Note: Descriptions are shown in the official language in which they were submitted.
CA 02428619 2003-05-21
PARALLEL OFF-HOOK DETECTION FOR BOTH
LINE-AVAILABLE AND PHONE PICK-UP
DETECTION
Technical Field
The present invention relates to a device for
detecting the status of a telephone line so that
communication equipment sharing the line with a telephone
will not interfere with normal usage of the telephone via
the line.
Background of the Disclosure
Many automated devices, such as answering machines,
facsimile machines, modems and the like, share a standard
telephone line with a subscriber's telephone set.
Generally, priority should be given to use of the line
by the telephone set. Problems have arisen, however, in
detecting a subscriber's attempts to access the line
through the telephone set. for purposes of discussion
we will consider a data reporting device as one such
automated system which shares a telephone line with a
subscriber's telephone set.
An automated data reporting device transmits utility
meter data or similar data from a subscriber's premises
to a central computer database and/or receives
programming and control data from the central computer.
CA 02428619 2003-05-21
2
Such devices eliminate the manual labor required to go
out to the subscriber's premises and read or program the
meter.
Many such data reporting systems use the
subscriber's telephone line and the public switched
telephone network to carry the data between the
subscriber's premises and the central database. F'or
example, the data reporting device might include an
autodialing modem and periodically initiate a call in to
the central database to transfer the latest meter data.
Some of these systems can answer calls from th.e central
database and accept programming or control data. Data
reporting devices which use the subscriber line ~ however,
share the line with the subscriber's telephones) and/or
any other customer premises equipment connected to the
line.
In such telephone line systems, the subscriber's
telephone or other equipment is the primary communication
device connected to the line and must be given priority
over the secondary communication device, e.g. the data
reporting device. ~peration of the data reporting device
or other automated device connected to share the line
therefore must not interfere with the subscriber's use
of the telephone set on the telephone line. 'fo prevent
interference' two problems must be addressed. first, the
automated device must not attempt to seize the line if
CA 02428619 2003-05-21
3
the telephone is using the line at a time when the
automated device is ready to access the line, e.g., to
report data. Second, during use of the line by the
automated device, if the subscriber picks up the
telephone handset to use the telephone, the automated
device must relinquish the line to the telephone.
Circuits have been proposed which detect an existing
off-hook condition, and devices have been proposed to
detect a pick-up of a telephone connected in parallel
with the data reporting device. Such circuits, however,
have tended to be rather complex, particularly if capable
of both existing line use detection and pick-up
detection. Due to such complexity, these circuits have
been expensive to manufacture and subject to false
detection results.
Detection of a pick-up of a telephone connected in
parallel with the data reporting dev~_ce~or the like can
be particularly difficult because the resultant voltage
drop tends to be quite small. As a result, devices
intended to detect parallel pick-up often have been
ineffective.
A circuit has been proposed which would detect
parallel pick-up by sensing changes in AC loading on the
telephone line (U. S. Patent 4,958,371 to Damoci et al).
The approach in that patent, however, requires generation
and application of a 300 Hz test signal to the line.
CA 02428619 2003-05-21
4
Another problem with prior art systems arises
because many such systems detect line voltage directly,
as an indication of the on-hook or off-hook state of the
associated customer premises equi~ament. Different lines,
however, exhibit different off-hook and on-hook voltages.
For example, a residential line will.typically have an
on-hook, voltage of 48 volts DC. The off-hook voltage
typically will be as low as 3 volts DC, but the off-hook
voltage may be as high as 26 volts DC. In contrast, the
on-hook voltage for a PBX line typically is 22-26 volts
DC. In view of this voltage overlap between off-hook of
residential lines and on-hook for PBX line, it is
necessary to use a different voltage detector for each
different type of line~
Disclosure of the Invention
The present invention provides efficient and
effective means for solving the above noted problems of
sharing the use of the subscriber's telephone line
between the subscriber's primary telephone equipment and
some secondary communication device, without applying any
additional signals to the line.
The invention provides method and apparatus for
detecting whether or not the primary telephone equipment
is off-hook. This detection can be used to prevent or
inhibit the secondary device from seizing the line if the
CA 02428619 2003-05-21
primary communication device is off-hook.
The invention also detects parallel pick-up; e.g.
of the telephone handset, during a period when the
secondary device is already using the line. This
5 detection can be used to control the secondary device to
terminate its use of the line and thereby make the line
available for use by the primary telephone device.
As part of the line availability detection, an
average interval for charging a capacitor to a threshold
in'response to an on-hook voltage appearing across flue
telephone line is determined and periodically updated.
Typically, the average is determined by periodically
applying the line voltage to charge the capacitor.
