How Modems Use Guard Time to Switch Between Data and Commands
This patent describes a method for modems to safely switch from sending data to accepting commands without accidentally triggering that switch while transmitting normal files.
Original patent title: “Modem with improved escape sequence mechanism to prevent escape in response to random occurrence of escape character in transmitted data”
This patent describes a method for modems to safely switch from sending data to accepting commands without accidentally triggering that switch while transmitting normal files. Granted to Hayes Microcomputer Products Inc in 1985 with 9 claims and 42 forward citations, and it is now in the public domain.
Key facts
Coverage
What does this patent actually cover?
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.
The gap
What does this patent NOT cover?
- 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.
These exclusions are unique to PatentBrief — derived from the actual claim language, not patent-office boilerplate.
What made this novel
The invention treats 'silence' as a data signal itself, using it as a gatekeeper to distinguish between intentional user commands and accidental patterns within a data stream.
The Patent Drawing
Schematic visualization of the patent's claim structure. Hand-drawn diagrams in progress for each landmark patent.
Where you've seen this
Real-world examples
Hayes Smartmodem series
Early dial-up internet connections
Bulletin Board System (BBS) communication software
Why it matters
The bigger picture
This invention solved a major frustration in the early days of dial-up internet and bulletin board systems. Before this, sending a file that randomly contained the 'escape' character string would cause the modem to drop the connection or stop sending data, effectively killing the session. It became the industry standard for the Hayes AT command set, which defined how computers talked to modems for decades.
Filed
October 11, 1983
Granted
October 22, 1985
Market context
Who's building on this
Companies in this space
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.
Market impact
This patent solidified the Hayes AT command set as the de facto industry standard. It effectively prevented competitors from creating compatible modems without licensing or finding complex workarounds, cementing Hayes Microcomputer Products as the dominant force in the 1980s modem market.
Claim 1 — Plain English
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.
The clever bit
The invention treats 'silence' as a data signal itself, using it as a gatekeeper to distinguish between intentional user commands and accidental patterns within a data stream.
What it does not cover
- 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.
Patent Journey
From filing to expiry
PatentBrief Score
Impact Score
Early stage
Citation count
33/40
Moderately cited
Claim breadth
6/20
Moderate scope
Recency
0/20
Older than 20 years
Assignee scale
0/20
Independent or smaller assigneeassigneeThe entity that owns the patent — usually the inventor's employer or a company.Read more →
PatentBrief Impact Score — based on citation count, claim breadth, recency, and assignee scale. Not a legal assessment.
Heuristic Value Estimate
What this patent might be worth
$19K – $60K
Midpoint $38K · expired or expiring · industry ×1.4
Heuristic only — blends forward/backward citation counts, claim scope, time remaining, litigation history, and CPC-derived industry baseline. Real valuations need a professional appraisal.
The original legal language
Original claims
9 claims as filed with the patent office.
Concepts involved
Citations
Patent lineage
Cite this patent
Heatherington, D. A. (1985). How Modems Use Guard Time to Switch Between Data and Commands (U.S. Patent No. 4,549,302). U.S. Patent and Trademark Office. https://patentbrief.org/patent/us/4549302/hayes-modem-escape-sequence
Auto-generated from the patent record. Double-check author order and the issue date against the official USPTO document before submitting.
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Common Questions
Frequently Asked Questions
What does How Modems Use Guard Time to Switch Between Data and Commands cover?
This patent describes a method for modems to safely switch from sending data to accepting commands without accidentally triggering that switch while transmitting normal files.
Who owns patent US 4549302?
Hayes Microcomputer Products Inc owns this patent, granted in 1985.
When does this patent expire?
This patent has expired and is now in the public domain — anyone can use the invention freely.
What is patent US 4549302 cited by?
This patent has been cited by 42 later patents that build on its ideas.
What problem does this patent solve?
This invention solved a major frustration in the early days of dial-up internet and bulletin board systems. Before this, sending a file that randomly contained the 'escape' character string would cause the modem to drop the connection or stop sending data, effectively killing the session. It became the industry standard for the Hayes AT command set, which defined how computers talked to modems for decades.
What does this patent NOT cover?
Does not cover switching modes without a time-based guard interval.
Patent monitoring