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.
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.
Key facts
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 timeline
Application submitted to the patent office
Application published, typically 18 months after filing
Patent officially issued
Patent enters public domain
This patent is in the public domain
See the Freedom to Build guide — what is free to use, what is not, and how to cite this patent.
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.
Patent Claims
0 independent claims · 1 dependent
Claims are the legal boundaries of the patent. An independent claim stands alone. A dependent claim adds limitations to its parent, narrowing — but not broadening — the scope.
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.
Embed
Add this patent to your site
Drop this plain-English patent card into any blog post or article — free, no signup. It always links back to the full breakdown here.
<div data-patentlens-widget data-patent-number="US4549302"></div> <script src="https://patentbrief.org/embed.js" async></script>
Stay in the loop
Get a weekly digest of new patents.
One email per week. No spam. Unsubscribe anytime.
Keep exploring
Related patents you should know
US 4683195 · 1987
How to Make Billions of Copies of a DNA Segment
This patent describes the Polymerase Chain Reaction (PCR), a method to rapidly create many copies of a specific piece of DNA or RNA, enabling its detection and analysis.
Cetus Corp
US 8697359 · 2014
How to Edit Genes in Human Cells Using an Engineered CRISPR System
This patent describes an engineered CRISPR-Cas9 system for precisely cutting DNA in eukaryotic cells to change how genes work, opening the door for gene editing in complex organisms.
Massachusetts Institute of Technology
US 7657849 · 2010
How the iPhone's Slide-to-Unlock Gesture Works
Apple's 2010 patent describes unlocking a device by dragging a specific graphical image across the touchscreen along a predefined path, a gesture that became iconic with the original iPhone.
Apple Inc
US 4733665 · 1988
How Doctors Implant a Permanent Stent Using a Balloon
This patent describes the method for placing a permanent, expandable wire mesh tube inside a blood vessel or other body tube using a balloon-tipped catheter to widen it and keep it open.
Expandable Grafts Partnership
US 4965188 · 1990
How to Make Many Copies of a DNA Piece with Heat
This patent describes the Polymerase Chain Reaction (PCR) method, a technique to make millions of copies of a specific DNA segment using a heat-resistant enzyme and repeated temperature changes.
Cetus Corp
US 4235871 · 1980
How to Encapsulate Active Materials in Lipid Bubbles Efficiently
This patent describes a method for trapping biologically active substances inside tiny, multi-layered fat bubbles called liposomes, using a specific water-in-oil emulsion and gel-forming process to improve how much material gets captured.
Individual
Semantically similar
You might also find these interesting
US 6665308 · 2003 · Terayon Communication Systems Inc
How Cable Modems Fix Signal Distortions Before Sending Data
US 4063220 · 1977 · Xerox Corp
How Multiple Computers Share a Network Cable Without Crashing
US 4677614 · 1987 · Process Systems Inc
How Self-Healing Data Loops Automatically Elect a New Master Controller
US 3906166 · 1975 · Motorola Inc
How Early Cell Phones Handled Calls Across Different Towers
More to explore
More in Telecom & Wireless
US 5347632 · 1994 · Prodigy Services Co
Prodigy's System for Interactive Online Information and Shopping
US 3906166 · 1975 · Motorola Inc
How Early Cell Phones Handled Calls Across Different Towers
US 4063220 · 1977 · Xerox Corp
How Multiple Computers Share a Network Cable Without Crashing
US 4200770 · 1980 · Leland Stanford Junior University
How to Create a Secret Code Key Without Meeting First
New to patents?
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