The Data Test Set (DTS) provides a means for a technician at a remote site to implement a full external loopback at the terminal port of a DSU or modem and to obtain visual indication of the circuit operation. The DTS has two physical DB-25 male connectors which support EIA-530, RS-232 (sync and async), and a variant of the V.35 interface standards. The DTS may be attached directly to the female DB-25 connector on a DSU or modem, or attached at the end of a cable. Only one connector on the DTS may be used at one time. The DTS is not intended to be used with DTE’s such as terminals or PCs

Four light-emitting diode (LED) indicators provide a means for the technician to view the status of data, clock, and certain control signals from the DSU or modem. Indicators for clock and data provide status on the activity of the corresponding signal, while those four control signals provide status on the signal level. All power to the DTS is supplied via the DSU or modem from the interface signal drivers.