Proton test and space-flight single event effect data for NASA's first fiber optic data bus are presented. Bit error rate predictions based on a proton direct ionization model agree well with flight data for proton-belt and solar-flare effects. Specifically, the authors discuss the SEUs (single event upsets) seen in space during the first months of the SAMPEX (Solar Anomalous Magnetospheric Particle Explorer) mission, including a solar flare from October 30 through November 6, 1992, their impact, and comparison to predicted SEU rates. The fact that the interpretation of test results leads to a flight model which assumes direct ionization effects suggests that the occurrence of retransmissions could be significantly reduced with design changes to the receiver. A comparison of the measured and predicted rates for the present implementation with those expected for a III-IV based detector showed that improvement would follow from both a smaller physical cross-section, and greatly reduced particle pathlengths through a thinner direct band-gap detector. >