  18  or over a separate dedicated bus (not shown). The security panel  12  may also be joined to wireless sensors  38  through a wireless link  40 . The wireless link  40  may represent an RF link, an IR link and the like. The number of video cameras  36 , key pads  20 , exterior audio stations  22 , interior audio stations  24 , GSM cellular communicators  26 , video verification modules  32 , modems  42  and wireless sensors  38  may vary. The security panel  12  affords integrated audio and video features through the use of the communications bus  18  which carries control, event and configuration data, as well as audio and video data. Examples of audio and video features include audio intercom, video surveillance, video for intercom, audio verification of the alarm events, video verification of the alarm events and remote access of audio and video data. It is understood that all, or only a portion, of the audio and video features, and components illustrated in  FIG. 1 , may be provided and/or connected through the bus  18 .  
       FIG. 2  illustrates a block diagram of a portion of the TLCI module  44 . An opto-coupler (OC) module  50 , AC impedance matching network  52 , and high impedance DC load  54  are formed within. The OC module  50  comprises a receive opto-coupler  56  interconnected in parallel with a transmit opto-coupler  58 . By placing the receive and transmit opto-couplers  56  and  58  in parallel, the dynamic range over which the TLCI module  44  can operate is increased. Performance may be improved by approximately 40% over configurations having the receive and transmit opto-couplers interconnected serially. Therefore, the TLCI module  44  will operate over longer phone lines  46  which have lower tip-to-ring voltages. In other words, the TLCI module  44  will work at lower voltages than previous configurations having the receive and transmit opto-couplers  56  and  58  interconnected serially with one another, and thus supports the use of longer phone lines  46 .  
      A hook control opto-coupler  60  is connected in series with the receive opto-coupler  56 . The hook opto-coupler  60  is used to turn hook switch  70  ( FIG. 3 ) on for off-hook and dial-pulse control. The receive and hook control opto-couplers  56  and  60  form a receive loop bias circuit.  
      The AC impedance matching network  52  comprises components having values which may be changed, if necessary, to provide the signal characteristics necessary for operation in different countries and areas. By carefully choosing the values, a minimal number of build configurations may be established to meet the requirements of each country and area as discussed previously, which minimizes the cost associated with producing multiple build configurations. With the exception of the AC impedance matching network  52 , the TLCI module  44  may remain unchanged from one build to the next. Optionally, components within the high impedance DC load  54  may also be changed to vary the current characteristics and/or requirements of each country and area. Although illustrated separately in  FIGS. 2 and 3 , the AC impedance matching network  52  and the high impedance DC load  54  may comprise a sub-set of common components.  
       FIG. 5  illustrates a chart  130  of country/regions  132 , corresponding telephone line reference impedance  134 , and return loss requirements  136  over the associated frequency range. North America  138 , for example, has an approximate 600 ohm line impedance requirement. Europe  140  has complex impedance having a range of impedance from approximately 466 ohms to approximately 889 ohms with an associated 150 nano-farad capacitance. By way of example, the 466 ohms is based on 270 ohms plus 750 ohms in parallel with 150 nF at a frequency of 4000 Hz, and the 889 ohms is based on 270 ohms plus 750 ohms in parallel with 150 nF at a frequency of 300 hz. Therefore, North America  138  has a first range of line impedance and Europe  140  has a second range of line impedance. By carefully choosing components within the impedance matching circuit  78 , a first build option can meet the requirements of North America  138  and Europe  140 , as well as the other countries listed in Section One  142 . Second and third build options are used for South Africa  144  and Australia  146 .  
       FIG. 3  illustrates a block diagram of the TLCI module  44  interconnected with the internal phone lines  28 , phone lines  46 , and the security panel  12 . The TLCI module  44  may optionally be connected with the modem  42 , processor module  14 , and/or other interfacing and/or controlling component.  
      The receive, transmit, and hook control opto-couplers  56 ,  58  and  60  each include a light emitting diode (LED) that is located in proximity to a photosensitive transistor. For example, current is supplied to the LED in the receive opto-coupler  56  through receive input line  90  and flows out through return line  92 . As the current varies, the brightness of the LED varies proportionally. The transistor in the receive opto-coupler  56  adjusts its conductivity based on the amount of exposed light. As the light from the LED increases, the current flow passed by the transistor increases linearly. The LEDs and transistors in the transmit and hook control opto-couplers  58  and  60  operate in a similar manner.  
