FOUNDATION fieldbus has moved much of the control functionality once found in the proprietary computer of a DCS into FF interoperable field devices, based on an open specification. End users need a way to recognize those devices that adhere to the FF specification so as to assure a functional fit into their control network. The Interoperability Test System and FOUNDATION Fieldbus's compliance registration program thoroughly test intelligent field devices from the electrical circuit interface characteristics through the user layer function blocks. This testing allows users to apply "Best-in-Class" measurement and control technology with confidence that devices from various manufactures will work together.