| 摘要 |
Test means 6 plugged into the switch 2 for assigning the line to be tested 4 generate a test signal of predetermined frequency and predetermined level and a prior signal with a frequency which differs from that of the test signal, which signals can be sent over the two outward-bound wires of the line to be tested and return via the two inward-bound wires of the said line. At the datacom subscriber set, a local box 10, plugged into the line to be tested, comprises means for receiving the prior signal and the test signal as well as automatic switching means which establish at least one mode of interconnection of the two outward-bound wires of the line with the two inward-bound wires of the said line and whose opening/closing is controlled by the prior signal. The local box 10 also comprises control means which are able, in response to a detected test signal lying within a predetermined range of levels, to send an additional test signal over the line 4 to the test means 6. <IMAGE>
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