Code selector

Claim 1

A CODE SELECTOR OF THE TYPE USED IN A SELECTIVE CALLING SYSTEM IN WHICH AN ALARM IS ACTUATED AT A PARTICULAR RECEIVER IN RESPONSE TO A PARTICULAR TWO-TONE CODE MADE UP OF A PLURALITY OF BINARY BITS, INITIATION OF ACTUATION OF SAID SELECTOR OCCURRING IN RESPONSE TO A GIVEN CONDITION, COMPRISING A RING COUNTER, SAID RING COUNTER COMPRISING A PLURALITY OF STAGES, EACH STAGE CORRESPONDING TO ONE BIT OF THE RECEIVED CODE, CLOCK CIRCUITRY FOR DRIVING SAID RING COUNTER THROUGH ITS CYCLE IN RESPONSE TO THE FIRST RECEIVED BIT OF THE CODE, MEANS FOR COMPARING EACH BIT OF THE RECEIVED CODE WITH THE OUTPUT OF THE CORRESPONDING RING COUNTER STAGE TO DETERMINE IF THAT BIT IS THE SAME AS THE CORRESPONDING BIT OF THE CODE WHICH THE SELECTOR IS DESIGNED TO RECOGNIZE, TONE ERROR LOGIC CIRCUITRY FOR DETERMINING WHETHER THERE HAS BEEN AN ERROR IN THE TRANSMISSION OF THE RECEIVED CODE DUE TO THE PRESENCE OF A CONDITION OTHER THAN SAID GIVEN CONDITION, MEANS RESPONSIVE TO AN ERROR IN THE TRANSMISSION OF THE CODE AND RESPONSIVE TO THE DETERMINATION OF A DIFFERENCE BETWEEN ONE OF THE RECEIVED BITS AND THE CORRESPONDING BIT OF THE CODE WHICH THE SELECTOR IS DESIGNED TO RECOGNIZE FOR PREVENTING SAID RING COUNTER FROM BEING DRIVEN THROUGH A COMPLETE CYCLE, AN ALARM CONNECTED TO THE LAST STAGE OF SAID RING COUNTER, AND MEANS FOR ACTUATING SAID ALARM ONLY WHEN SAID RING COUNTER HAS BEEN DRIVEN THROUGH ONE COMPLETE CYCLE.