{
  "patent_number": "US 4549302",
  "country": "US",
  "title": "How Modems Use Guard Time to Switch Between Data and Commands",
  "original_title": "Modem with improved escape sequence mechanism to prevent escape in response to random occurrence of escape character in transmitted data",
  "summary": "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.",
  "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."
  ],
  "filed": "1983-10-11",
  "granted": "1985-10-22",
  "expires": "2003-10-11",
  "status": "expired",
  "holder": "Hayes Microcomputer Products Inc",
  "holder_url": "https://patentbrief.org/company/hayes-microcomputer-products-inc",
  "inventors": [
    {
      "name": "Dale A. Heatherington",
      "url": "https://patentbrief.org/inventor/dale-a-heatherington"
    }
  ],
  "times_cited": 42,
  "tags": [
    "telecommunications",
    "consumer_electronics",
    "mechanical"
  ],
  "abstract": "An FSK modem for coupling a standard data port (50) to a telephone line (45) in which the signal path between the phone line and the data port passes through a processor (55). When the modem is in a transparent mode of operation, for transmitting data, a timer (190) detects the passage of a predetermined period of time after the most recent data input signal input to a data port (50). If the next sequence of data signals corresponds to an escape character, the program (112) controlling the modem waits to see if a second occurrence of the predetermined period of time follows provision of the escape character. If this occurs, the modem switches to a command mode of operation in which it ceases to modulate and transmit signals and waits for commands to be entered at the data port. The same frequency detection apparatus (83, 192, 180) is used to both demodulate FSK encoded signals and detect the presence of a telephone ringing signal on the line. A pair of impedances (161, 162) provide alternate paths either through an input filtering network (167) or bypassing the network (135, 69) in a manner which allows the ring signals to pass directly to the input (69) of the frequency detector while assuring that only filtered FSK signals reach the input. Also shown is the use of the same frequency synthesizing apparatus (86, 253, 259) for generating FSK output signals and high group DTMF signals with appropriate switchable attenuation (88) provided in this signal path.",
  "url": "https://patentbrief.org/patent/us/4549302/hayes-modem-escape-sequence",
  "markdown_url": "https://patentbrief.org/patent/us/4549302/hayes-modem-escape-sequence/md",
  "google_patents_url": "https://patents.google.com/patent/US4549302",
  "relatedPatents": [
    {
      "patentNumber": "6665308",
      "countryCode": "US",
      "title": "How Cable Modems Fix Signal Distortions Before Sending Data",
      "url": "https://patentbrief.org/patent/us/6665308/apparatus-and-method-for-equalization-in-distributed-digital-data-transmission-systems"
    },
    {
      "patentNumber": "4063220",
      "countryCode": "US",
      "title": "How Multiple Computers Share a Network Cable Without Crashing",
      "url": "https://patentbrief.org/patent/us/4063220/ethernet-packet-network"
    },
    {
      "patentNumber": "4677614",
      "countryCode": "US",
      "title": "How Self-Healing Data Loops Automatically Elect a New Master Controller",
      "url": "https://patentbrief.org/patent/us/4677614/data-communication-system-and-method-and-communication-controller-and-method-therefor-having-a-dataclock-synchronizer-and-method"
    },
    {
      "patentNumber": "3906166",
      "countryCode": "US",
      "title": "How Early Cell Phones Handled Calls Across Different Towers",
      "url": "https://patentbrief.org/patent/us/3906166/cellular-mobile-phone-radio-telephone"
    },
    {
      "patentNumber": "5440632",
      "countryCode": "US",
      "title": "How Cable Boxes Download Software Updates Remotely",
      "url": "https://patentbrief.org/patent/us/5440632/reprogrammable-subscriber-terminal"
    }
  ]
}