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Claims  |
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What is claimed is:
1. A telemetry device for collecting and transmitting data over a phone line to a central station, said phone line conducting a loop current, said telemetry device comprising:
data collecting means for collecting data at a remote location;
transmitting means, coupled to said collecting means, for transmitting said data over said phone line to said central station during an active state of said device in which said telemetry device seizes the phone line;
voltage deriving means, coupled to said phone line, for deriving a voltage dependent on the loop current of said phone line;
variable load pre-loading means, situated in said telemetry device, for variably loading said telemetry device down during said active state to draw an amount of loop current which is varied until said voltage reaches a first predetermined
voltage level, and
voltage drop detecting means, coupled to said voltage deriving means, for detecting when said voltage decreases to a second predetermined voltage level less than said first predetermined voltage level, thus indicating that a contending telephone
device coupled to said phone line has come off-hook during said active state.
2. The telemetry device of claim I further comprising a real time clock, coupled to said transmitting means, for causing said telemetry device to enter said active state at an appointed alarm time to transmit said data, said telemetry device
otherwise being in a static state.
3. A telemetry device for collecting data and transmitting said data over a phone line to a central station during an active state of said telemetry device during which said device seizes the phone line, said telemetry device otherwise remaining
in a static state, said phone line conducting a loop current, said telemetry device exhibiting a current load on said phone line, said telemetry device comprising:
voltage deriving means, coupled to said phone line, for deriving a voltage dependent on the loop current of said phone line, said voltage being designated the derived voltage;
a microprocessor for controlling said telemetry device and for collecting data from devices external to said telemetry device;
a peripheral bus coupled to said microprocessor, said bus including a plurality of data lines;
a plurality of pull up/load resistors respectively coupled to said plurality of data lines;
control means, within said microprocessor, for activating during said active state an increasing number of said pull up/load resistors to increase the current load exhibited by said telemetry device on the phone line until said derived voltage
reaches a first predetermined voltage level, and
voltage drop detecting means, coupled to said voltage deriving means, for detecting when said derived voltage decreases to a second predetermined voltage level less than said first predetermined voltage level, thus indicating that a contending
telephone device coupled to said phone line has come off-hook during said active state.
4. The telemetry device of claim 3 wherein said telemetry device includes a phone line side and a meter side coupled together by an electrically isolative barrier therebetween, said line side of said telemetry device being coupled to said phone
line, said telemetry device further including a power supply situated on said meter side of said telemetry device for drawing power from said line side to provide a power supply voltage to said meter side.
5. A telemetry device for collecting data and transmitting said data over a phone line to a central station during an active state of said telemetry device during which said telemetry device seizes the phone line, said telemetry device otherwise
remaining in a static state, said telemetry device including a phone line side and a meter side coupled together by an electrically isolative barrier therebetween, said line side of said telemetry device being coupled to said phone line, said phone line
conducting a loop current, said telemetry device exhibiting a current load on said phone line, said telemetry device comprising:
a power supply situated on said meter side of said telemetry device for drawing power from said line side to provide a power supply voltage to said meter side;
a microprocessor for controlling said telemetry device and for collecting data from devices external to said meter side;
a peripheral bus coupled to said microprocessor, said bus including a plurality of data lines;
a plurality of pull up/load resistors respectively coupled to said plurality of data lines;
control means, within said microprocessor, for activating during said active state an increasing number of said pull up/load resistors to increase the current load exhibited by said telemetry device on the phone line until said power supply
voltage reaches a first predetermined voltage level, and
voltage drop detecting means, coupled to said power supply, for detecting when said power supply voltage decreases to a second predetermined voltage level less than said first predetermined voltage level, thus indicating that a contending
telephone device coupled to said phone line has come off-hook during said active state.
6. The telemetry device of claim 5 further comprising memory means for storing the number of resistors activated to reach said first predetermined voltage level during a first time when said telemetry device enters said active state.
7. The telemetry device of claim 6 further comprising means for recalling said number of resistors from said memory means the next time said device enters the active state and re-using that number of resistors to again load said telemetry
device.
8. The telemetry device of claim 6 further comprising means for recalling said number of resistors from said memory means the next time said telemetry device enters the active state and using this recalled number of resistors as a starting value
to determine the number of pull-up resistors activated to again load said telemetry device.