During each periodic sampling, the time to charge the
I5 capacitor is measured and the time is recorded as a
sample interval. The current sample interval and a
previously stored average interval are averaged together,
and the new average value is stored. Subsequently, at
a time when the secondary communication device is ready
to use the line, the time interval for charging the
capacitor in response to the voltage appearing across the
telephone is measured. If the secondary communication
device determines that the measured time interval exceeds
the stored average interval by more than a predetermined
amount, the secondary device recognizes that the
telephone or other primary customer premises equipment
CA 02428619 2003-05-21
is off-hook. The secondary device seizes the line only
if the measured interval does not exceed the average
interval by more than the predetermined amount. The
periodic averaging of the charging interval allows the
detector to learn the typical on-hook voltage for the
particular line on which the detector is installed. The
interval averaging and comparison process thereby
eliminates the need for a voltage detector selected or
designed to operate solely on the particular type of line
1~ on which the detector is installed. This apt~roach allows
one line state detector circuit to operate on different
types of lines exhibiting different voltages, including
PBX lines and residential lines.
During line seizure by the secondary
communication device, a steady-state condition is brought
about between the secondary device, the line steady-state
impedance, and the central office voltage source. If the
line impedance drops from the steady-state value, the
detector assumes that the subscriber has taken the
telephone off-hook or that other primary customer
premises equipment has gone off-hook, and the line
seizure by the secondary communication device is
terminated.
CA 02428619 2005-06-29
6a
In accordance with an aspect of the present invention, there
is provided a communication device for seizing a telephone line
and communicating over the telephone line, said communication
device including an apparatus for detecting that a subscriber's
telephone connected to the telephone line is off-hook at a time
when the communication device is ready to seize the telephone line
and for detecting if the telephone is taken off-hook during a time
when the communication device has seized the telephone line, said
apparatus comprising(a)means for determining an average interval
for charging a capacitance to a threshold voltage in response to
an on-hook voltage appearing across the telephone line; (b) means
for measuring an interval for charging the capacitance to the
threshold voltage in response to a voltage appearing across the
telephone line at the time when the reporting device is ready to
seize the telephone line; (c)means for recognizing that the
telephone is off-hook and inhibiting line seizure by the
communication device if the measured interval exceeds the average
interval by more than a predetermined amount; (d) means for
detecting a steady-state impedance across the telephone line when
the communication device has seized the telephone line; (e) means
for detecting a drop in impedance across the telephone line from
the steady-state while the communication device has seized the
telephone line and terminating seizure of the line by the
communication device.
Additional objects, advantages and novel features of the
invention will be set forth in part in the description which
follows, and in part will become
CA 02428619 2003-05-21
7
apparent to those skilled in the art upon examination of
the following or may be learned by practice of the
invention. The objects and advantages of the invention
may be realized and attained by means of the
instrumentalities and combinations particularly pointed
out iri the appended claims.
Brief Description of ~rawincrs
Figure 1 is a simplified block diagram of an
automated communication device, incorporating the
1~ parallel off-hook detector of the present invention.
Figure 2 is a simplified block diagram of a
preferred embodiment of the present invention, wherein
the parallel off-hook detector is incorporated into a
data reporting device such as might be used for automated
reading of a utility meter.
Figure 3 is a detailed circuit diagram depicting the
parallel off-hook detector of the present invention
together with the transformer and the line seizure relay
which may be connected to the subscriber's telephone line
in the embodiment of Figure 2.
Figure 4 is an equivalent circuit diagram showing
the parallel connection of the data reporting device of
Figure 2 (incorporating the parallel off-hook detector
of the present inventions and a subscriber's telephone
CA 02428619 2003-05-21
to the line to a central office, for use in explaining
the operation of the present invention.
Figures 5 to 10 are flow charts illustrating various
processing operations performed in accord with the line
availability determination of the present invention.
Figure 11 is a flow chart depicting processing
responsive to an interrupt signal7.ing indicating a
parallel pick-up of the primary equipment.
Best Mode for Carr_yina out the invention
Figure 1 illustrates a generic embodiment of the
present invention. In this embodiment, the primary
communications equipment includes at least a standard
telephone set 122. The telephone 122 shares the
telephone line 11~ to central. office 120 with some form
of secondary communications equipment 112. Although the
primary communications equipment is shown as a telephone
set 122, the primary communications equipment can be any
telephone line compatible device which is to have
priority over the secondary equipment 112 for purposes
of using the telephone line 114. Similarly, the
secondary communications equipment can take a variety of
forms, such as a voice answering machine, a facsimile
machine, a PC with a modem, a meter reader, etc., which
are to have lower priority. For' example, if the
secondary device were an answering machine, the primary
CA 02428619 2003-05-21
9
device typically would be a telephone set. In another
example, if the secondary device reports meter data or
the like, the primary equipment may include both the
telephone 122 and' any other customer p~°emises equipment,
such as a voice answering machine, a facsimile machine,
a PC with a modem, etc. , which has priority over the data
reporting device.
