Abstract: An adapter terminal unit is designed to connect to the time-division switching center of which it forms part at least one digital trunk to a standard different to that normally applicable to the digital trunks associated with the switching center. Each adapter terminal unit, equipped with a microprocessor-based control circuit, is connected by a switching matrix to the input/output lines of the switching center's switching matrix. It further comprises a data and signaling transfer device connected to the switching matrix and to a transmitter device and to a receiver device, one at the end of a transmit multiplex link and the other at the end of a receive multiplex link of the digital trunk. In this way the standard changes necessary for exchanges between the digital trunk and the switching center are effected.