In a preferred embodiment (FIG. 6), an identification circuit comprises an input terminal (40), an output terminal (43), a combined counter and comparator circuit (39) with counter input terminals (38), an OR gate circuit (42) and a trigger circuit consisting of an inverter (44) and a multivibrator (41)....http://www.google.com.au/patents/US4827257?utm_source=gb-gplus-sharePatent US4827257 - Identification circuit