9. The telemetry device of claim 5 wherein the pull up/load resistors activated by the control means during a first time when said telemetry device enters said active state define a resistor configuration, said telemetry device further
comprising memory means for storing the resistor configuration.
10. The telemetry device of claim 9 further comprising means for recalling said resistor configuration from said memory means the next time said telemetry device enters said active state and re-using that resistor configuration to again load
said telemetry device.
11. The telemetry device of claim 5 further comprising at least one peripheral device coupled to said peripheral bus.
12. The telemetry device of claim 11 wherein said peripheral device comprises a real time clock.
13. A telemetry device for collecting data at a remote location and transmitting said data over a phone line to a central station, at least one other contending telephone device being coupled to said phone line at said remote location, said
phone line conducting a loop current, said telemetry device comprising:
data collecting means for collecting data at said remote location during a static state of said telemetry device;
transmitting means, coupled to said collecting means, for transmitting said data over said phone line to said central station during an active state of said telemetry device during which said telemetry device seizes said phone line;
a power supply, coupled to said phone line, for supplying power supply voltage to said telemetry device;
static off-hook detector means for monitoring said phone line during the static state of said telemetry device to determine when said contending telephone device goes off-hook and preventing said telemetry device from entering said active state
when said contending telephone device goes off-hook during said static state;
dynamic off-hook detector means for detecting, during said active state, when said contending telephone device goes off-hook, said dynamic off-hook detector means including:
variable load preloading means for variably loading said telemetry device to draw progressively higher amounts of loop current from said phone line until said power supply voltage decreases to a first predetermined level, and
voltage drop detecting means, coupled to said power supply for detecting when said power supply voltage decreases to a second predetermined voltage level less than said first predetermined voltage level, thus indicating that a contending
telephone device coupled to said phone line has come off-hook during said active state.
14. The telemetry device of claim 13 further comprising phone line disengaging means, coupled to said voltage drop detecting means, for causing said telemetry device to disengage said phone line when said dynamic off-hook detector determines
that said contending telephone device has come off-hook during said active state.
15. A method of operating a telemetry device which is coupled to a phone line exhibiting a loop current, said telemetry device including a voltage deriving circuit coupled to the phone line for deriving a voltage dependent on the loop current of
the phone line, said method comprising the steps of:
entering an active state, by said telemetry device, during which said telemetry device seizes said phone line and collected data is to be transmitted back to a central station;
variably pre-loading said telemetry device down to draw an amount of loop current which is varied until said voltage reaches a first predetermined voltage level, and
detecting when said voltage decreases to a second predetermined voltage level less than said first predetermined voltage level to indicate that a contending telephone device coupled to said phone line has come off-hook during said active state.
16. A method of operating a telemetry device, said telemetry device including a power supply coupled to a phone line to derive power supply voltage therefrom, said telemetry device including a microprocessor coupled to a bus including a
plurality of data lines to which a plurality of pull-up resistors are respectively coupled, said method comprising the steps of:
entering an active state, by said telemetry device, during which said telemetry device seizes said phone line and collected data is to be transmitted back to a central station;
activating, by said microprocessor during said active state, said pull-up resistors to increasingly load said telemetry device to draw progressively higher amounts of current from said phone line until said power supply voltage decreases to a
first predetermined level, and
detecting when said power supply voltage decreases to a second predetermined voltage level less than said first predetermined voltage level to indicate that a contending telephone device coupled to said phone line has come off-hook during said
active state.
17. The method of claim 16 wherein said activating step includes activating an increasing number of said pull-up resistors to increasingly load said telemetry device to draw progressively higher amounts of current from said phone line until said
power supply voltage decreases to said first predetermined level.
18. The method of claim 17 including the step of storing the number of resistors activated in said activating step when said telemetry device enters the active state.
19. The method of claim 18 including the step of reactivating, during the next time said telemetry device enters the active state, the same number of resistors as said telemetry device employed the last time it entered the active state.
20. The method of claim 18 including the step of recalling the number of resistors stored in said storing step and using this recalled number of resistors as a starting value to determine the number of pull-up resistors activated the next time
said telemetry device enters the active state.
21. The method of claim 17 including the step of reactivating, during the next time said telemetry device enters the active state, the same resistors as said telemetry device employed to load said telemetry device the last time it entered the
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Description |
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CROSS REFERENCE TO RELATED PATENT APPLICATIONS
This patent application relates to my copending patent application entitled "Telemetry Device Including A Dynamic Off-Hook Detector Capable Of Operating In A Pulse-Dialing Environment", Ser. No. 08/128,864 filed concurrently herewith and
assigned to the same assignee.
