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This paper addresses the problem of improving immunization of programmable logic controllers (PLC) to electromagnetic interference with impulsive characteristics. A filtering structure, based on weighted median (WM) filters, that does not require additional hardware and can be implemented in legacy PLC is proposed. The filtering operation is implemented in the binary domain and removes the impulsive noise present in the discrete input adding thus robustness to PLC. By modifying the sampling clock structure, two variants of the filter are obtained. Both structures exploit the cyclic nature of the PLC to form an N-sample observation window of the discrete input, hence, a status change on it is determined by the filter output taking into account all the N samples avoiding thus that a single impulse affects the PLC functionality. A comparative study, based on a statistical analysis, of the different filter's performances is presented.