Interface system between mixed-signal electronic components, assemblies, and systems, and external or built-in-test equipment to provide those components, assemblies, and systems with testability attributes. To define and describe the signals, functions, and characteristics of the testability bus and to describe how the bus shall be implemented to improve the controllability and observability of mixed-signal designs and to support mixed-signal built-in test structures in order to reduce test development time, testing costs, and improve test quality. New IEEE Standard - Superseded. The testability structure for digital circuits described in IEEE Std 1149.1-1990 has been extended to provide similar facilities for mixed-signal circuits. The architecture is described, together with the means of control of and access to both analog and digital test data. Sample implementation and application details (which are not part of the standard) are included for illustration.