BACKGROUND OF THE INVENTION
This invention relates in general to data collection systems which use remotely located telemetry devices to transfer telemetry data from a remote site to a central processing location. More particularly, the invention relates to automatic meter
reading (AMR) systems which use conventional subscriber telephone lines to transfer telemetry data, in the form of utility meter readings, from a customer's premises to a central processing location.
Prior to the existence of automatic meter reading (AMR) systems, the most common method for determining the amount of commodity delivered to a utility customer was to manually read a meter at, or in close proximity to, the consumer's premises.
Because the utility meters were located at the point where the utility commodity was dispensed to the consumer, it became necessary for utility companies to establish routes where a "meter-reader" periodically visited each meter to record the amount of
utility product consumed.
At present, many utilities, including gas, electric and water companies, continue to send meter-readers to consumer residences' to collect utility meter readings. However, there are practical limitations as to how often and how efficiently this
procedure can be manually performed. For instance, weather and the ability to gain access to meters themselves (which are often inside the consumer's residence) directly impact the efficiency of this manual procedure. Today, where it is desirable for
the utility to have almost instantaneous access to any meter, the manual method for collecting these readings is becoming both economically and operationally obsolete in view of the more sophisticated and reliable automatic techniques now available.
One very practical method for automating the process of collecting utility meter readings uses the existing telephone system and takes advantage of the already widespread availability of telephone service to both residential and business
premises. Using this existing infrastructure, remotely located telemetry devices (at each consumer's premises) electronically upload utility meter readings as telemetry data to a central processing location via the subscriber telephone lines. This
process is analogous to the procedure used by many PC users to electronically upload files by the use of a modem connected to the subscriber's telephone line, except that the AMR procedure is fully automatic. This invention relates to those AMR systems
which utilize telephone line telemetry techniques.
In practice, the actual telemetry device used in an AMR system is called a meter interface unit (MIU). Located at the customer's premises, the MIU, as the name implies, is an interface between two different electrical environments. One side of
the MIU, called the meter side, is connected to one or more utility meters while the remaining side, or line side, of the MIU is connected in parallel with, or across, the subscriber telephone line. In this regard, the telephone line side of the MIU is
connected to the phone line in a manner similar to that used to add an additional extension phone, answering or FAX machine. Other than connecting the MIU to the subscriber line, no modification of the existing telephone line wiring is required.
In one particular type of AMR system, a real time clock within the MIU activates the device at a prescribed date and time. Once activated, the MIU seizes the phone line by taking the phone line off-hook, dials a preprogrammed telephone number to
connect the MIU to a central processing station, reads the utility meters connected to it and then uploads the telemetry data from the MIU via the telephone line to the central station. Automatic meter reading systems which utilize this technique are
known as dial-inbound systems, since the MIU dials into a central data processing center.
Since the telemetry transaction typically takes only a few seconds to complete, the MIU normally resides in a low power, standby condition (on hook) until such time as the programming of the MIU's real time clock causes the device to be activated
again. Therefore, the MIU is said to be in a static mode between telemetry transactions and in a dynamic mode while engaged in the transfer of telemetry data across the telephone line.
An MIU may be viewed as an interface between two different electrical environments, namely the telephone line side and the meter side of the MIU. In order to protect both telephone company personnel and their counterparts in the utility industry
from possible accidental electrocution, there is no DC continuity between the meter and telephone line sides of the MIU. Isolation is also required because the telephone subscriber loop must remain balanced, since translongitudinal imbalance would
result in hum, noise and unacceptable loading of the telephone line. This isolation is obtained by using transformer (magnetic) coupling or optical coupling of signals on opposite sides of the isolation barrier.
With regard to the MIU itself, the terms meter and telephone line side more accurately describe how the telemetry device is electrically connected than how the MIU is operationally partitioned. For instance, because the meters attached to the
MIU are typically read by a serial communication protocol similar to RS-232, a low power microprocessor is best suited to performing this task. Consequently, the same microprocessor may be used to format and transmit the telemetry data packet while
simultaneously controlling the entire operation of the MIU. Therefore, functionally the meter side of the MIU is also called the control side.
