Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Transient or portion of cyclic
Patent
1984-06-29
1986-12-23
Kreitman, Stephen A.
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Transient or portion of cyclic
379 24, 379 26, H04B 346
Patent
active
046314750
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
The present invention relates to a method of detecting hazardous line voltages over a two-wire line, e.g. for transferring digital information (data transmission). The method is only applicable when the two-wire line is terminated at one end with a hybrid coupler and associated circuits for adaptive echo elimination.
BACKGROUND
Methods and apparatus for detecting hazardous line voltages known in the prior art involve connecting fusing resistors and threshold sensing elements into the two-wire line, overvoltages then causing rupture or short-circuiting. Such protection in an electronic telephony installation is illustrated in Swedish Patent Application No. 7600255-9, for example. Protection here comprises fusing resistors and Zener diode combinations for preventing overvoltages arising between the line wires and ground. Furthermore, it is already known to connect test relays to both wires, e.g. in the vicinity of the hybrid coupler. When the line condition is to be tested, the relay opens the line and its impedance is measured, whereby information is given as to possible abnormal conditions such as short-circuiting or rupture.
SUMMARY OF INVENTION
The method in accordance with the invention has the aim of utilizing properties of an apparatus for adaptive echo elimination, known per se, and described in, for example, the Swedish Patent Application No. 8106444-6. As is described in this application, a correction unit in the apparatus gives a digital signal to a balance filter for the two-wire line. As is well known in the art, this signal is an unambiguous measure of the difference between the actual line impedance and the hybrid impedance. The hybrid impedance is dependent on the design of the hybrid coupler and is defined as the impedance that, when connected to the two-wire port instead of the two-wire line, gives high attenuation between the four-wire ports. However, the impedance of the two-wire line is dependent on the actual cables and is usually never equal to the hybrid impedance. The line condition can be detected by comparing said digital value with one or more stipulated digital limiting values. The need of the above mentioned test relays is thus precluded, but for protecting the line there are still protective resistors and special components, e.g. Zener diode combinations, for short-circuiting the line wires should there be overvoltage.
An object of the present invention is thus to provide a method of detecting abnormal conditions in a two-wire line, which is terminated by a hybrid coupler and associated circuitry for adaptive echo elimination and which obviates the need for special test relays.
In accordance with the invention and as will be shown in detail hereinafter, there is provided a method of detecting hazardous line voltages occurring over a two-wire connection resulting from a cutoff or a short circuiting, the connection being terminated by a hybrid coupler and an apparatus for adaptive echo elimination, which sends a digital signal which stands in an unambiguous relation to the line impedance of the connection, said method comprising comparing said digital signal with at least one digital limit value corresponding to an upper or lower limit of the normal value of the line impedance. In response to the thusly made comparison, an alarm signal is transmitted.
In further accordance with the invention, the aforesaid digital signal is compared with a first digital limiting value corresponding to a break in the line and also with a second digital limiting value corresponding to a short-circuit in the line. In further accordance with the invention, the aforesaid first and second digital limiting values correspond to a first and a second finite value of line impedance.
BRIEF DESCRIPTION OF DRAWINGS
The invention will now be described in detail with reference to the accompanying drawing, in which:
FIG. 1 schematically illustrates a two-wire line and its connection to an apparatus for adaptive echo elimination, and
FIG. 2 is a block diagram, for more closely describi
REFERENCES:
patent: 4203012 (1980-05-01), Boxall
patent: 4539674 (1985-09-01), Carlquist et al.
patent: 4547633 (1985-10-01), Szechenyl
patent: 4549049 (1985-10-01), Guidoux
Kreitman Stephen A.
Oldham Scott M.
Telefonaktiebolaget LM Ericsson
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