Testability is a very important aspect of VLSI circuits. Numerous design for testability (DFT) methods exist. Often designers face the complex problem of selecting the best DFT techniques for a particular chip under a set of design constraints and goals. In order to aid in designing testable circuits, a prototype knowledge based system has been developed which simulates a human expert on design of testable PLAs. The system, described in this paper, has knowledge about testable PLA design methodologies and is able to negotiate with the user so as to lead the user through the design space to find a satisfactory solution. A new search strategy, called reason analysis, is introduced.