When inventing the first Smartmodem, Dale Heatherington was faced with the problem of distinguishing commands from data given that there were only two serial pins available for use in the RS-232C port. He eventually settled on having two modes of operation, switching between them with an escape sequence, +++. Of course, files being sent over the modem could contain the string +++. For instance, this page includes the sequence several times. If the modem simply looked for the string to arrive in the data, it would switch to command mode at inappropriate times. There needed to be some way to distinguish a deliberate command from random data.
Heatherington's solution to this problem was to introduce a delay on either side of the escape sequence, meaning that only a
Hayes had initially filed for a patent on the electronic design of the Smartmodem in June 1981. In September 1983 they purchased a patent from Dr. Eaton of Bizcomp for $2 million, which included both a circuit design as well as a description of an escape sequence to trigger it. Hayes then updated the patent and re-filed it, this time including a lengthy abstract that focused entirely on the guard time and escape sequence, something that was mentioned only in passing in the original version. They received patent #4,549,302 in October 1985, Modem With Improved Escape Sequence With Guard Time Mechanism, commonly known as the "Hayes '302 patent" or the "Heatherington patent". It was this version that contained the description of the guard time.
A year after receiving the patent, Hayes decided to charge a $1 per modem license fee to use it. This included any modem already manufactured, and Hayes sent bills for millions of dollars to a number of major manufacturers. Lawsuits immediately started flying. Within a month of the license being made public U.S. Robotics and Prometheus Products started a lawsuit against Hayes in an attempt to break the patent, followed immediately by Hayes suing both for infringement. Microcom, Multi-Tech, and Ven-Tel then sued Hayes, and Hayes in turn sued Everex and Omnitel for patent infringement. Microcom and U.S. Robotics settled out of court and agreed to license the patent, but Everex, Ven-Tel, and Omnitel stuck it out in court where the Hayes patent was upheld.
Read more about this topic: Time Independent Escape Sequence
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