To understand security vulnerabilities of communication protocols used in power systems, a real-time framework can be developed to conduct vulnerability studies. The framework should implement protection mechanisms against vulnerabilities and study their effectiveness. In this paper, a realtime cyber-physical framework or test bed is presented. It integrates a real-time power system simulator and a communication system simulator to study the cyber and physical system vulnerabilities in smart power grids. The power system simulation is implemented using the Real-Time Digital Simulator (RTDS®) power grid simulator, with LabVIEW and PXI modules that simulate the supervisory control and data acquisition (SCADA) system and intelligent electronic devices (IEDs). The communication system simulation is implemented using Opnet's System-in-the-Loop (SITL) simulator and open source Linux tools and servers. Results of two cyber-attacks on the Modbus/TCPprotocol are discussed and improvements to the test bed for protocol attack detection and mitigation are proposed.