Hardware-in-the-loop (HIL) real-time simulation is becoming a significant tool in prototyping complex, highly available systems. The HIL approach permits testing of hardware prototypes of components that would be extremely costly or difficult to test in the deployed environment. In power system simulation, key issues are the ability to wrap the systems of equations (such as Partial Differential Equations) describing the deployed environment into real-time software models, provide low synchronization overhead between the hardware and software, and reduce reliance on proprietary platforms. This paper introduces an open source HIL simulation framework that can be ported to any standard Unix-like system on any shared-memory multiprocessor computer, requires minimal operating system scheduler controls, enables an asynchronous user interface, and allows for an arbitrary number of secondary control components. The framework is implemented in a soft real-time HIL simulation of a power transmission network with physical Flexible AC Transmission System (FACTS) devices. Performance results are given that demonstrate a low synchronization overhead of the framework.