A integrated virtual learning system is being researched and developed to teach students about programmable logic controllers (PLCs). This system, called the Virtual PLC, incorporates intelligent tutoring system, simulation, and animation technologies. This article describes the development and evaluation of modules on PLC timer and counter instructions. These modules were first developed using an intelligent tutoring system (ITS) authoring tool and animation tools. After the concept was proved positively, a Web-based ITS was developed to incorporate both modules. The authoring tool-based ITS timer modules were evaluated with 90 undergraduate manufacturing engineering students in 2002. The Web-based ITS timer and counter modules were evaluated by 38 undergraduate students in 2003. In both cases, students made statistically significant learning gains as a result of taking the modules, and rated the modules positively in terms of ease of use and understanding, clear objectives, amount of interaction, ability to motivate, relevance, and pace.