# 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. - **Patent:** US 4549302 - **Original title:** Modem with improved escape sequence mechanism to prevent escape in response to random occurrence of escape character in transmitted data - **Owner:** Hayes Microcomputer Products Inc - **Granted:** 1985 - **Status:** Public domain (expired) - **Times cited:** 42 - **Field:** telecommunications, consumer_electronics, mechanical ## What it does 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 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. ## 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. ## Real-world examples 1. Hayes Smartmodem series 2. Early dial-up internet connections 3. Bulletin Board System (BBS) communication software ## Why it matters 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. ## 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. **Full plain-English explainer:** https://patentbrief.org/patent/us/4549302/hayes-modem-escape-sequence **Original patent:** https://patents.google.com/patent/US4549302 --- _Source: PatentBrief — https://patentbrief.org. Patent facts are from public records; the plain-English explanation is PatentBrief's._ ## Related patents Semantically similar inventions in the PatentBrief corpus: - [How Cable Modems Fix Signal Distortions Before Sending Data](https://patentbrief.org/patent/us/6665308/apparatus-and-method-for-equalization-in-distributed-digital-data-transmission-systems) — A method for cable modems to pre-filter data so it arrives clearly at the central office, preventing signal errors caused by the messy physical wires between them. - [How Multiple Computers Share a Network Cable Without Crashing](https://patentbrief.org/patent/us/4063220/ethernet-packet-network) — This patent describes how multiple computers can share a single communication cable by listening for other transmissions and stopping their own if a collision occurs, then trying again later. - [How Self-Healing Data Loops Automatically Elect a New Master Controller](https://patentbrief.org/patent/us/4677614/data-communication-system-and-method-and-communication-controller-and-method-therefor-having-a-dataclock-synchronizer-and-method) — A communication system where multiple nodes in a loop can automatically take over as the master controller if the current one fails, ensuring the network stays synchronized. - [How Early Cell Phones Handled Calls Across Different Towers](https://patentbrief.org/patent/us/3906166/cellular-mobile-phone-radio-telephone) — This patent describes a system for early portable phones to automatically find the strongest signal from a base station and switch channels as the user moves, reducing battery drain and interference. - [How Cable Boxes Download Software Updates Remotely](https://patentbrief.org/patent/us/5440632/reprogrammable-subscriber-terminal) — A method for cable television boxes to automatically download and install new software updates sent over the air from the cable provider's main office.