You can freely build on How Modems Use Guard Time to Switch Between Data and Commands

What this patent covers

When a modem is in 'transparent mode,' it simply passes data through to a phone line. If the data stream happens to contain the specific character sequence used to tell the modem to stop and listen for commands, the modem might accidentally disconnect or stop transmitting. This patent introduces a 'guard time' requirement. The modem only switches to command mode if it detects the escape sequence surrounded by a specific period of silence, ensuring the sequence was intended as a command and not just part of a data file.

What is now free to use

All 1 claims of US 4549302 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 switching modes without a time-based guard interval.

• Does not cover hardware that lacks a processor to monitor data timing.

• Does not cover command mode entry triggered by physical hardware switches or buttons.

Who is building on this today

While the specific Hayes modem hardware is largely obsolete, the logic of using timing buffers to distinguish between data and control signals remains a fundamental concept in network protocol design and serial communication interfaces.

Products built on expired version of this technology

How to cite this patent in your documentation

Hayes Microcomputer Products Inc. US Patent 4549302. Modem with improved escape sequence mechanism to prevent escape in response to random occurrence of escape character in transmitted data. Granted 1985, expired 2003. 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.