Similarly, since it is desirable to have the MIU be self-powered from the phone line (with the exception of a small battery to sustain the real time clock in the static standby mode), the loop current from the central office switch provides the
primary power source for the MIU. Consequently, the phone line side of the MIU is referred to as the primary side.
Because it is not acceptable for the MIU to disrupt or otherwise interfere with the normal operation of the subscriber telephone line, the MIU must include an "off-hook detector" or "line-status indicator" which is capable of detecting when the
subscriber phone line is or is not in use. As noted before, the MIU is in either a static mode or a dynamic mode. As a result, the off-hook detector consists of not one function but two, namely a "dynamic off-hook detector" and a "static off-hook
detector". The terms dynamic and static describe the current mode of the MIU.
Briefly, a "static off-hook detector" is employed when the MIU is in the static mode (standby state) to determine when the telephone subscriber commences use of the phone by lifting the telephone receiver or other telephone subscriber equipment
seizes the phone line for communications use. In contrast, a "dynamic off-hook detector" is employed when the MIU is in the active state (MIU has seized the phone line and is actively transmitting data) to determine when the telephone subscriber
commences use of the phone by lifting the telephone receiver or other telephone subscriber equipment seizes the phone line for communications use.
At first glance, the design of off-hook detectors may seem deceptively simple. However, getting them to function reliably, in practice, is a task requiring specialized design knowledge and skill. My patent, entitled Signal Processing Circuit
For Use In Telemetry Devices, U.S. Pat. No. 5,202,916, the disclosure thereof being incorporated herein by reference, describes some of the complexities involved in designing a static off-hook detector for an MFG. Whereas the static off-hook detector
serves to prevent the MIU from going to the dynamic (off-hook) mode while the subscriber line is in use, the dynamic off-hook detector permits the MIU to immediately disengage itself from the phone line should the subscriber attempt to use the telephone
while the MIU is actively engaged in a telemetry transaction.
In practice, the dynamic off-hook detector is considerably more difficult to design than the static off-hook detector. The wide variation in operational parameters found within the telephone system complicates the task of designing a generic
dynamic off-hook detector which functions reliably. Should the dynamic off-hook detector erroneously trigger during a telemetry session, the repeating malfunction may make it impossible to ever complete the telemetry transaction. In that case, the MIU
would perpetually disengage itself from the line prior to the session's completion. At the other extreme, a dynamic off-hook detector which cannot detect a subscriber off-hook condition will not disengage itself from the telephone, thereby refusing to
surrender up the phone line to the telephone subscriber, as it should. In fact, the task of designing this detector is so formidable that some MIU vendors do not even incorporate a dynamic off-hook detector function in their products. Since the
functional requirements for static and dynamic off-hook detectors are different, each detector is customarily implemented as a separate device in conventional off-hook detectors.
One prior art dynamic off-hook detector approach is to sense the change in phone line loop current which occurs when the user takes a telephone device off hook. For example, the Schlumberger Model MIU T-3000 meter interface unit incorporates an
analog to digital (A/D) converter for the sole purpose of periodically measuring the loop current on the primary side of the MIU. Although technically elegant, this is not a particularly cost effective implementation. Not only is the A/D converter
expensive but additional components are required to transport the A/D's output (an 8-10 bit digital number representing the loop current) from the primary side of the telemetry device to the microprocessor on the control side of the isolation barrier.
In addition, those skilled in the art will recognize that the environment under which line-powered telemetry devices must operate is one which is extremely power conscious. The A/D approach therefore has the added disadvantage that the A/D conversion
consumes precious power resources.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a telemetry device including a dynamic off-hook detector which detects, with high reliability, when the telephone user has taken the telephone set off-hook during the active state of the MIU.
Another object of the present invention is to provide a telemetry device including a dynamic off-hook detector which is capable of adapting itself to changing telephone line conditions.
Yet another object of the present invention is to provide a telemetry device including a dynamic off-hook detector which allows an MIU to recognize, and subsequently disengage itself from the telephone line, should the telephone subscriber
interrupt a telemetry session by taking the telephone set off-hook.
