MECHANISM FOR CONTROLLABLY ENABLING TEST SET TO ASSERT OFF-HOOK CONDITION ON TELEPHONE LINE IN DEPENDENCE UPON DETECTION OF PRESCRIBED VOLTAGE LEVEL AND ABSENCE OF DATA TRAFFIC

摘要

The possibility of corrupting digital data traffic carried by a data line (11, 12) (powered or unpowered), by the connection of a telephone test set to the data line (11,12) is effectively prevented by augmenting the telephone test set with a mechanism that prevents the test set from asserting an OFF-HOOK condition on the line, unless it has been determined that each of two conditions are satisfied. These conditions are: 1- that no digital data traffic is being carried by the line (11, 12); and 2- that the electrical voltage condition of the line (11,12) satisfies a prescribed minimum voltage level criterion. If either of these conditions is notsatisfied, a switch circuit installed in the test set's connection path to the line is maintained in a default, open condition, so that it is effectively impossible for the line to be presented with an OFF-HOOK condition through operation of the test set. This default condition remains in effect until both criterion are fulfilled. As a further safety precaution, should the line to which the test set is connected be determined to be carrying digital data traffic, the invention causes an audible alert tone to be generated as a warning signal to the craftsperson.