As the electric grid evolves into the smart grid, the ability to adequately model and test components going on the grid becomes more critical. As such, the role of large scale testbeds capable of testing utility-scale equipment will play an ever increasing role in the development of smart grid technologies. A flexible and robust communication and control system is of paramount importance in the successful operation of such testbeds. This paper describes one approach to control of a large-scale hardware-in-the-loop power system testbed. Clemson University's Duke Energy eGRID is a multi-megawatt electric power laboratory combining real time grid simulation capabilities with a highly configurable, medium voltage three phase experimental grid. Control of this laboratory is achieved through the use of real-time hardware, a custom serial communication protocol, and FPGA-based data acquisition.Index Terms--power system simulation, power hardware in the loop, power hardware testbed, smart grid testbed