In accordance with one embodiment of the present invention, a telemetry device is provided for collecting and transmitting data over a phone line back to a central station. The telemetry device includes a data collector for collecting data at a
remote location. The device further includes a transmitter, coupled to the data collector, for transmitting the data over the phone line to the central station during an active state of the device in which the device seizes the phone line. The device
also includes a voltage deriving circuit, coupled to the phone line, for deriving a voltage which is dependent on the loop current of the phone line. The telemetry device still further includes a pre-loading circuit, situated in the device, for loading
the device down during the active state until this voltage reaches a first predetermined voltage level. The device also includes a voltage drop detector, coupled to the voltage deriving circuit, for detecting when this voltage decreases to a second
predetermined voltage level less than the first predetermined voltage level, thus indicating that a contending telephone device coupled to the phone line has come off-hook during the active state.
BRIEF DESCRIPTION OF THE DRAWINGS
The features of the invention believed to be novel are specifically set forth in the appended claims. However, the invention itself, both as to its structure and method of operation, may best be understood by referring to the following
description and accompanying drawings.
FIG. 1A is a simplified schematic diagram of the telephone line side of a telemetry device in accordance with the present invention.
FIG. 1B is a simplified schematic diagram of the control side or meter side of a telemetry device in accordance with the present invention.
FIG. 2 is a flow chart depicting the operational flow of one embodiment of the present invention.
FIG. 3 is a flow chart depicting the operational flow of a more efficient embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
I. Operational Environment
As discussed above, AMR systems utilize the existing telephone system to collect telemetry data from a plurality of remotely located meter interface units (MIU's). To be cost effective, the MIU shares the existing telephone line with the
subscriber telephone set without significantly interfering with the subscriber's usage of the telephone system.
Functionally, the MIU can be considered to be a "smart" telephone in that it automatically determines when the phone line is available for the MIU's use (via the static off-hook detector), takes itself off-hook, dials a preprogrammed telephone
number, communicates over the phone line, and then hangs up. The telephone central office system cannot distinguish the operation of the MIU from a manual telephone call which the subscriber might make.
When a telephone device is taken off-hook, the impedance at the terminals of that device drops from a very high on-hook impedance to a few hundred ohms. With the normally open end of the subscriber line now terminated with this low impedance,
the telephone set draws a loop current from the central office switch of approximately 40 mA. As long as loop current is being drawn, the central office switch sees the subscriber line as being off-hook. The telephone audio signal appears as a direct
modulation of the loop current, namely as an AC signal superimposed on the DC loop current.
Although the loop current is nominally 40 mA, the actual value of the loop current depends upon a multitude of factors. For instance, each wire in the two-wire cable pair which runs from the central office to the subscriber's premises has a
series impedance associated with it. The actual resistance value depends upon the length of the cable run, the type of wire and other cable characteristics. Additional factors, such as the age and condition of the cable, also affect the series
impedance of the telephone line. Furthermore, the off-hook terminating impedance of the subscriber telephone set significantly affects the value of the line impedance seen at the terminating end (at the central office switch) of the cable. When these
external variations are coupled with the tolerances of the central office equipment itself, the magnitude of the loop current can vary greatly from circuit to circuit.
Recalling that the MIU must be able to detect when another telephone device, ostensibly the subscriber telephone set, has interrupted a telemetry session, it can be seen that the dynamic off-hook detector function is unique to the MIU. That is,
the subscriber telephone set is always manually disengaged, by hanging up the telephone, and does not require a dynamic off-hook detector. In contrast, a properly designed MIU must automatically disengage itself from the phone line when a contention for
the circuit arises. When the MIU telemetry session is interrupted by a contending telephone device on the same phone line, the contending telephone device 5 appears in parallel across the MIU's input terminals, as a shunt load. Therefore, if both
devices had identical impedances, half the loop current would be shunted away from the active MIU. The dynamic off-hook detector function can be implemented by sensing this "current robbing" which is caused when the contending device comes on line.
A telemetry device including a dynamic off-hook detector which senses current robbing by the contending device to determine line status is described and claimed in my patent entitled "Outbound Telemetry, Device", U.S. Pat. No. 5,204,896, the
disclosure of which is incorporated herein by reference. Contending telephone devices include the standard subscriber telephone set, modems, fax machines or any other subscriber device hooked to the telephone line.
Unfortunately in practice, both the loop current and the shunting impedance of the contending device can vary greatly, making it difficult to set a reliable trigger point for such a dynamic off-hook detector. The problem is compounded still
further by certain telephone configurations, such as party lines and remote office extensions, where the contending device can be miles away from the MIU.
