IES930703A2 - A monitoring device which detects when a two-wire loop¹start telephone circuit is in use - Google Patents

Abstract

Subscriber telephones (3) are connected to the central office (1) batteries that supply direct current to operate through a local loop of two wires (5). The aim of the invention (4) is to detect if any subscriber telephone connected to the loop is in the off-hook condition which occurs when the telephone circuit is in-use (loop closed). When the loop is closed the invention performs timing functions (10) and provides an audible and/or visual indication (11) at periodic intervals. When the loop is open and in proper order the invention is not active. The invention is connected across the loop (ό) and maintains an extremely high resistance to the loop at all times. The invention comprises a "on-hook / off-hook closed detection" circuit (Θ), an electronic "power switch" (9), an internal "timer" (10), audio and visual "indicators" (11) and a "power supply source" (7)

Description

Α MONITORING DEVICE WHICH DETECTS WHEN A TWO-WIRE LOOP-START TELEPHONE CIRCUIT IS IN USE.

The invention is concerned with the DC operating parameters of a two- wire subscriber loop-start telephone circuit. The present invention relates to the detection of an open or closed loop condition. The invention is particularly concerned with detecting the closed state (off-hook condition) of the loop circuit independent of and not associated with any particular telephone apparatus connected to the loop.

Prior art teaches us of a current sensing method to determine when a particular telephone apparatus is in-use or when connected between the telephone line and subscriber telephones to detect if any telephone is in-use.

The present invention connects across (in parallel with) the local loop in the same way as a standard telephone apparatus and maintains an extremely high input resistance to the loop at all times. The invention contains its own power source and with the exception of a very small leakage current the invention does not drain any current from the telephone loop. According to the present invention only a very small amount of current is required from the invention's own power source when the telephone loop is open (on-hook condition).

According to the present invention when the telephone circuit loop is closed (off-hook condition) an electronic timer is powered on and remains on while the loop remains closed. At periodic intervals an audio and/or visual indicator associated with the invention is momentarily activated to mark the end of a 1 S93070 —2— period af telephone use.

The invention also seeks to detect and indicate when the local loop ( or telephone line) or telephone company central office DC loop equipment are abnormally open and inoperative (telephone line down condition).

An embodiment of the invention will now be described with reference to accompanying drawings, in which! FIGURE A illustrates the main functional components of a two-wire subscriber telephone circuit with the invention attached.

FIGURE 1 shows the equivalent DC loop resistances within the subscriber telephone circuit loop.

FIGURE 2 is a representative schematic diagram of the invention and the internal circuit loops.

DESCRIPTION OF THE PREFERRED EMBODIMENT The basic two-wire subscriber loop telephone circuit is comprised of a local loop (i.e. telephone line) which connects subscriber telephone apparatus to a central office that contains switching equipment and batteries that supply DC current to operate the telephone circuit.

The invention relates to the DC operating parameters of the two-wire telephone loop circuit. All references to the AC operating characteristics (i.e. ring and voice signalling) have been excluded from the description of the preferred embodiment ' >030703 -3and are mentioned only where deemed significant or necessary.

Refering to FIGURE 1, the telephone apparatus (109) is isolated from the local loop by the open contacts of switch-hook Sla and Sib (104) when the telephone handset is on-hook. The invention (110) is connected across the two wire local loop as shown in FIGURE 1. The invention is virtually isolated from the loop circuit due to the very high equivalent resistance Rinv (106). No DC current flows except for leakage current due to Rinv (106).

When the handset is lifted to make or answer a call, the switch-hook contacts (104) close. Loop current flows from the central office battery (101), through the telephone set equivalent resistance Rtele (105) via the local loop Rline (103), and through the switching relay coil equivalent resistance Rrelay (102). When sufficient current flows through the relay (102) the relay is energised and its closed contacts (not shown) signal to the central office switching equipment that a telephone (109) is off-hook.

Whether the loop circuit is opened or closed, equivalent resistance Rinv (106), which is across the local loop (108), maintains a very high resistance and has no significant affect in any way with normal telephone circuit operation.

The basic DC operating parameters and characteristics of the loop circuit include a typical battery feed arrangement (101) shown in FIGURE 1 that is located used in the central office (107) and to connect to the local loop (108). In the central office feed arrangement LS93 0 703 J -4the only significant DC equivalent resistance is that of the sensing relay Rrelay (102). The loop circuit also includes the equivalent resistance of the local loop Rline (103) and the resistance of the telephone apparatus Rtele (105) when the switch-hook (104) is closed (off-hook condition).

Using Ohm's law, the total off-hook loop circuit resistance RT_offhook is therefore: RT_0-f-f—hook = Rrelay * Rline * Rtele Note that Rtele (105) is in parallel with Rinv (106). However, the extremely high resistance of Rinv (106) is not significant when compared to Rtele (105). For that reason Rinv (106) has been excluded from the equation.

...The on-hook loop circuit resistance RT_onhook RT_onhoofe = Rrelay * Rline + Rinv Note when the switch-hook (104) is open no current flows through Rtele (105), therefore the equivanent resistance at the subscriber telephone end of the circuit is effectively the extremely high Rinv (106).

The two-wire subscriber telephone circuit operating parameters observed in Europe and the United States are listed below.

Parameter Typical Value Operating Limits Common Battery Voltage (101) -48 VDC -43 to -165 VDC Operating Current 20 to 80 mA 20 to 120 Ma Subscriber Loop Resi stance 0 to 1300 ohms 0 to 3600 ohms -5The Subscriber Loop DC Resistance can be -further broken down as -follows: Parameter Typical Value Operating Limits Central Office Sensing Relay < 107) 400 ohms 380 to 780 ohms Local Loop (108) 0 to 1300 ohms 0 to 3600 ohms Tel ephone Apparatus (109) 400 ohms about 100 ohms min. Invention (110) 20 megohms 10 megohms minimum.