      The receive opto-coupler  56  has a receive input side  102  (diode-side) and a receive output side  104  (transistor-side). The receive input side  102  has the receive input line  90  that receives the signal from the phone line  46  and the return line  92 . The receive output side  104  has a receive output line  100  which conveys the signal to the security panel  12 . The transmit opto-coupler  58  has a transmit input side  106  and a transmit output side  108 . The transmit input side  106  has a transmit input line  110  that receives signals form the security panel  12 , and transmit output line  112  which conveys the signal to the phone line  46 . The hook control opto-coupler  60  has a hook input side  114  and a hook output side  116 . The hook input side  114  includes a hook control line  68  that receives a hook signal from the security panel  12 . The hook output side  116  has a hook input line  118  and a hook control output line  120 .  
      The hook output side  116  is joined serially with the receive input side  102  of the receive opto-coupler  56 , providing it with a constant current that remains the same regardless of telephone line voltage. Also, the AC signal from the phone line  46  from the negative voltage side  96  of the diode bridge  62  passes through the hook switch  70 , the impedance matching network  52  via C 91  to the receive input side  102  of the receive opto-coupler  56 , then to the positive voltage side  94  of the diode bridge  62 .  
      The receive opto-coupler  56  and the hook control opto-coupler  60  are connected serially at node  122  via the return line  92  and the hook input line  118 . The transmit output side  108  of the transmit opto-coupler  58  in connected in parallel with the receive input side  102  of the receive opto-coupler  56  at node  124  via receive input line  90  and transmit input line  110 , and also at the hook switch  70  across the high impedance DC load  54 .  
       FIG. 4  illustrates a schematic diagram of the TLCI module  44 . The schematic diagram represents a configuration having at least resistors, diodes, capacitors and transistors, each of which is denoted with an R, D, C or Q label, respectively, followed by a unique number. The receive, transmit and hook control opto-couplers  56 ,  58  and  60  within the OC module  50  are schematically represented as discrete components that are labeled U 12 , U 13 , and U 14 , respectively. The following discussion of the components in  FIG. 4  will use some, but not all of these unique labels.  FIGS. 3 and 4  will be discussed together.  
      In general, when the security panel  12  detects an alarm condition, it wishes to communicate with the monitoring station  30 . The security panel  12  utilizes the TLCI module  44  to seize control of the phone line  46  and set an off-hook condition. The off-hook condition causes current to flow through TIP  72  and RING  74 . The high impedance DC load  54  is connected across TIP  72  and RING  74  of the phone line  46 , causing enough loop current to flow through the phone line  46  to indicate an off-hook condition to the phone network CO  48 . The phone network CO  48  detects the off-hook condition and sends a dial tone on the phone line  46 . When the security panel  12  detects the dial tone, the security panel  12  may use the modem  42  to dial the monitoring station  30  ( FIG. 1 ) using DTMF, dial pulsing, and the like, depending upon the programming and requirements of the phone network CO  48 . The AC impedance matching network  52  provides the impedance matching between the TLCI module  44  and the phone lines  46 .  
      When the monitoring station  30  receives the call on the phone line  46 , the monitoring station  30  transmits a hand shake tone. The security panel  12  detects the hand shake tone through the receive loop formed by receive and hook control opto-couplers  56  (U 12 ) and  60  (U 14 ), which are connected in series. The security panel  12  then may transmit and receive data using the transmit and receive opto-couplers  58  (U 13 ) and  56  (U 12 ). The monitoring station  30  may transmit one or more acknowledge signals to acknowledge receipt of transmitted data, as well as transmit other data as necessary. Bias for the transmit opto-coupler  58  is provided through transmit buffer/amplifier  80 . The receive opto-coupler  56  is biased through the phone line  46  via the hook control opto-coupler  60 .  
      More specifically, to gain control of the internal phone line  28 , the security panel  12  sets line seize control  64  to HIGH which turns on Q 41   b . This energizes line seize relay  66  (RLY 1 ) which transfers the phone line voltage of the phone lines  46  to diode or steering bridge  62  (comprising D 26 , D 27 , D 30  and D 31 ). The diode bridge  62  may also be referred to as a receive circuit. Thus, the line seize relay  66  (RLY 1 ) transfers control of the phone line  46  from the house phone  34  to the security panel  12 . If the house phone  34  is in use, it is disconnected by this transfer. This prevents anyone from compromising the communication of the alarm event to the monitoring station  30 , either by accident or on purpose.  
