A solar ultraviolet detector prototype for the GOES spacecraft has been calibrated using the X24C beamline at the Brookhaven NSLS. Similar in design to the 3-channel 50110 CELIAS SEM, the GOES EUV uses a combination of transmission gratings and silicon photodiodes with thin-film metal overcoats to provide the required bandpasses. Four of the channels position the photodiodes at the first to fourth orders of 2500 and 5000 L/mm transmission gratings to provide spectral information over four wavelength bands from approximately 5-80 nm. The fifth channel positions the photodiode at first order of a 1667 L/mm transmission grating in combination with a bandpass filter centered at approximately 120 nm to provide coverage in the Lyman alpha region of the solar spectrum. The GOES EUV will provide continuous monitoring of solar EUV in bandpasses that are known to have a large variability in the amount of energy deposition in the earth's ionosphere over a solar cycle. Prototype detector design and calibration procedure are discussed. Absolute responses of the design model and synchrotron beamline properties relevant to calibration are presented.