Abstract: A communication system includes a plurality of requestors and a plurality of responders representing resources. A protocol exercised by all elements provides accurate and consistent interconnection of all elements in a system where messages may be delayed or lost. Requestors are provided with means to transmit a request for use of a plurality of resources, respond to acceptances from one or more responders and create a cooperative relationship by transmitting a confirmation to one responder and cancellation of the request to all other responders. Responders become committed to a requestor by signalling an acceptance to a request if a previous acceptance has not been transmitted, or if the resource is not already committed to another requestor, and a subsequent confirmation signal is received. The responder does not become committed after transmitting an acceptance if the next signal directed to it from the requestor is a request cancellation.

Abstract: Digital data communication system uses a common channel to carry both normal messages and supervisory messages, the latter being inserted into the message stream during intervals between normal messages. Each transmitting location has a switch or equivalent controller with three settings. Setting 1 enables data to pass from the normal message generator directly to the channel. Setting 2 enables data to pass indirectly from the normal message generator through a delay line to the channel. Setting 3 enables data to pass from the supervisory message generator to the channel. Message status means associated with the delay line enables the switch to assume setting 3 when no normal message is being generated and a supervisory message is awaiting transmission.