US 4,539,677 A · Multiple access data communication system

Abstract

In this data communication system, stations linked to a single line bus exchange information in a bit-serial asynchronous format in which transitions between stop and start signals mark beginnings of information bytes. The stations use a modified CSMA/CD protocol (carrier sense multiple access with collision detection) to obtain sending access to the bus. During information transfers, durations of stop signals are less than a predetermined limiting time length, but long enough to allow general purpose processing equipment at a station to participate directly in the real time process of information reception, thereby avoiding the need for having complex and costly adapting equipment interface between the bus and such processing equipment. When any transmission concludes, the bus remains at the stop signalling level. By conditioning detection of bus availability on timeouts conducted while the bus is in this condition, stations then ready to transmit avoid interfering with on-going transmissions. After timing out, any station may begin transmitting. At this same time, all stations monitor the bus receptively. If a station not currently sending detects a start-stop transition, its processor is interrupted and directly examines the following information byte signals. If a sending station detects a transition, its processor acts to sense for bus collision by comparing the byte last transmitted with the byte last received from the bus. If collision is detected, the sending process is aborted and the timeout process for detecting bus availability is repeated. If collision is not detected, the sending process continues.

Claims11

Dependent claims

A method of transmitting multi-byte information messages between transceivers linked in a multiple access mode to a common bus, each transceiver interfacing between the bus and a respective user device, each device containing a general purpose computer adapted for multiple applications, comprising:

transmitting digital signals representing a said message from any one of said user devices through a respective one of said transceivers to said bus, said signals being arranged in a bit-serial asynchronous baseband form, said signals containing groups of first signals representing information bytes, each such byte group preceded by a respective group of second signals defining the beginning of the respective byte group, each said second group consisting of paired stop and start signal elements;
the stop and start elements in each pair having different amplitude levels providing a transition on said bus which forms a time reference for reception of the bits in the following byte group;
receiving and temporarily storing said byte groups one at a time at transceivers connected to other said user devices;
and
timing durations of stop signals in said second groups to allow sufficient time between receptions of consecutive byte groups at said other devices to permit respective computers of said other devices to be interrupted and perform real time processing operations relative to each received byte group while a next byte group is being received, as required for handling reception of the respective message.

2 more elements

In a data communication system for transferring signals constituting variable length data packets bit serially over a bus between a plurality of transceivers having multiple access passively tapped connections to said bus, each packet containing a plurality of bytes, each transceiver interfacing between the bus and a respective user device which device contains a general purpose computer, "primitive" transceiver apparatus for conducting said packet transfers with buffer storage capacity only sufficient for storing one incoming or outgoing byte at a time, said apparatus comprising:

means for transferring data byte signals from said device to said bus in a bit-serial baseband asynchronous form;
each group of bit signals which form a byte signal being preceded by a pair of oppositely directed stop and start signals whose transition forms a reference time for reception of the bits in that group;
the durations of said stop signals being tailored to functional characteristics of the associated user device computer;
timing means receivably coupled to said bus for timing intervals during which the signal amplitude on the bus is that associated with stop signal transmission, said timing means activating an associated bus available indication when said amplitude persists continuously for a predetermined limit time longer than the time allotted for transmission of one byte signal and an associated pair of stop-start signals;
and
means for applying the bus available indication to the user device for enabling that device to initiate transmission.

3 more elements

Dependencies are inferred from the claim text.

Multiple access data communication system

Life of this patent

Multiple access data communication system

Life of this patent

Abstract

Claims11