Built-in self-test is a functional requirement that a complex electronic system must satisfy in order to provide efficient production testing and system maintenance. Effective solutions with minimum possible hardware overhead are needed. By the advent of application specific integrated circuits, integration of functions conventionally performed by external test instrumentation in a single test IC on a board has become a reality. The paper gives an example how the advantage of user programmable logic was taken in the design of a built-in self-test of an asynchronous time division multiplex unit, supporting both second-order and third-order multiplex equipment, as recommended by the CCITT.