A façade to a system or device implementing Boolean logic. It can represent anything from a simple relay contact to complex relay circuitry or a PLC that converts high/low electric input signals from any source into Boolean true/false outputs. It can exchange binary i/o with the interlocking. Use this for ancillary equipment connected to the interlocking, e.g. bascule bridges, tunnel equipment, detectors (such as earthquake and flooding detectors). Also useful for reading inputs from field-activated level crossings.