The Open Architecture Test System provides a method and framework under which software and instruments of different vendors can be developed and integrated into an ATE. In this paper, we describe the overall architecture and design of the system. First we describe the architecture and the control mechanism for the overall system and for individual test-sites. Data and command communication mechanism among these control elements is explained. After this general structure, we describe how this architecture allows deployment of third party instruments and modules, what are the interfaces, how system is configured and how correct operation of the system is ensured after pluggingin a third party module.