The secondary communications equipment 112 connects
to the subscriber premises end of telephone line 114
through a coupling transformer T1 and a line seizure
relay 116. The parallel off--hook detection device 111
shown in Figure 1 includes a programmable microprocessor
based controller 110, a Line Available Detector 124 and
a Pick-up Detector 126. The controller 110 connects to
secondary communications equipment 112 to exchange
information relating to line seizure. The controller 110
also connects to the relay 116 to control operation
thereof for opening and closing the telephone loop.
The line seizure relay 116 provides selective
connection to one of the leads of telephone line 114.
The primary of the coupling transformer T1 is connected
to the relay 116 and the other lead of the telephone line
114. As illustrated in Figure 1, the line seizure relay
16 connects between the TIP lead and one lead of the
primary of the transformer T1. The other lead of the
primary of the transformer Tl connects to the RING lead.
CA 02428619 2003-05-21
1~
Closure of the line seizure relay 16 permits current flow
through the primary of the coupling transformer T1 and
thereby creates an off-hook condition.
The Line Available Detector 124 and the Pick-up.
Detector 126 each connect to both the TIP and RING leads
of the telephone line 14, but in a slightly different
manner. In this embodiment, the Line.Available Detector
124 and the Pick-up Detector 126 both connect to the RING
lead. The Line Available Detector 124 connects to the
TIP lead at a point on the Central Office side of the
Line seizure relay 116, and the Pick-°up Detector 126
connects to the TIP lead on the same side of the relay
as the coupling transformer T1.
At a time when the secondary communications
equipment 112 has not yet accessed or seized the line by
closure of the relay 116, if the associated subscriber's
telephone 122 is on-hook, the loop of telephone line 114
is open. The entire central office loop voltage appears
across the TIP and RING of the subscriber's line. If the
subscriber's telephone 112 is off-hook, however, the
telephone would appear as a 600 Ohm impedance across the
line closing the loop through the TIP and RING pair of
the telephone line. The line voltage would be
substantially lower.
To seize the line, the secondary communications
equipment provides a line seize signal to the controller
CA 02428619 2003-05-21
11
110. The controller applies an activation signal to the
Line Available Detector 124, in response to which the
Detector 124 provides signals which the controller 110
can interpret to determine if the telephone 122 is
off-hook or if the line 114 is available so that the
secondary communications equipment.can use the line 114.
If the telephone 122 is off-hook, the controller 110
waits for some predetermined period and repeats the line
available checking procedure. When the line 114 becomes
1o available for use by the secondary equipment, the
controller 110 activates relay 116 to seize the line and
provides appropriate signals to the secondary
communications equipment 112 informing it that the line
is now ready for use.
At a time when the secondary communications
equipment 112 is using the line 114, the line seizure
relay 116 is in a closed state, and the impedance of the
coupling transformer T1 appears across the telephone
line. If a primary piece of subscriber premises
equipment, such as telephone 122, attempts to use the
line at such a time, for example because the subscriber
lifts the handset of telephone set 122 off-hook, the
impedance of the primary equipment would appear across
the line in parallel with that of the transformer T1 and
associated secondary communications equipment 112. The
Pick-up Detector 126 senses a sudden drop in impedance,
CA 02428619 2003-05-21
12
and provides a signal to the controller 110 indicating
the drop in impedance.
In response to the indication signal from the
Pick-up Detector 126 signifying that primary equipment
has attempted to sere the line 114, the controller 110
deactivates relay 116 and interrupts communications,by
equipment 112. The subscriber picking up the handset 122
will hear silence on the line. A fairly natural reaction
of a subscriber who hears silence 'anstead of the expected
dial tone is to hang up briefly and try again or to
actuate the hookswitch briefly one or more times and wait
to see if dial tone appears. In either case, the
telephone network central office 1.20 senses an
interruption of line current indicating the end of the
existing call connection followed by a new off-hook
state. In response, the central office 120 will apply
a dial tone to the telephone line 114. ~ The subscriber
can then dial out to place a call in the normal manner.
Figure 2 depicts a preferred embodiment of the
present invention, wherein the parallel off-hook detector
is incorporated into a data reporting device for
automated reading of a utility meter.
