[en] This poster describes the control system upgrade of NSTX (National Spherical Torus Experiment) and its impact on plasma stability. NSTX began operation in 1999 and performed with typical parameters Ip <1.5 MA, BT <= 0.55 T, Neutral Beam Injection (NBI) heating <7 MW and a 1.5 s maximum pulse length. Plasma control on NSTX was until recently accomplished using a SKY computer consisting of eight 333 MHz G4 processors. Scheduled programmatic goals including improved MHD mode stabilization and increased plasma stability for longer pulses motivated the addition of an RWM feedback algorithm and a beta feedback NBI control algorithm requiring the addition of 64 new real-time data acquisition diagnostic channels and 6 new output control commands. The increased demands on processing power and timing constraints given the additional I/O latency, required a major system upgrade. To accommodate these new demands we devised a plan to replace this system with more modern technology utilizing readily available commodity computers, which are more easily maintainable. In addition to several independent real-time processes the General Atomics developed PCS system infrastructure continues to be used on NSTX. While maintaining previous functionality, improvements in the control system software include: an RWM feedback algorithm, beta feedback NBI control, more comprehensive error logging and trapping, more user-friendly interface, more complete archiving and restoring functionality, and better status reporting and diagnostic tools. Once completed, we succeeded in increasing overall plasma stability and decreasing control system latency by several times