The Grid Explicit Congestion Notification control mechanism (GECN) is a broadcast-based real-time demand- response mechanism designed for primary voltage control in Active Distribution Networks (ADNs) [1,2]. An extensive set of off-line simulations has indicated that GECN is a promising candidate for deployment in the real field. However, prior to the actual deployment of the control mechanism, it is crucial to validate its performance when controlling a real grid. For this purpose we design and develop a dedicated experimental Hardware-in-the-Loop (HIL) test platform for the real-time val- idation of GECN. The HIL architecture consists of a Real-Time Simulator (RTS) where a real distribution feeder is modeled, together with controllable loads and the associated measurement infrastructure composed by virtual PMUs. These virtual metering devices stream data, via Ethernet, to a local Phasor Data Con- centrator suitably coupled with a Discrete Kalman Filter State Estimator. The estimated network state is received by a GECN network controller. We close the control loop by transmitting the computed broadcast control signals back to the network buses in the RTS using a micro-controller. By using this experimental setup we are able to (i) assess the performance of the whole control process in terms of voltage optimality and time latencies in a realistic setting and (ii) implement the GECN controllers into dedicated equipment that with the proper ruggedization can be readily deployed in the real field.