Abstract: A key telephone system includes a main unit, a DSS interface, and a detector. The main unit includes a speech path switch, a plurality of call processing interfaces, an operating data memory, operation mode setting switches, and a controller for controlling the speech path switch by referring to the operating data memory when data transmission is periodically performed between the plurality of key telephone sets through the call processing interface in the call processing mode, and for updating contents of the operating data memory according to operating data externally input in the data setting mode. The DSS interface connects at least one DSS console. The detector detects that first normal data received, after a periodic data transmission with the DSS console through the DSS interface is an abnormal transmission for a predetermined period of time, is data unique to the key telephone set.

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.

Abstract: Disclosed is a system for transmitting data by a data pulse train obtained by a combination of "0" or "1" bit pulse signals through a pair of lines connected beween a main unit and a key telephone set. The system includes a counter for sampling the pulse train and counting "0" or "1" bit pulse signals; a comparator for comparing a count of the counter with a reference value; and a correcting section for adding "0" or "1" correction pulses so as to set the count to be the reference value when the count is different from the reference value.