The Ramaty High Energy Spectroscopic Imager (RHESSI) spacecraft is a NASA Small Explorer (SMEX) class mission. RHESSI is designed to image solar x-rays and gamma rays with high energy resolution. The Instrument Data Processing Unit (IDPU) serves as the central RHESSI instrument on-board data processing element. It controls and monitors the instrument operations, and provides a flexible telemetry collection and formatting system. The system responds autonomously to optimize science data collection over a wide dynamic range of conditions, handling up to 40Mbps of telemetry during solar flares. This paper presents an overview of the IDPU hardware and software design.