Data collection system using telephone lines
First Claim
1. Apparatus for sensing at a central location events occurring at a plurality of remote sites comprising:
- one or several switches at each remote site, each switch adapted to change state at the occurrence of an event to be recorded;
resistors associated with said switches for developing a resistance which is a function of said switch settings;
telephone lines coupling said switches and resistors to said central location;
a multiplexer for sequentially selecting each pair of telephone lines;
a means for reading the electrical signal on the selected pair of telephone lines; and
a computer for(1) computing from said signal the selected line impedance;
(2) adding to said line impedance an impedance correction factor equivalent to the telephone line impedance deviation from nominal, resulting in a computed resistance which is a function of the switched resistance;
(3) using large changes in said computed resistance as a data input; and
(4) using small changes in said computed resistance to improve said impedance correction factor, thereby correcting for the change in telephone line impedance.
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Accused Products
Abstract
A circuit and a computer program for enabling the monitoring of a plurality of switch settings at remote sites by central telephone exchange equipment through regular telephone lines is described. To compensate for slow drift variations in line impedance, the impedance of each line is continually calculated from the measurements, and any small change is used to update a line impedance correction factor, while any large instantaneous change is used to update the data counter. Telephone exchange battery variations which may be large and instantaneous, are compensated for by measuring the battery voltage immediately prior to measuring the line impedances, and correcting the line impedance readings with a battery voltage correction factor. Other types of temporary signal interference which are frequently present in telephone switching equipment are identified and rejected by applying consistency criteria to multiple readings which have been corrected by the aforementioned method. Using these techniques, long distance telephone lines may be used to monitor the settings of switches in parallel or series with selected resistances even where the variations in battery voltage, line impedance and signal interference may produce voltage variations on the telephone lines which may exceed the voltage variations produced by the switched resistance by a factor of five or more.
122 Citations
15 Claims
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1. Apparatus for sensing at a central location events occurring at a plurality of remote sites comprising:
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one or several switches at each remote site, each switch adapted to change state at the occurrence of an event to be recorded; resistors associated with said switches for developing a resistance which is a function of said switch settings; telephone lines coupling said switches and resistors to said central location; a multiplexer for sequentially selecting each pair of telephone lines; a means for reading the electrical signal on the selected pair of telephone lines; and a computer for (1) computing from said signal the selected line impedance; (2) adding to said line impedance an impedance correction factor equivalent to the telephone line impedance deviation from nominal, resulting in a computed resistance which is a function of the switched resistance; (3) using large changes in said computed resistance as a data input; and (4) using small changes in said computed resistance to improve said impedance correction factor, thereby correcting for the change in telephone line impedance. - View Dependent Claims (2, 3)
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4. Apparatus for sensing at a central location events occurring at a plurality of remote sites comprising:
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one or several switches in series at each remote site, each switch adapted to change state at the occurrence of an event to be recorded; a resistor in parallel with each switch; telephone lines coupling said switches and resistors to said central location; a multiplexer at said central location for sequentially selecting each pair of telephone lines; a means for reading the electrical signal on the selected pair of telephone lines; and a computer for (1) computing from said signal the selected line impedance; (2) correcting said line impedance by means of an impedance correction factor representing the telephone line impedance deviation from nominal, resulting in a computed line impedance which is a function of the switched resistance; (3) using large changes in said computed impedance as an event input; and (4) using small changes in said computed impedance to improve said impedance correction factor, thereby correcting for the changes in telephone line impedance. - View Dependent Claims (5, 6, 7, 8)
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9. The method of using telephone lines at a central location to monitor switch settings at a plurality of remote sites, where the switches at each remote site are associated with resistors for developing a resistance which is a function of said switch settings, comprising the steps of:
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sequentially measuring the total impedance of each remote site comprising the impedance of the telephone lines and each corresponding set of switches and resistors; correcting said total impedance with one or more impedance correction factors, the result being a function of the switched resistance and the nominal telephone line impedances; using large variations in corrected total impedance to indicate a changed switch setting; and using small variations in corrected total impedance to improve one or more of said correction factors thereby correcting for the slow drift of impedance in the associated telephone line. - View Dependent Claims (10)
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11. In a central system for monitoring the setting of switches in parallel or in series with resistances at a plurality of remote sites, using telephone lines for coupling the switches to said central system, where said central system comprises a multiplexer for sequentially selecting each set of telephone lines, apparatus comprising:
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an analog to digital converter for receiving the multiplexer voltage output coupled from each selected telephone line and for converting said voltage into a computer-readable form; and a computer (1) for storing the analog to digital converter output, said output being a binary representation of the telephone line voltage which is a function of the remote site switch settings; (2) for applying telephone line impedance correction factors (S) to the value of each telephone line voltage reading to compensate for differences in telephone line impedance from nominal values; and (3) for using small changes in telephone line voltage between successive readings on the same telephone line to update the impedance correction factors (S) to compensate for slow changes in telephone line impedance. - View Dependent Claims (12, 13, 14, 15)
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Specification