One approach to accommodate these wide variations in off-hook loop current would be to provide an adjustable component in the MIU which could be manually adjusted by the installer to set the trigger point for the off-hook detector. This approach
is not economically desirable because each MIU would require substantially more installation time at the user's premises. Moreover, the MIU expense would be increased by the cost of the adjustable component which typically costs more than a fixed value
component. The MIU expense would also be increased because a more well-sealed, weather fight (and expensive) MIU housing would have to be used to assure that the setting of the adjustable component is not disturbed by a hostile environment.
However, even if the use of a manually adjustable component were not the issue, the telephone company often changes, unbeknownst to the subscriber, the physical cable pair used by the subscriber, as it maintains or upgrades the phone lines. Such
a change would render useless the initial adjustment of the adjustable component performed by the installer.
II. Theory Of Operation
The present invention overcomes the wide variation in operating parameters found in the telephone system by automatically tuning itself to the telephone line. Prior to the beginning of each telemetry transaction, the MIU dynamically adjusts the
amount of loop current it draws to a predetermined optimum value to set the trigger point for the off-hook detector. Essentially, this pre-biases the MIU to an operating point where the dynamic off-hook detector is most capable of detecting when a
contending telephone device has come off-hook.
As mentioned briefly above, the dynamic off-hook detector described in my patent entitled "Outbound Telemetry Device", U.S. Pat. No. 5,204,896, senses the current robbing effect of the shunt load of the contending device to disengage an active
MIU. Fundamentally, this MIU was designed to require a nominal amount of current to operate; this was not necessarily the minimal amount of current. Furthermore, the design of the MIU was such that it could not operate unless the loop current was in
excess of this nominal value. In other words, the MIU would return to the static mode unless it could pull sufficient current from the subscriber loop to retain it in the dynamic mode.
In principle, by judiciously selecting the nominal loop current required to operate the MIU, it could be assured that another off-hook device (contending device) would shunt away enough current to cause the active MI to "intentionally
malfunction" and cease transmitting data, thus returning it automatically to the static mode and thereby releasing the subscriber line. In that MIU, the basic design of the MIU partially determined the required operating current and a fixed value load
resistor was used to increase the operating current to the nominally desired value. Although this technique worked well for the average subscriber loop, it had the disadvantage that the fixed value load resistor wasted power which is especially precious
for line powered devices.
In practice, variations in both the central office loop current and the off-hook impedance of the contending telephone devices limited the dynamic range of this technique. On weak current loops the nominal loop current required by the MIU proved
to be excessive and prevented the MIU from operating. Conversely, on strong loops, another off-hook contending telephone device, especially one with a high off-hook impedance, could not steal away enough current from-the MIU to current starve it back
into the static mode. To compensate for this limitation, one embodiment of the present invention replaces the aforementioned fixed value resistor with a resistor whose value can be adaptively adjusted under program control of the MIU microprocessor.
The MIU of the invention operates by adaptively loading the subscriber loop to compensate for wide variations in the loop current, thus permitting a predictable loop current change to occur when another telephone device comes off-hook.
Irrespective of the loop characteristics, this current change becomes both predictable and constant thereby allowing the presence of the contending telephone device to be detected.
III. Schematic Description: MIU Overview
FIGS. 1A and 1B show a simplified schematic diagram of an MIU as being composed of two parts. FIG. 1A shows the components on the telephone line side of the MIU (alternatively referenced as the primary side), while FIG. 1B shows the components
on the meter side (alternatively referenced as the control side). It is noted that each side is electrically isolated from the other, either optically, by the use of opto-couplers, or magnetically, by the use of transformers. This point of demarcation
is collectively referred to as an isolation barrier and is designated as barrier 10 in FIG. 1A. Functionally, isolation barrier 10 protects equipment and personnel on either side of the MIU and also assures that the telephone line remains electrically
balanced and isolated from ground, as it must for proper operation.
Most of the novel elements of the invention are contained on the control side of the MIU, in FIG. 1B. However, FIG. 1A is included for completeness to show how a complete MIU is configured when operating in conjunction with the invention.
Accordingly, FIG. 1A is a simplified representation of the line side of a functional MIU.
Although FIG. 1B shows the preferred embodiment of the invention, the techniques taught by this invention can be implemented in several different ways. Therefore, it should be understood that the embodiment shown represents just one
implementation. Other implementations are possible and examples of these implementations will be discussed after the discussion of the preferred embodiment.
