The paper introduces a new technique for on-line checking of FPGA based Control Units (Cus). This technique is based on the architecture comprising two portions: a self-checking CU and a separate totally self-checking (TSC) checker. Each of these portions is implemented as a combination of an Evolution block and an Execution block. Comparison of a code vectors being transerred between the blocks of the portions enables providing a totally self-checking property. The self-checking CU is implemented in a form of one-rail network of interconnected pre-designed LUT-based configurable logical blocks. The self-checking checker is a Sum-Of-Minterms based checker. The proposed technique: a) does not require any encoding of output words; b) uses one-rail design, thereby drastically decreasing the required overhead.