The secondary communication device in this
embodiment includes a programmable microprocessor based
controller 10, which includes a central processing unit
(CPU) ~ random access memory (RAM) , read only memory (ROM)
CA 02428619 2003-05-21
13
etc. The Input/output ~I/~) ports of the controller 10
send and receive digital signals to and from an
associated digital device. For ea~~mple, in an
electricity metering and control system, the Input/Owtput
(I/O) ports of the controller 10 connect to a
programmable meter/reader and control device (not shown)
for two-way transfer of inforanation. The controller 10
performs the function of controller 110 in the first
embodiment and performs certain data processing and
control functions of the secondary equipment :112.
A modem 12 connects to the subscriber premises end
of telephone line 14 through a coupling transformer Tl
and a line seizure relay 16. Although illustrated as
separate components for convenience, the line seizure
relay 16 and coupling transformer T1 may be components
of the modem. The controller 10 connects to the modem
12 via a standard serial interface for transmission and
reception of data via the telephone line 14. The
controller 10 also connects to the relay 16 via line 17,
to control operation thereof for opening and closing the
telephone loop by connecting the primary of transformer
T1 across telephone line 14. One or more pieces of
primary subscriber premises telephone equipment, such as
a telephone set 22, connect to the Subscriber premises
end of telephone line 14 ~n parallel with the data
reporting device. The primary device may include any
CA 02428619 2003-05-21
1~
customer premises equipment, such as a voice answering
machine, a facsimile machine, a PC 'with a modem, etc.
which is to be given higher priority for accessing the
line than the particular type of secondary equipment, in
this specific case a utility meter data reporting device.
In this embodiment, the line seizure relay ,16
connects between the RING lead of telephone line 14 and
one lead of the primary of the transformer T1. The other
primary lead of the transformer T1 connects directly to
the TIP lead. Closure of the line seizure relay 16
permits current flow through the ~0~ Ohm secondary
impedance of the coupling transformer T1 and thereby
creates an off-hook condition (see also Figure 3)..
The meter reading device of Figure 2 also
incorporates a Line Available Detector 24 and a Pick-up
Detector 26. The Llne Available Detector 24 arad the
Plck-up Detector 26 both connect t0 the TIP lead. The
Llne AVallable Detect~r 2~ cC~nnectS t0 the RING lead at
a point on the Central Office side of the line seizure
relay 16, and the Pick-up Detector 26 connects to the
RING lead on the same side of the relay as the primary
of coupling transformer T1.
At a time when the data reporting device has not yet
accessed or seized the line by closure of the relay 16,
if the associated subscriber's telephone 22 is on-hook,
the telephone loop is open. The entire central office
CA 02428619 2003-05-21
loop voltage (typically a 24-4~ volts DC battery voltage)
would appear across the TIP and RING of the subscriber's
line. If the subscriber's telephone is off-hook,
however, the telephone would appear as a 600 Ohm
5 impedance across the line closing the loop through the
TIP and RING pair of the telephone line (see also Figure
3). The line voltage would be substantially lower.
The Line Available Detector 24 responds to an
activation signal from the controller 10 and provides
10 signals which the controller 10 can interpret to
determine if the telephone 22 is off-hook or if the line
14 is available so that the data reporting device can
call the central computer, in a manner to be explained
in more detail below.
15 If the telephone 22 is off-hook, the line is not
available for use. by the data reporting device.
Therefore, the controller 10 waits for some predetermined
period and repeats the line available checking procedure.
The controller repeats this processing loop until it
determines that the telephone line 14 is available. When
the line becomes available, the controller 10 activates
relay 16 to sere the line and provides appropriate
signals to the modem 12 instructing it to send dual tone
multifrequency (DTMF) dialing signals t~ initiate a call
to the central computer. Alternatively, the controller
10 could intermittently activate the relay 16 to simulate
CA 02428619 2003-05-21
16
pulse dialing. When the call reaches the central
computer, the controller 10 executes appropriate
procedures for reporting current meter reading data to
the central computer and receives back any necessary
instructions or control data from the central computer.
At a time when the data reporting device is using
the line, the line seizure relay 16 i~ in a closed state,
and the 600 Ohm impedance of the coupling transformer T1
appears across the te7_ephone line. The subscriber's
customer premises equipment, however, is on-hook and
appears as a very large impedance across the line. If
a piece of subscriber premises equipment attempts to use
the line at such a tame, for example because the
subscriber lifts the handset of telephone set 22 off--
hook, a second 600 Ohm impedance would appear across the
line in parallel with that of the meter data reporting
device (see also Figure 3). The Pick-up Detector 26
senses a sudden drop in impedance, i.e. the change from
a very high parallel impedance to the 600 ohm parallel
impedance, and provides a signal to the control7_er 1.0
indicating the drop in impedance.
