Abstract: A protocol controls communications between a master unit and a plurality (M) of slave units connected thereto. The master unit sends a downlink message over a first communication path to the M slave units, the downlink message including a number N of information bytes, where N.gtoreq.M, each byte except a last byte comprising a data portion followed by a first predetermined control bit and the last byte comprising a status portion followed by a second predetermined control bit. The master unit receives uplink messages over a second communication path from the M slave units, the uplink messages having the same format as downlink messages. A third communication path connects between the master unit and the M slave units enabling a first slave unit to signal other slave units of the first slave unit's desire to send information to the master unit.

Abstract: A communication system operates compatibly with a public or private switching network using a Q.931 protocol for basic rate ISDN communications. Communications between a control unit and station terminals of the system use a local protocol which is a modification of the Q.931 protocol. This local protocol includes one segment including the mandatory elements of the Q.931 protocol and a second segment derived by converting the optional elements of the Q.931 protocol into a local codeset which controls the user interface circuits at each station terminal. The local protocol enables call control between the control unit and each station terminal to be managed on a bearer channel basis while call control between the system and the network is managed on a call appearance basis using the standard Q.931 protocol. The local protocol provides for communicating line selection and feature activation commands from the terminal to the control unit, using simple button commands.