In a mechanically operated keyboard employing keys each having a multiplicity of electrical switch contacts for providing an encoded electrical signal output corresponding to and uniquely identifying individual key stations, an electrical circuit provides a dummy load across the multiple switch contacts which varies as different keys are actuated in a manner to provide a constant load across the multiple switch contacts. The electrical circuit develops a control signal which can be employed in conjunction with associated circuits to inhibit erroneous data output signals from the keyboard in the event of improper switch closure.