In less then an hour (11/12/2014 @ approx 0835 GMT) 511,000,000 km from Earth the Philae lander of the Rosetta mission will detach and begin its decent to a comets surface. The orbiter is powered by a 1750A processor by Dynex (as we previously discussed). The lander is powered by two 8MHz Harris RTX2010 16-bit stack processors, again a design dating back to the 1980’s. These are used by the Philae CDMS (COmmand and Data Management System) to control all aspects of the lander.
All lander functions have to be pre programmed and executed by the CDMS with absolute fault tolerance as communications to Earth take over 28 minutes one way. The pair of RTX2010s run in a hot redundant set up, where one board (Data Processing Unit) runs as the primary, while the second monitors it, ready to take over if any anomaly is detected. The backup has been well tested as on each power cycle of Philae the backup computer has started, then handed control over to the primary. This technically is an anomaly, as the CDMS was not programmed to do so, but due to some unknown cause it is working in such a state. The fault tolerant programming handles such a situation gracefully and it will have no effect on Philae’s mission.
Why was the RTX2010 chosen? Simply put the RTX2010 is the lowest power budget processor available that is radiation hardened, and powerful enough to handle the complex landing procedure. Philae runs on batteries for the first phase of its mission (later it will switch to solar/back up batteries) so the power budget is critical. The RTX2010 is a Forth based stack processor which allows for very efficient coding, again useful for a low power budget.
Eight of the instruments are also powered by a RTX2010s, making 10 total (running at between 8-10MHz). The lander also includes an Analog Devices ADSP-21020 and a pair of 80C3x microcontrollers as well as multiple FPGAs.