Simultaneous switching noise (SSN) is an important issue for the design and test and actual ICs. In particular, SSN that originates from the internal logic circuitry becomes a serious problem as the speed and density of the internal circuit increase. In this paper, an on-chip monitor is proposed to detect potential logic errors in digital circuits due to the presence of SSN. This monitor checks the variations of power/ground lines at the interface between noncoherent logic blocks in order to warn that a logic error is likely to occur. This information can then be used for any scheme that takes corrective actions.