You can freely build on How Multiple Computers Share a Network Cable Without Crashing

What this patent covers

The patent describes a system for multiple data processing stations to communicate over a shared cable, like a bus. Each station has a "transceiver" (a device that transmits and receives) connected to the cable. When a station wants to send data, it first listens to the cable (Claim 5, "signal detecting means"). If the cable is busy, it waits. If the cable is clear, it starts transmitting. Crucially, if another station starts transmitting at the same time, causing a "collision," the system detects this (Claim 1, "collision detecting means"). Upon detecting a collision, the transmitting station immediately stops sending data (Claim 1, "interrupting the transmission"). It then waits a random amount of time before attempting to transmit again, with the wait time increasing if collisions keep happening (Abstract, "random number generator"). For example, if two computers try to send data at the exact same moment on a shared cable, this system detects the garbled signal, tells both computers to stop, and makes them wait different random times before trying again, preventing a continuous jam.

What is now free to use

All 1 claims of US 4063220 are in the public domain. Specifically:

The 1 dependent claim add narrowing limitations and are also free.

What is NOT covered

Patent expiry frees this specific invention. Separately-patented improvements made after expiry may still be protected.

• Does not cover communication systems where each device has its own dedicated connection (e.g., point-to-point links).

• Does not cover network protocols that use tokens or central controllers to manage access to a shared medium.

• Does not cover wireless communication systems where collisions are handled by different methods like listen-before-talk (LBT) without explicit collision detection on the medium.

• Does not cover systems where the transmitting means does not interrupt its transmission upon detecting a collision.

• Does not cover systems that do not use a random backoff mechanism to reattempt transmission after a collision.

Who is building on this today

While the direct implementation of CSMA/CD is less common in modern switched Ethernet networks, the core principles of network access control and collision avoidance are built upon by companies like Cisco, Juniper Networks, and Broadcom in their networking hardware and chipsets. The fundamental ideas of shared media communication and robust error handling continue to influence network protocol design.

Products built on expired version of this technology

How to cite this patent in your documentation

Xerox Corp. US Patent 4063220. Multipoint data communication system with collision detection. Granted 1977, expired 1995. Now in the public domain.

Note: This is a convenience citation. Consult a patent attorney for formal freedom-to-operate analysis.

PatentBrief is an educational resource and does not provide legal advice. Patent expiration information is derived from USPTO records and may not reflect continuation patents, divisional filings, or separately-patented improvements. For commercial use or production decisions, obtain a formal freedom-to-operate (FTO) opinion from a registered patent attorney.