Before discussing the operation of the MIU, it is important to note that the MIU illustrated is one which is line-powered. That is, the MIU draws all of its operating power from the telephone line itself, with the exception of a small battery 15
which maintains the real time clock U24 as seen in FIG. 1B. In the static mode, this power is very minute, namely less than 10 .mu.A as per FCC requirements. To meet this stringent low current requirement, all the logic shown on the primary side of the
MIU (FIG. 1A) is static CMOS logic. Since CMOS logic consumes virtually no power unless some logical input changes state, the standby current of the MIU can be made extremely small. To appreciate how small this current is, it is noted that the value of
resistor R1, which limits the standby current drain of the CMOS logic, is approximately 6-10M.OMEGA..
Referring to FIG. 1A, the open circuit voltage, V.sub.IN, of the telephone line (approx 48 volts) appears at the input terminals 20A and 20B of the MIU when the line is first connected to the MIU. A diode bridge 25, consisting of four diodes, at
the MIU input assures that the MIU is polarity insensitive since the bridge assures that the more positive voltage potential always appears on the V.sub.p side of the bridge. For reasons explained shortly, when the phone line is first connected to the
MIU or when the MIU is first turned on and initialized, power FET Q1 is off so no current is drawn through that device. Hence, only a small amount of current is drawn through zener diode D1 and resistor R1. Zener diode D1 and parallel capacitor C1 form
a simple voltage regulator to stabilize the operating voltage of the MIU's CMOS logic (later described) at approximately 5 volts, which is the zener voltage of diode D1. Recalling that the value of R1 is approximately 6-10M.OMEGA., the current drain of
the MIU in the static, quiescent state is minuscule.
When the MIU is first connected to the telephone line, i.e. when the MIU is first powered up, the series resistor R and capacitor C together act as an R-C circuit which momentarily holds the input to OR gate U4 at a logic high state. This
provides a "one-shot" power-up pulse to the reset input of SR (set-reset) flip-flop U5 so as to initialize all the logic on the primary side of the MIU (FIG. 1A). The width of this power-up pulse is proportional to the R-C time constant and causes SR
flip-flops U5, U8 and U2 to be reset in sequence; the reset condition is one where the Q output of the flip-flop is a logic low and the Q (or not-Q) is a logic high. Since the SR flip-flop U8 is reset, the Q output is a logic low assuring that power
ZFET Q1 is turned off at initialization as earlier stipulated.
The above described logical state of the MIU is the static or quiescent mode, as referenced earlier. In this static state, with FET Q1 off and diode D3 blocking any return current, only components to the left of capacitor C1 are actively biased
by leakage current from the subscriber loop or phone line. The minuscule standby current drawn by the MIU in this static state is sufficient to operate the CMOS logic and establish an operating voltage across zener diode D1 prior to returning to the
phone line through the resistor R1. (The logic devices of the MIU are implemented in CMOS logic to keep any current drain from the phone line as low as possible.)
To place the MIU in the active mode, power FET Q1 is turned on. In the on state, the series impedance of the FET is only a few ohms, thus causing the MIU to draw substantial loop current (20-80 mA) through zener diode D2 and the low DC
resistance of the primary winding of transformer T2. Zener diode D2 in conjunction with the filter capacitor C2 forms a voltage regulator to stabilize the operating voltage for a power inverter 30 formed by oscillator U9, inverter U10, transistors Q2
and Q3, and center-tapped transformer T1 (FIG. 1A).
Shortly after the MIU goes off-hook, drawing current from the subscriber loop, the voltage appearing across the filter capacitor C2 causes the oscillator U9 of power inverter 30 to become operative. The output of oscillator U9 is essentially a
square wave with approximately a 50% duty cycle. The oscillator U9 turns on transistors Q3 and Q2 (via inverter U10) on opposite phases of the square wave, alternately energizing each side of the center-tapped transformer T1. The switching action of
transistors Q2 and Q3 on the primary side of transformer T1 (FIG. 1A) drives an AC power signal into the transformer which is magnetically coupled to the transformer's secondary winding (FIG. 1B) on the other side (meter side) of the isolation barrier
10. The recovered AC power signal appearing across the secondary winding is then full wave rectified by diodes D5 and D6 and filtered so as to provide DC operating power (power supply voltage V.sub.s) for the components on the meter side of the MIU.
Thus, operating power for the MIU in the active mode is derived entirely from the telephone line loop current. (It is also possible to use a single phase power inverter, instead of a center-tapped design shown here, but the conversion efficiency would
be less.)
