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In Xilinx Field Programmable Gate Arrays two types of logical constants, implicit and explicit, are used to prevent unspecified signals from floating. Implicit logical constants are implemented with a weak keeper circuit, called a half latch, and are used to tie off unspecified input signals to user flip-flops. Explicit logical constants in the earlier devices are implemented using look up tables (LUTs) set to a constant value (constant LUTs) and in the newer devices are implemented using posts that provide access to the ground plane. Explicit logical constants often are used in adders and multipliers. In this paper, we will present radiation test data and analysis of the three types of logical constants.