PURPOSE: To realize the perfect prevention for the leak of the transmission information between the information source and the information reception part.

CONSTITUTION: The transmission information is divided with the numbers given. Dividing assembly control part 4 allots the 1st to 4th divided transmission information to A∼D respectively and then edit them into a format for transmission to the circuit. Each of transmission/reception parts A∼D receives the telegram transmitted from corresponding extension and then extracts the divided transmission information and the sequence numbers. Part 4 stores the extracted divided transmission information into the fixed position within buffer 5 based on the sequence numbers within each transmission/reception part.