Again referring to FIG. 1A, several other components are present on the telephone line side of the MIU. Most noticeably, a static off-hook detector U1 monitors the telephone line input to the MIU and provides a logic high indication if the
telephone line is available for use. (My patent entitled "Outbound Telemetry Device", U.S. Pat. No. 5,204,896, describes in detail one static off-hook detector circuit which may be employed as detector U1.) Also, a timer U6 provides a logic high
output once 2.8 seconds, or more, have elapsed since the timer was enabled by removal of a logic high resetting signal. Finally, two receptor transistors Q4 and Q5 for a pair of opto-couplers, 35 and 40, respectively, allow logic signals to be
transferred across the required isolation barrier 10, from the meter side (FIG. 1B) to the line side (FIG. 1A). Each opto-coupler includes a sender, namely a light emitting diode (LED) which is optically coupled to, but electrically isolated from, the
receptor transistor. When the LED is activated, the light falling on the receptor transistor causes the transistor to turn on.
Previously, it was noted that to switch the MIU from a static mode to an active mode required FET transistor Q1 to be turned on, from its normally off condition. It will be shown that, at an appointed alarm time, the MIU's real time clock U24
(FIG. 1B) will generate a short start pulse. This start pulse will be transferred across the isolation barrier 10 via opto-coupler 35, momentarily activating transistor Q4 (FIG. 1A) on the line side of the MIU. Recalling that in the static mode all SR
flip-flops are initially reset (Q=logic low), the momentary logic high appearing on the set input of SR flip-flop U5 will set the Q output to a logic high.
If at this time the telephone line is not being used, the output from the static off-hook detector U1 will also be a logic high causing AND gate's U7 output to go high. If, however, the telephone line is in use by a contending telephone device,
the logic high output from AND gate U7 will be delayed until such time as the static off-hook detector U1 indicates that it is available. Subsequently, SR flip-flop U8 will be set, turning on FET Q1. So long as FET Q1 is on, the MIU is in the active
mode (off-hook) and pulls current from the telephone subscriber loop. At the same time as FET Q1 is turned on, the Q output of SR flip-flop U8 goes to a logic low, enabling 2.8 second timer U6 and removing the forced reset condition on SR flip-flop U2.
With the MIU active, oscillator U9 is activated and supplies power to the meter side of the MIU, per the prior discussion. It will be shown subsequently, that transformer T2 couples DTMF (dual tone multi-frequency) dialing tones and telemetry
data across isolation barrier 10 and onto the phone line, thereby dialing and transferring the telemetry data to a remote site via the subscriber loop.
When the telemetry data transfer is complete, the logic on the meter side (control side) of the MIU (FIG. 1B) will generate a short stop pulse which is coupled across isolation barrier 10, in a manner analogous to that described for the start
pulse. If 2.8 seconds, or more, have elapsed since the MIU entered the active mode, this momentary logic high pulse will set the Q output of SR flip-flop U2 to a logic high, via AND gate U3. The timer U6 provides a power-up guard time of 2.8 seconds to
permit the MIU logic on the control side to power up and become functional before enabling one of the AND gate U3 inputs; once the guard time has elapsed a stop pulse can set SR flip-flop U2. Should this occur, the logic high Q output of SR flip-flop U2
will cause SR flip-flop U5 to be reset (via OR gate U4) which will, in turn (via the logic high Q output) then reset RS flip-flop US. When SR flip-flop U8 is reset the Q output goes low, turning off FET Q1, while its Q output goes to a logic high state,
disabling timer U6 and resetting SR flip-flop U2 which initiated the described active mode. It can be seen that all SR flip-flops are now reset and that the MIU is logically in the static mode described earlier.
IV. Schematic Description: Dynamic Off-Hook Detector On The Meter Side Of The MIU-Overview
Having discussed the primary side or line side of the telemetry device of FIG. 1A, the discussion now turns a more detailed discussion of the meter side or primary side of the telemetry device as seen in FIG. 1B.
Referring to FIG. 1B, the full wave rectifier and filter capacitor described earlier, consisting of diodes D5 and D6 and capacitor C7, can be seen. The zener diode D7 in conjunction with capacitor C7 stabilizes the DC operating voltage V.sub.s
on the meter side (control side) of the MIU. For reasons explained shortly, zener diode D7 has a zener voltage (approx 5.6 volts) sligh | | |
