Data transmission system of key telephone system

Abstract

A system for transmissing data by a pulse signal through a pair of lines connected between a main unit and a key telephone set is disclosed. The main unit includes a transmitter for sending, prior to transmission of pulse train data, a start signal of a pulse train including a pulse of an n-bit (where n is an integer) specific logic value and a pulse of an (n+1)-bit logic value opposite to the specific logic value. The key telephone set includes a change detector for detecting a change in the pulse train of the start signal in a direction of the logic value opposite to the specific logic value, a logic value detector for detecting a logic value of an (n+1)th bit pulse from a detection timing of the change detector, a discriminator for discriminating on the basis of the logic value of the (n+1)th bit pulse whether a line connection for the key telephone set is normal, and a reader for reading data following the start signal according to a logic determined by a discrimination result.

Dependencies are inferred from the claim text.