In response to the indication signal from the
Pick-up Detector 26 signifying that other equipment has
attempted to seize the line, the controller 10
deactivates relay 16 and interrupts modem transmissions.
This results in loss of carrier at the centra7_ office
CA 02428619 2003-05-21
17
computer, and therefore the modem at the central office
computer assumes that the data reporting device has hung
up. In response, the modem at the central computer will
terminate its connection to its telephone line, and the
subscriber picking up the handset will hear silence on
the line. A fairly natural reaction of a subscriber who
hears silence instead of the expected dial tone as to
hang up briefly and try again or to actuate the
hookswitch briefly one or more times and wait to see if
dial tone appears. In either case, the telephone network
central office sees an interruption of line current
indicating the end of the existing call connection
followed by a new off-hook state. In response, the
central office will apply a dial tone to the telephone
line 14. The subscriber can then dial out to place a
call in the normal manner.
After the interruption of the data reporting call,
the controller 10 will periodically activate the Line
Available Detector 24 to check the line status. When the
line becomes available again, the controller 10 can
proceed with a new data reporting call to the central
computer, as discussed above.
Line Available Detection
The present invention determines line availability
based on the voltage appearing across the telephone line.
CA 02428619 2003-05-21
1. 8
This voltage is sensed by measuring the time required to
charge a capacitor. With reference now to Figure 3, the
Line Available Detector 24 comprises the circuit
COmpOneritS U7, Uf, e:20, R4, R6, R14, R15, R24, R25, R34,
R43, CR4, and CR7.
To sense line voltage, the CPU. of. the controller 10
will gate the opto-coupler U7 on to allow sampl ing of the
telephone line voltage. Specifically, the CPU will apply
an activating voltage to the light emitting diode of
opto-coupler U7. This activates the transistor of
opto-coupler U7 and applies the voltage between the TIP
and RING leads across the RC circuit formed by resistors
R24, R25, and R34 and capacitor C20. The line voltage
will charge the capacitor C20.
The Programmable Unijunction Transistor (PUT) CR7
will trigger when the voltage applied thereto passes a
threshold value thereof established by resistor R25 and
zener diode CR4. Thus, when the voltage on capacitor C20
exceeds the threshold of the PUT CR7, the PUT CR7 turns
on momentarily and discharges the voltage from the
capacitor C20 through the light emitting diode of .the
opto-coupler U8. This activates the transistor of opto-
-coupler U8 causing a pulse output to the CPU.
If the voltage across the TIP and RII3G is high, the
current flowing through the RC circuit will be higher
than when the voltage across the TIP and RING is low.
CA 02428619 2003-05-21
19
As a result, the rate of chaxging will vary as a function
of the voltage across the line. Different charging rates
will result in different periods of time between. the
start of charging by gating opto-coupler U7 on and the
pulse output by opto-coupler U8 indicating that the
capacitor voltage exceeds the threshold. It has been
found that the time difference (c:harging interval)
between gating opto-coupler U7 on and the pulse output
from opto-coupler U8 is exponentially related to the DC
voltage across the TIP and RING pair of the line. For
example, if the line voltage is approximately 48 volts
DC, the average charging interval will be around 60 ~s;
if the line voltage is approximately 22 volts DC, the
average charging interval will be around 1.00 ACS; if the
line voltage is approximately 15 volts DC, the average
charging interval will be around 140 ~a~ and if the line
voltage is approximately 14 volts DC, the average
charging interval will be around 166 ~s~ Therefore, the
charging time can be measured as an indication of line
voltage.
When the subscriber's telephone is on-hook, a high
voltage appears across the telephone line, and the
capacitor C20 will charge relatively quickly. The
capacitor voltage will exceed the threshold shortly after
the~CPU gates the opto-coupler U7 on. When subscriber's
telephone is off-hook, a low voltage appears across the
CA 02428619 2003-05-21
telephone line, and the capacitor C20 charges at a slower
rate. It therefore will take longer for the voltage on
the capacitor C20 to reach the threshold value and
trigger the PUT C2'7 and opto-coupler U8. The present
5 invention detects this timing difference to determine
whether the associated telephane is on-hook or off-hook.
In the present invention, the CPU learns a typical
average charging interval and assumes that the telephone
set is off-hook if a measured charging interval exceeds
10 the typical charging interval by more than a
predetermined amount. Specifically, the CPU periodically
(e.g. every thirty minutes) gates the opto-coupler U~ on
and measures the time between the pulse it applied to the
opto-coupler U7 and the output pulse received from the
15 opto-coupler U8. After each sample interval measurement,
the CPU averages the current sampled interval with the
previously stored average value. Specifically, the CPU
adds the current sample to the average and divides by 2.
