Modsim is a tool for simulating Modbus devices, while Modscan is a tool for testing and debugging Modbus-based systems. 2. Modsim allows users to create and configure virtual Modbus devices, while Modscan allows users to send and receive Modbus messages and monitor Modbus communication. 3. Modsim is primarily used for testing and development, while Modscan is primarily used for troubleshooting and debugging.