The description o-f the preferred embodiment will assume a typical loop voltage (101) of -48 VDC and a minimum loop relay sensing current of 20 mA when closed (off-hook).

The dc voltage potential observed across the switch-hook (104) and the invention Rinv (106) is the same, and : in the on-hook state (as per equation 2 above) |7ini/ = (Rinv/ (Rrelay+Rline+Rinv)) X Vbat = 48 VOC ...where Vinv is the voltage across the invention.

In the off-hook state (as per equation 1 above) Vinv = 20aa X Rtele = 0.02 X 400 = 8 VOC The on-hook and off-hook voltages, found in equation 3 and 4 respectively can now be applied to the invention as seen in the schematic diagram in FIGURE 2.

Loop 2 shows the forward bias circuit of the on—hook / off- hook detection circuit which includes the detection circuit super—beta transistor pair (208 and 209), the power switch (211), and the power source (214). The power source battery provides the bias voltage to allow bias current to flow through the base -S930703 -ώresistor (206) through the super-beta bipolar transistor pair (208 and 209) which in turn drive the bipolar power switch (211). A limiting resistance (210) is included to prevent excessive drive of the power switch transistor (211).

The base resistor (206) controls the bias current by allowing only a very small amount of current to flow through the super—beta transistor (20B and 209) pair base-emitter junctions. The super-beta transistor arrangements provide a high gain needed to drive the power switch (211) into saturation.

Loop 1 provides a very high resistance to the telephone circuit loop ¢201) while at the same time providing a means of detecting the on and off hook condition of the telephone circuit. The local loop provides an on and off hook potential of -48VDC and -8VDC respectively. A very small amount of current flows through loop 1 due to the series combination of the two 10 megohm resistors (202 and 204) and the small signal switching diode (205), resulting in a input resistance of at least 20 megohms as asserted in claim 1 of the disclosure.

Loop 1 and loop 2 intersect at the base of the first transistor (20B) of the super-beta pair. When in the on hook condition the local loop is open and about -48VDC is applied to loop 1. The resulting current flow that occurs in loop 1 due to the high open-loop voltage is significantly more negative than that of the bias current of loop 2. This results in the loopl 1 diode (205) being forward biased. In this condition, a voltage drop of about 0.7 VDC (as character!stic of silicon semiconductor devices) occurs across the diode (205). The potential voltage realised at the base of super-beta transistor pair (20Θ and 209) 3 0 7 0 3 -7is well below the potential necessary to forward bias the superbeta pair <208 and 209) and power switch transistor (211) arrangement. Thus no base current flows through these transistors and the power switch (211) collector circuit remains open (off condition). In this state no current flows through loop 3 and the timer (212) and indicator (213) circuit remain switched off.

When a telephone apparatus connected to the telephone line is taken off-hook the local loop (201) is closed. This condition results in a -SVDC potential across loop 1, a much lower potential as compared with the -48VDC potential when on-hook.

This significantly decreased potential in turn reduces the current flawing through loop 1 to a much lower level. So much so that the bias circuit of loop 2 over-rides the effect that loop 1 had in the on-hook state. The loop 1 diode (205) is now reverse biased and the super-beta <208 and 209) and power switch (211) transistor arrangement are forward biased. The power switch (211) is switched on current is allowed to flow through loop 3 to power on the timer (212) and indicator (213) circuits. While the local loop (201) remains in the closed or off-hook state, current flows through loop 3 and the timer (212) counts to a predetermined and periodic interval value and results in the momentary activation of the audio and visual indicators (213) as asserted by claim 3 of the disclosure.

When the telephone connected to the local loop (201) is placed in the on-hook state the voltage potential applied to loop returns to -48VDC and the power switch (211) is turned off (as previously discussed). No current flows through loop 3 and the timer (212) and indicators (213) are inactive. The very high S9307O3 -810 resistance of the loop 2 base resistor (206) (several megohms) allow only a very small leakage current to flow from the power source as asserted in claim 2 of the disclosure. No other current drain of the power source (214) occurs in the on-hook state.

In the event of an abnormal open occurs within the central office equipment or the local loop (telephone line), the resultant potential applied to the local loop ¢201) connection of loop 1 is OVDC (none). In this case, loop 1 is effectively open and eliminated from having any effect on the operation of the bias circuit of loop 2. The super-beta (208 and 209) and power switch (211) transistors are turned on and the collector of the power switch (211) is driven into saturation. Loop 3 is now closed and the power source (214) potenial is applied to the timer (212) circuits and momentary activation of the audio and visual indicators (213) occur at periodic interval while the abnormal open condition exits, as asserted in claim 4 of the disclosure

Claims (3)

1. CLAIMS (1) A device (110) that is connected across a two-wire telephone local loop (108) and maintains a very high impedance (106) to the telephone circuit loop at all times and can detect 5 if the loop is open (on-hook condition) or closed (104) (offhook condition)

2. (2) A device claimed in 1 which has an extremely low current drain from the power source (7) when the two-wire telephone circuit (6) is open (on-hook condition) and can switch on power 10 to other circuits when the telephone circuit (6) is closed (offhook condition).

3. (3) A device claimed in 1 that incorporates an electronic timer (10) which activates an audio and/or visual indicator (11) at periodic intervals while the loop circuit (6) remains closed. 15 (4) A device claimed in 1 which detects if the DC loop circuit is abnormally open at the Central Office (1) or in the local loop (2), and controls a power switch (9) to provide power to other circuits (10 and 11) when the dc loop is determined to be abnormally open.