The CPU stores this latest average in RAM as a new
20 repreSentatlOn Of the typiCal On-h~ok time.
When it is desired for the meter to make a call to
the central computer to report reading data, the CPU must
first determine if the subscriber's telephone is
currently in use. Figure 5 depicts this main processing
routine. When the main cyclic loop of the CPU indicates
a time to report data, the CPU calls a parallel off-hook
CA 02428619 2003-05-21
21
detection (PARALLEL OH~) routine. When the parallel
off-hook detection routine indicates that the line is
available, the CPU calls up its call origination routine
(CALL ORIGINATE) routine to seize the line and instruct
the modem to autodial the central computer. To determine
if the line is available, the CPU executes a sampling ~f
the line voltage as discussed above, and measures the
current time between the pulse applied to gate the opto-
-coupler U7 on and the output pulse received from the
l0 onto-coupler U8. This current sampled time measurement
is compared against the average or "typical" on-hook time
which the CPU stored in RAM. ~Cf the current sampled time
measurement exceeds the "typical" on-hook time by a
predetermined amount, the time sample indicates that the
TIP and RING line voltage decreased a substantial amount
representative of an off-hook state.
In the preferred embodiment, the predetermined
amount is 40 ACS. For example, assume that the telephone
line is a residential line. The on-hook voltage will be
approximately 48 volts DC, and the average charging
interval will be around 60 ~.s. The 40 ACS increase in
charging interval corresponds to a l~.ne voltage drop of
2 6 volts DC, and the interval threshold ( average plus the
predetermined amount) will be 1~0 us~ Thus, if the
charging interval at the time when the data reporting
device is ready to use the line is greater than 1o0 ~s,
CA 02428619 2003-05-21
22
the line voltage has dropped 26 volts or more (i.e. to
a value less than 22 volts). For example, if the line
voltage is approximately 14 volts DC, the measured
interval will be 166 ,~s, which is grater than the 100
,us threshold, and indicates that the telephone set 22 is
off-hook.
As a second example, assume telephone line 14 is a
PBX line. In this case, the ora-hook voltage is
approximately 22 volts DC, and the average charging
interval will be around '100 ~s. The 40 ,~s change
corresponds to a line voltage drop of around 7 volts DC,
e.g., from 22 volts DC to 15 volts DC. The interval
threshold (average plus the predetermined amount) for
this example will be 140 ACS. of the current sampled time
measurement exceeds the "typical'° on-hook time by the
predetermined amount (e.g. is greater than 140 ~,s), the
CPU assumes that a telephone set or the like is off-hook
and using the PBX line.
If the charging interval measurement at the time
when the data reporting device is ready to access the
line indicates that the telephone line is being used by
some primary piece of customer premises equipment, such
as the associated subscriber's teleph~~ne set 22, the CPU
does not initiate the call to the central computer. The
CPU will wait for a fixed time interval and check again
to determine if the subscriber's telephone is off-hook.
CA 02428619 2003-05-21
23
If the current sampled time measurement does not exceed
the "typical" on-hook time by the predetermined amount,
the time sample indicates that the line voltage
corresponds to an on-hook state. In this case, the CPU
assumes that the telephone line is not currently in use
and initiates the call to the central computer. An
example of a 'CALL ORIGINATE' routine using this
procedure appears in Figure 9.
As shown in Figure 9, when the call origination
routine starts, the CPU 'checks to determine if an
appropriate command has been received. If so, the CPU
sets the flag to execute the POHD check routine described
in more detail below. Then, if the POHD measurement is
done, the CPU analyzes the results to determine if the
primary equipment (e.g. the telephone) is on-hook. If
the primary equipment is off-hook the OPU terminates the
call origination procedure and the program ends. If the
telephone is on-hook, however, the CPU provides a signal
to the relay to seize the line by connecting the coupling
transformer to the telephone line, and call origination
continues until dialing is complete and data
communication is established with the central computer.
Figures 6 to 8 illustrate the processing routines
of one actual example of the ;~ara11e1 off-hook
(PARALLEL OHD) or line availability determination in
accord with the present invention. Figure 6 provides an
CA 02428619 2003-05-21
24
overview of the processing operations, and Figures '7 and
8 illustrate specific subroutines used in the process
flow of Figure 6.
Referring to Figure 6, when the PARALLEL OHD routine
starts, the CPU checks to determine if it is time for a
periodic measurement check of the charging interval. If
s~, the CPU sets a POND Check request flag. Iri the neXt
processing step, if the POND check flag is set, the CPL1
determines if the modem is Currently using the line for
a call to the central Computer or if the telephone is
ringing. If either of these two determinations is true,
then the CPU stops the parallel off-hook measurement and
Waits until the line is free to begin again. If the
modem is not using the line and the telephone is not
ringing, processing flOWS t0 the determination at step
SZ In Figure C ~ In thlS Step, the CPU determlneS Whether
or not the measurement has already started.