      Receive and on/off-hook operations are controlled by the receive opto-coupler  56  (U 12 ) and the hook control opto-coupler  60  (U 14 ). An off-hook condition is initiated by setting hook control line  68  to LOW or zero volts. This causes current to flow through the diode of the hook control opto-coupler  60 . Assuming a Current-Transfer-Ratio (CTR) of 100%, the current through the diode of the hook control opto-coupler  60  will be transferred to the collector (pin  4 ) of the hook control opto-coupler  60 . The current passes through the diode of the receive opto-coupler  56 , biasing the receive opto-coupler  56 , and also into the base of the hook switch  70  (Q 45 ). When the hook switch  70  turns on, the high impedance DC load  54  (comprising Q 43 , Q 44 , R 213 , R 214 , R 216 , R 219 , R 221 , R 223 , and C 93 ) is connected across TIP  72  and RING  74  of the phone line  46 , increasing the amount of loop current flowing through the phone line  46  to indicate an off-hook condition to the phone network CO  48 . The AC signal sent through the phone line loop is picked up by the diode in the receive opto-coupler  56  (U 12 ), transferred to the collector (pin  4 ) of the receive opto-coupler  56  (U 12 ) (transistor side) and converted to a voltage, which is then received by output line  100  and detected by receive amplifier/high-pass filter  76 , then passed to the security panel  12 . The hand shake tone from the monitoring station  30  modulates current in the telephone loop.  
      The AC impedance matching network  52  to the phone line  46  comprises R 221 , R 223  and C 93 . As illustrated in  FIG. 4 , the components R 221 , R 223  and C 93  are shared by the AC impedance matching network  52  and the high impedance DC load  54 .  
      As discussed previously, different countries and areas require different impedance matching to work with the phone lines  72  when the phone line  46  is in an off-hook condition. Each country or geographic area has specified impedance parameters or ranges within which the equipment must work as illustrated in  FIG. 5 . For example, the phone line impedance may be 600 ohms in North America, and thus the impedance of the AC impedance matching network  52  is 600 ohms when the phone line  46  is in an off-hook condition. In contrast, Europe may have complex impedance in which the resistance value of the phone line  46  changes depending upon the frequency being transmitted. The frequencies across the range may be correlated to actual impedance values, such as along a curve. Components within the AC impedance matching network  52 , as well as the high impedance DC load  54 , are selected to satisfy the actual impedance.  
       FIG. 6  illustrates a chart  150  of build options  152 , associated country/regions  154 , and examples of component values which may be used for the impedance matching network  156 . Within the examples of the impedance matching network  156 , each country/region has two corresponding resistor values indicated for R 221  and R 223  of the impedance matching network  52  ( FIG. 4 ) and one capacitor value indicated for C 93 . For example, North America and Europe may both use resistor values of 510 ohms and 390 ohms with a capacitor value of 100 nano-farad. Each country/region  154  within Section One  142  may use the same resistor and capacitor values, even though the telephone line reference impedances  134  ( FIG. 5 ) are not the same. This greatly reduces the overall number of different build options needed. Different resistor and capacitor values are used for South Africa  144  and Australia  146  to meet their particular requirements.  
      It should be understood that other countries/regions which are not listed may be included within any of the builds  152  of Section One  142 , South Africa  144 , and Australia  146  if the line impedance requirements are met. Also, different resistor and capacitor values may be used, as well as more than two resistors and/or more than one capacitor to meet the line impedance requirements. In addition, other components may be used within the impedance matching network  52  to meet the line impedance requirements.  
