ency was maximized by tuning and monitoring the photodiode output on a digital sampling oscilloscope. Signals were amplified by a Trans-Impedance Amplifier (TIA) and evaluated for errors resulting from proton strikes. With protons incident on the photodiode, we monitored the BER and recorded the number of errors. Measurements of BER were typically made with >100 total errors to assure good statistics. This usually covered a time interval of minutes. By logging the percent of error free intervals, we verified that for protons the errors were due to individual events and not contiguous errors from a single strike. Similarly, for higher LET He ions, we determined the average number of errors per strike using this method. These measurements described here and in [Mars-94a] were performed at the Naval Research Laboratory beam-line (beam-line 2) at the Crocker Laboratory, University of California. For in situ measurements of data transmission with bit periods of only a few nanoseconds, one must carefully consider the beam's temporal structure and its relation to the data stream. We examined the impact of the 22 MHz cyclotron frequency (at 63 MeV) which provides micro-pulses of approximately 1.3 ns duration every 44 ns. Our experiments were IV-33 conducted in a manner to assure this did not influence bit-error cross section measurements. Consideration of the microstructure of the timing of proton arrivals may be an important experimental issue is some situations [LaBe-93], especially when high data rates are involved. Most high energy proton facilities have similar concerns. 4.2.2.2 Analysis and Indication of the Role of Direct Ionization in Photodetectors: Next we consider the example of errors from proton-induced direct ionization in photodetectors to show that they can be quantified with the well-developed tools used in more conventional single event investigations. As is customary with spatially separated arrays of memory elements in Random Access Memories (RAMs), we define bit error cross sections for temporally se