Abstract: A fault-location operating system for use with transmission networks using repeaters with loopback capabilities. A plurality of bidirectional transmission links L.sub.i, for i=1 through N, are used for transmission of data between the terminals. A test control unit is connected at the first terminal to the first end of the first transmission link L.sub.1, and controls transmission of control signals across the first transmission link L.sub.1. A plurality of repeaters R.sub.i, for i=1 through N-1, are used for interconnecting the plurality of bidirectional links. Each repeater R.sub.i is connected to the first end of one transmission link L.sub.i and to the second end of another transmission link L.sub.i+1. Each repeater includes a first circuit for repeating data transmitted from the one transmission link L.sub.i for transmission on the other transmission link L.sub.i+1, and a second circuit for repeating data transmitted from the other transmission link L.sub.

Abstract: Centralized telephone circuit testing equipment with receptacles for selectively receiving a wide variety of electronic circuit modules, various modules being provided to make the equipment capable of performing a large number of commonly employed telephone tests. A scanning system is disclosed as part of the central measuring equipment for determining which of a plurality of remote testing stations has requested access to the central measuring equipment, one requesting remote station at a time being given complete control over a test function of the central measuring equipment for conducting tests on a telephone trunk.

Abstract: A movable telephone circuit test instrument is provided, indicators mounted thereon which show the communicating state of a telephone circuit being tested. Supervision of the circuit under test is thus provided at the same location as the tester, thereby reducing the number of persons required to carry out the test. The supervising indicating means are preferably light emitting diodes and are provided in a unique driving circuit that includes a variable conductance parallel current path for maintaining the brightness of the light emitting diode substantially constant regardless of the magnitude of current being monitored by the supervising circuits.