Modem Escape Sequences What is a Modem Escape Sequence? A modem escape sequence allows a modem to change or "escape" from the receive/transmit mode of operation to the command mode of operation. Prior to 1981, modems used various escape sequences, such as the Eaton escape sequence, but these escape sequences were found to be unreliable in actual use because they could not prevent the modem from unexpectedly escaping into command mode when the data being transmitted contained the escape code. Improved Escape Sequence with Guard Time The Improved Escape Sequence with Guard Time was first used in a Hayes modem in 1981. The particular improvement allows a modem to escape from the receive/transmit mode of operation to the command mode of operation in a very reliable manner that does not rely at all on the probability of character occurrence in the data. It is important that the modem not escape if the characters used in the escape sequence appear at any time in the data being transmitted. Dale Heatherington was not satisfied with an escape mechanism which caused some data to be unsendable and therefore made the modem not be truly transparent to some data. He solved the problem by choosing predetermined characters for the escape code (such as +++) and surrounded them on either side by a predetermined guard time to alert the modem that the sequence is set off from a typical data string transmission. Dale Heatherington redefined the problem, and his resulting invention lead to U.S. Patent # 4,549,302 and corresponding patents in a number of countries. Hayes has licensed many modem manufacturers to make this technology readily available to the market. Currently, manufacturers such as Amstrad, Compaq, GPT, IBM, Megahertz, OKI, Practical Peripherals, US Robotics, and others license this technology from Hayes and have provided reliable escape mechanisms in their products. Time Independent Escape Sequence A new escape sequence, known as Time Independent Escape Sequence (TIES), has recently appeared on the market. TIES is a non- standard escape sequence which is not the same as the Improved Escape Sequence with Guard Time that was first used in a Hayes modem and is now used as the defacto standard for reliable modem operation by modem manufacturers worldwide. How do I know if my modem supports TIES? Hayes has developed a binary file (TIES_ID.BIN) that can assist you in determining if your modem or the modems which you are evaluating support TIES. If you transfer the TIES_ID.BIN file using XMODEM or YMODEM and your modem supports TIES, the file transfer will halt at a certain point or the modem will revert to Command State where it will not transmit data until an appropriate AT command is typed. Unlike the Improved Escape Sequence with Guard Time, TIES will cause a file transfer to abort if certain sequences of characters are present. The simplest TIES default escape sequence is "+++AT" where represents "carriage return." In TIES there are no required guard times. Other TIES sequences might have a lower case A or T (e.g. "at") or be of the form "+++AT", where is any valid AT command. The particular TIES sequences for a TIES modem depend on whether the communications software changes the value of the "escape character" (the +) and the end-of-command character (the ). The TIES_ID.BIN file includes all possible sequences of the form "xxxATy" where the ASCII value for x is varied from 0 to 127 and the ASCII value for y is varied from 0 to 127. This results in 16,384 sequences which are each repeated twice because the protocol does interrupt a sequence every so often (e.g., every 128 or 1024 bytes). A shorter file, TIESQUIK.BIN, is also available and will identify the existence of TIES if any Hayes Smartcom communications software is used. TIESQUIK.BIN can also be used with any other XMODEM or YMODEM file transfer software that does not reprogram the end-of- command character (most widely used communications software fall into this category). This shorter file will upload in 6 to 12 seconds at 2400 bps. Note: this procedure checks for the TIES escape mechanism only - it does not check for the Hayes Improved Escape Sequence with Guard Time. How can I obtain these TIES identification files? If you are reading this from a TIES Identification Kit diskette, then the files are also on this diskette. The diskette also contains a text file containing a White Paper on TIES. The White Paper file can be read with almost any word processor. On a DOS machine, select the diskette as the default drive, and type "BR PAPER" at the DOS Prompt. To obtain a disk with the TIES test files or to learn more about escape sequences, call the Online with Hayes (electronic bulletin board system) in United States at 800/US HAYES (800/874- 2937) or 404/HI MODEM (404/446-6336); in Europe at +44 81 569-1774 and in Asia at 852-887-7590. Select TIES Info from the main menu. The files TIES_ID.BIN and TIES_QID.BIN can be also downloaded if you wish. Information on TIES and copies of TIES_ID.BIN and TIES_QID.BIN can also be obtained on Compuserve by accessing the Hayes Forum (GO HAYES).