the first tester 20 and the second tester 22 with the use of the two signal thresholds allow for simultaneous testing of the off hook detect circuit and ring trip detector without disabling the subscriber loop interface circuit during testing the output signal rtd may also be received by the dc feed circuit portion not shown of the subscriber loop interface circuit in response to receiving the output signal rtd the dc feed circuit may control the voltage supplied to the subscriber line terminals tip and ring terminals 36 of the subscriber loop interface circuit to thereby maintain the voltage across the terminals 26 at a constant predetermined value during testing the voltage across the subscriber line terminals 36 is defined to be a constant voltage during testing so that the full loop voltage is available for loop feeding one technique for providing a constant voltage across the terminals 36 loop voltage is described in copending application ser no 09 145 182 entitled u201c programmable subscriber loop interface circuit and method u201d by christopher ludeman the inventor hereof which is incorporated herein by reference maintaining the loop voltage at a constant during testing allows for the subscriber loop interface circuit to have the same off hook detection performance as during non testing the loop feed characteristic of the subscriber loop interface circuit may be the same during testing and during other.