      When the security panel  12  wants to transmit data, the security panel  12  uses transmit buffer/amplifier  80  to bias on the transmit opto-coupler  58  (U 13 ). The line seize control  64  is pulled HIGH, which transfers the phone line voltage to and through the diode bridge  62 . Activating the hook control line  68  causes bias current to flow through the receive opto-coupler  56 , hook control opto-coupler  60 , and the hook switch  70 . This turns on the high impedance DC load  54  which draws enough current to cause a detectable off-hook condition at the phone network CO  48 . Due to the biasing of the transmit opto-coupler  58  and the current-transfer-ratio, the current that flows through the LED of the transmit opto-coupler  58  also flows from positive voltage side  94  ( FIG. 3 ) of the diode bridge  62  through transmit opto-coupler  58 , R 222  ( FIG. 4 ), and hook switch  70  to negative voltage side  96  of the diode bridge  62 , out to RING  74 , thus creating a transmit signal path or transmit loop to the phone line  46 . As data is transmit through the transmit opto-coupler  58 , the current flowing through the diode of the transmit opto-coupler  58  is modulated. This modulation is passed through to the transistor side where it modulates the current in the DC path as described above. This current modulation is then applied to the phone line  46  through the diode bridge  62 . The data may be transmit in packets, pulse per second, F-tones, or other transmission protocol.  
      The security panel  12  also uses the TLCI module  44  to detect an incoming call or ring on the phone line  46 . A ring detect or ring coupling circuit  82  may comprise the components C 98 , C 99 , R 209 , R 211 , TVS 25 , U 12 , and D 28 . The ring coupling circuit  82  indicated on  FIG. 4  is for reference, and it should be noted that not all of the components are included for clarity. C 98  and C 99  are connected to RING  74  and TIP  72 , respectively. The ring coupling circuit  82  may be changed to meet certain build requirements, such as for South Africa. When the security panel  12  is in an on-hook condition and a ring signal occurs across TIP  72  and RING  74 , it is coupled into the TLCI module  44  through C 98  and C 99  of the ring coupling circuit  82 . For example, the ring signal may be a sinewave, such as a 20 Hz sinewave having a 253 V peak-to-peak signal, typical. The ring current is limited by R 209  and R 211 , and the sensitivity level of the ring signal is set by TVS 25 . When the ring signal is greater than the breakdown threshold of TVS 25  (in the positive voltage direction) ring current flows through the LED of the receive opto-coupler  56 . The ring signal is then coupled to the receive input side  102  of the receive opto-coupler  56  which drives Q 42  of telephone line monitoring (TLM)/ring detect circuit  84  (which may be used as a level shifter). The output of the TLM/ring detect circuit  84  on line  98  is monitored by the security panel  12  to determine if a ring signal has occurred.  
      On the negative voltage cycle of the ring signal, TVS 25  (of ring coupling circuit  82 ) and ring detect diode  86  (D 28 ) are forward biased. The forward voltage across the ring detect diode  86 , which is in parallel with the LED of the receive opto-coupler  56  makes sure that the LED of the receive opto-coupler  56  is off during the negative voltage cycle which then turns off the drive to the TLM/Ring detect circuit  84 . In this way, the ring frequency across TIP  72  and RING  74  is isolated and coupled to the low voltage side and also level shifted for interfacing to the processor module  14 .  
      The TLCI module  44  also provides for a telephone line monitoring (TLM) operation, which may be accomplished by TLM monitoring module  88  (comprising D 26 , D 27 , D 30 , D 31 , R 215 , D 29 , C 92 , R 224 , R 227 , and R 226 ), TLM input  89  (comprising R 217  and R 220 ), as well as receive opto-coupler  56  and hook control opto-coupler  60 . When the security panel  12  is in the on-hook condition, the phone line  46  will be monitored. During this time, TIP  72  and RING  74  voltage is applied to the TLM input  89 , allowing a small amount of current to charge C 92 . When the security panel  12  wants to perform a TLM function, such as to determine if the phone line  46  is present, the hook control line  68  is pulled LOW to turn on the LED of the hook control opto-coupler, which then turns on the transistor of the hook control opto-coupler  60 , thus providing a discharge path for C 92  through the LED of the receive opto-coupler  56 . The discharge of C 92  through the LED of the receive opto-coupler  56  causes the LED to turn on, generates a pulse across pins  3  and  4  of the receive opto-coupler  56 , and drives the TLM/ring detect circuit  84  (Q 42 ) on. This pulse duration is applied to the security panel  12  via line  98 . If the security panel  12  detects a pulse, the phone line  46  is deemed operable. If no pulse is detected at this time, the phone line  46  is deemed to be in a fault condition. The TLM function may be performed periodically, and the results displayed and/or logged at the security panel  12 .  
      While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.