We present the development of T-UPPAAL — a new tool for online black-box testing of real-time embedded systems from non-deterministic timed automata specifications. It is based on a sound and complete randomized online testing algorithm and is implemented using symbolic state representation and manipulation techniques. We propose the notion of relativized timed input/output conformance as the formal implementation relation. A novelty of this relation and our testing algorithm is that they explicitly take environment assumptions into account, generate, execute and verify the result online using the UPPAAL on-the-fly model-checking tool engine. This paper introduces the principles behind the tool, describes the present implementation status, and future work directions.