A full swing CMOS logic circuit provides fault tolerant, cold sparing of VLSI logic devices attached to a high speed bus. P-channel FET transistors are formed in an N-well which has a biasing transistor which effectively decouples the circuit when the circuit is not powered. The input/output interface of the cold spares have a high impedance and do not corrupt an interconnected electronic bus. The final drive transistors are reverse biased or clamped to zero to prevent any leakage paths.
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