If the charging interval measurement has not
started, the CPU Calls the subroutine POHD START to begin
the measurement. This subroutine, Shown in Figure 7,
first Causes the CPU to assert a POH ON signal to
initiate the actual measurement. This gates the opto-
-coupler U7 on to allow sampling of the telephone line
Voltage and Charging Of the CapaC3tOr C2~, aS dlSCUSSed
above. The CPU stores the start time in a register
POHD_STARTrTIP~tE. The CPU also arms an interrupt signal
CA 02428619 2003-05-21
(POH TIME) routine. The CPU then awaits for the
interrupt (POH TIME). The POH TIME interrupt occurs when
the PUT CR7 triggers, indicating that the voltage on the
capacitor C20 exceeds the threshold. The CPU stores the
5 time that the interrupt occurred in register
POHD STOP TIME.
Returning to Figure 6, if the determination at step
S1 indicates that the charging interval measurement has
started, the CPU calls the subroutine POND CHECK. This
10 subroutine, shown in Figure 8, performs the actual
calculations summarized above. The CPU first determines
if the POH TIME interrupt has occurred. If not, the
routine results in an indication that the measurement is
not done yet. If this interrupt has occurred, the CPU
15 proceeds to calculate the current voltage level as
represented by the measured charging :interval.
Specifically, the CPU calculates
CURRENT LEVEL - POHD STOP TIME -
POHD START TIME.
20 Using the calculated level, the CPU next calculates
the change or DELTA. Specifically,
DELTA = CURRENT LEVEL - CAL LEVEL
where the value CAL LEVEL is the average charging
interval calculated during the last previous execution
25 of the POND CHECK subroutine. The CPU next compares the
interval change value DELTA to the threshold value. If
CA 02428619 2003-05-21
2~
the change in the charging,interval is greater than or
equal to the threshold (DELTA >_ THRESHOLD), the routine
results in an indication that the telephone is off-hook.
If the change in the charging interval is not
greater than or equal to the threshold (DELTA _<
THRESHOLD) , the routine results in an indication that the
telephone is on-hook. The CPU next checks to determine
if this POHD CHECK is the first measurement of the
charging interval. If so, the CPU stores the currently
measured charging interval (CURRENT LEVEL) as the average
value (CAL LEVEL) for use in the next measurement. If
this POHD CHECK is not the first interval measurement,
the CPU calculates a new average charging interval value
using the currently measured charging interval
CURRENT LEVEL and stores the new average as the value
CAL LEVEL. After storing an average value, an indication
is produced that the POHD measurement is complete and the
subroutine ends.
Returning to Figure 6, after the POHD~CHECK
subroutine, the CPU checks to determine if the particular
execution of that subroutine indicated that the
measurement was complete. If so the CPU clears the POHD
check flag and the off-hook detection routine ends.
A special case exists when the data reporting device
is first powered up and has not learned the
characteristic of the telephone line. Figure l0, shows
CA 02428619 2003-05-21
2~
the initialization routine. As shown, when the data
reporting device first powers up, the CPU sets a flag to
request an immediate parallel off-hook detection (POND)
check. For purposes of determining the change in the
charging interval during this immediate parallel off-hook
detection check, the CPU will use .a default value
(CAL DEFAULT) for the previously calculated charging
interval value (CAL-LEVEL). The CPU will use a time
interval of approximately 140 ~.s as a threshold value.
This interval essentially corresponds to a line voltage
of 15 volts DC. If the measured charging interval is
longer than 140 ACS, the CPU assumes that the telephone
set is off-hook~ and if the measured charging interval
is shorter than 140 ACS, the CPU assumes that the
telephone set is on-hook. If the CPU detects an off--hook
in this manner, it will wait thirty minutes and take
another reading. This procedure will be repeated until
the line voltage exceeds 15 volts DC, indicating that the
customer premises equipment has gone on-hook. When the
detected interval first indicates that the line voltage
exceeds 15 volts DC, the CPU stores the sampled charging
interval value as the current average value for
subsequent use in the above discussed averaging
procedures.
The comparison of a currently measured time sample
to an average time sample reduces false indication that
CA 02428619 2003-05-21
2~
the telephone is off-hook,, which might otherwise be
caused by transient signals on the line. This approach
also allows the CPU to "learn" a time interval value
corresponding to the on-hook voltage actually appearing
on the particular line to which the data reporting device
connects.
