1.  Field of the Invention
The present invention relates to a key telephone system, and more particularly to a key telephone system whereby data is controlled to be transmitted among the key service unit of the key telephone system (hereinafter called KSU) and key telephone station set thereof (hereinafter called KTSS). 
2.  Description of the Related Art
There are two methods of transmitting data among the KSU and a plurality of KTSS of a key telephone system. 
According to one of the two methods, the KSU is provided with data interfaces to be used with respective KTSS, and the KSU and the key telephone station sets are connected via speech lines and data lines.  According to the other method, the KSU is provided with a common data interface shared by plural KTSSs, and the KSU and the KTSS are connected via each speech line and common data lines. 
The above conventional methods, however, have the following problems.  Namely, the former method requires as many data interfaces as the number of KTSS, and the latter method is associated with some complexity in that it becomes necessary to give an identification number to each KTSS in order to have time-allocation use for the common data interface connected to the respective data lines.