03939417 is referenced by 16 patents and cites 9 patents.

A control console including a transceiver, a plurality of terminal stations positioned at spaced apart locations remote from the console and each including a transceiver connected to the console by RF or a wire line, and a plurality of call boxes positioned at spaced apart locations adjacent to each terminal station and including a transceiver in communication with the terminal station. When the hand set of a call box is lifted from the cradle a tone is transmitted through the adjacent terminal station to the console which then keys on all terminal station transmitters to indicate a busy line at other call boxes and to provide a ringing signal at the call box being used. Subsequent to transmission of the tone, a signal indicative of the address of the box and the condition of predetermined portions of the box is automatically transmitted to the console. When the console operator answers, full duplex communication is provided between the call box and the console.

Title
Emergency communications system
Application Number
5/425908
Publication Number
3939417
Application Date
December 18, 1973
Publication Date
February 17, 1976
Inventor
Donald L Walker
Addison
IL, US
Thomas J Rollins
Arlington Heights
IL, US
Richard D Koenig
Elgin
IL, US
Richard J Crouse
Palatine
IL, US
Gary A Cannalte
Hoffman Estates
IL, US
Agent
Eugene A Parsons
James W Gillman
Assignee
Motorola
IL, US
IPC
H04B 1/40
H04B 7/14
View Original Source