Pick-up Detection
The present invention recognizes pick-up during use
of the line by the data reporting device by sensing
changes in the impedance appearing across the subscriber
premises end of the telephone line. The parallel off--
hook type Pick-up Detector 26 works on the principle that
the consumer line is in a high impedance state relative
to the impedance of the telephone company central office.
While the utility meter data reporting device is on-line
communicating with the central computer,~if the consumer
picks up the handset of telephone set 22 or some other
piece of customer premises equipment goes off-hook, a 600
Ohm impedance will appear across the line in parallel
. with that of the data reporting device ~ see I?figure 4 ) .
The Pick-up Detector 26 will sense this variation in
impedance, even though the change in impedance may
produce a relatively small variation in the line voltage.
With reference again to Figures 2 and 3, the Pick-up
Detector 26 comprises the circuit components U9, R29,
CA 02428619 2003-05-21
29
R37, R38, CR5, C21, and C22. When the date reporting
device is communicating over the telephone line, the line
seizure relay 16 is activated. With particular reference
t0 the c~.rcuit Of Figure 3, the relay Shorts LOAD-1 t0
LOAD-2 to connect the lower side of the coupling
transformer T1 through resistor R39 t.o the RING lead of
telephone line 14. The bulk of the off-hook tip-ring DC
current flows through R35, T1 and R39. A portion of the
off-hook current also flows through resistor R38 and
diode CR5 to charge capacitors C21 and C22. A short time
after initially closing the relay to seize the line, the
current will charge capacitors C21 and C22 to a
steady-state condition essentially corresponding to the
line voltage.
Subsequently if the subscriber picks up the handset
of telephone set 22 or some other piece of customer
premises equipment goes off-hook, a 600 ohm impedance
will appear across the line. This amounts to a
substantially drop in the impedance at the subscriber end
of the telephone line. When the impedance across the
telephone line drops suddenly, tree voltage on the
capacitors C21 and C22 will momentarily be higher than
the actual line voltage. The difference in the capacitor
voltage and the line voltage will discharge through the
opto-coupler U9 and the resistor R38. The opto-coupler
U9 is set up to be relatively sensitive to small swings
CA 02428619 2003-05-21
3d
in voltage. Also, the resistance value of resistor R38
is chosen so that the excess voltage charged on
capacitors C21 and C22 will discharge sufficiently slowly
that it appears as a voltage change which is detectable
by the sensitive opto-Coupler U9. Consequently, the
opto-coupler will output a pulse to the CPU indicating
that a piece of customer premises equipment has attempted
to seize the telephone line.
The CPU receives the pulse indicating a parallel
pick-up by the primary communications equipment as an
interrupt signal. As shawn in the flow chart of Figure
11, following the PICK UP interrupt, the CPU determines
if the modem is off-hook, which will be true during a
data reporting call. Processing therefore flows to a
determination of whether or not the modem is dialing out.
Dialing operations may produce changes in line impedance,
therefore the CPU will ignore impedance changes during
dialing. If the modem is not currently dialing, then the
modem is off-hook and the necessary dialing operations
to call the central computer are complete. The CPU
therefore checks to determine if the PICK UP signal is
still active, i.e., to determine if the change in
impedance has persisted. If so, then the change in
impedance represents a parallel pick-up or line seizure
attempt by the primary equipment, therefore the PICK UP
signal provides a signal to open the line seizure relay
CA 02428619 2003-05-21
32
and thereby place the secondary equipment on-hook, after
which the routine ends.
To recapitulate, the present invention detects
whether a telephone line is available by comparing a
sampled time period for charging a capacitor in response
to the line voltage to an average value. The invention
also detects a drop in impedance during use of the line
as an indication that a telephone or other customer
premises equipment has attempted to seize the line.
to These detection results control operation of a line
seizure means within a device, such a.s a data reporting
device, which shares use of the line with other
equipment. As a result, the device either will not seize
the line or will relinquish access to the line to the
telephone or other customer premises equipment.
The embodiment discussed in the above detailed
description should be interpreted as an example of the
invention only. The invention is subject to a variety
of modifications which will be apparent to a person
skilled in the relevant art. For example, the above
description assumes that the secondary communication
device reports utility meter data to a central computer
station and/or receives program and control information.
The invention can be used in other types of equipment,
such as systems to accumulate data regarding television
viewing habits. 1~1s, another example, the above
CA 02428619 2003-05-21
32
discussion assumes that the data reporting device
initiates calls to the central computer via the modem.
Other procedures could be used to initiate data
communication, such as by answering an incoming call at
certain times or if the incoming call exhibits a special
type of ringing signal. Alternatively, the secondary
device may not be a data device. The present invention,
for example, can be used in an answering machine which
shares the line with a primary telephone set.
