| 摘要 |
A telephone line testing circuit for determining whether a fault exists on a telephone line entering a customer's premises or whether the fault exits in the customer's wiring and equipment. The testing circuit comprises a test switch, a polarity detector, a current detector, a dial tone detector and an indicator. The polarity, current and dial tone of an incoming telephone line signal are tested sequentially in the same circuit and in one operation such that if any one of the three characteristics of the entering telephone line contains a fault, the indicator will not generate a signal. When the entering telephone line signal has the correct operating polarity, sufficient operating current draw and a dial tone of the correct operating frequency, the indicator will generate a signal indicating that no fault exits on the entering telephone line.
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