VDS is a Web-based HMI/SCADA application that consists of HMI Server/Data Server and an HMI client. The HMI Server/Data Server software has a simple three-layer hierarchical structure. In the bottom layer, an IO object transfers IO data to Data Server by means of a device-driver plug-in. In the second layer, Data Server runs as process management software, processing the data based on tags. VB applications run in this layer to access Data Server. In the top layer, HMI Server runs as message management software, serving as the HMI client's interface with Data Server.