The national spherical torus experiment (NSTX) currently uses a collection of analog signal processing solutions for coil protection. Part of the NSTX upgrade (NSTX-U) entails replacing these analog systems with a software solution running on a conventional computing platform. The new digital coil protection system (DCPS) will replace the old systems entirely, while also providing an extensible framework that allows adding new functionality as desired. The development of the DCPS was a multidiscipline engineering effort. The fact that long-trusted yet presently inadequate protection mechanisms were being replaced with a first-of-a-kind system at NSTX-U has led to a carefully crafted, full-featured software design. Real-time concurrent RedHawk Linux provides the deterministic environment in which the software runs, and the software architecture follows a unified modeling language design with industry standard patterns.