Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!cs.utexas.edu!yale!bunker!wtm From: rbarth@tumtum.cs.umd.edu (Richard Barth) Newsgroups: misc.handicap Subject: Re: News We've Been Waiting For Tdd/ascii Modem Message-ID: <9576@bunker.UUCP> Date: 15 Jan 90 03:33:28 GMT Sender: wtm@bunker.UUCP Reply-To: rbarth@tumtum.cs.umd.edu (Richard Barth) Distribution: misc Lines: 61 Approved: wtm@bunker.UUCP Index Number: 6234 In Digest #1025 Stephen White writes: >With the International Speeds on TTD's (sic!) what is the CCITT spec And with the US standard, where could I get the technical data, like >db levels for detect, etc etc... Just knowing the frequencies is not >enough. I wouldn't look too hard for a CCITT standard; finding it wouldn't help. TDDs use the Baudot code. The standard rate for domestic (U.S.) TDDs is 45.45 baud, which is the "60 words per minute" used on teleprinters back in the days when everybody used Baudot machines. In Europe and elsewhere, 50 baud is commonly used. 50 baud. Baudot is termed a "7.42 unit code", because there are one start bit, five data bits, and 1.42 stop bits. Granted 1.42 stop bits sounds a bit strange, but it derives from the design of the old mechanical printers. Today, electronic componentry is used and 1.5 stop bits is commonly used, being available from UARTs such as the 8250 used in the IBM-PC and many clones. There are several kinds of "Baudot" code. The one described in the CCITT standard (if there is one) is unlikely to describe the way TDDs are actually built. Find yourself a copy of the ITT handbook (formal title is the "Radio Engineer's Handbook" at a library. It will probably (my old copy does) list four or five versions of "Baudot". One is the "International" (European) standard; one is the weather standard used by meteorologists and contains wind-direction symbols and the like; there's another whose specialty I forget. There is finally one termed the "U.S. Military standard". That's the one you want. It's also the standard used by ham radio operators, and you can get it out of the ARRL Handbook (The Radio Amateur's Handbook, pubslished by the American Radio Relay League, Newington CT.) TDDs differ from this version primarily in ignoring the bell code (what's a deaf person gonna do with THAT?) and the substitution of a backspace for the Baudot null. The other characteristics of TDDs are, unfortunately, non-standard. The Electronic Industries Assn. (EIA) formed a special committee that labored mightily for several years to develop a TDD standard. Unfortunately some of the companies in the TDD business (who were participating in the committee) went out of the TDD business. Those remaining couldn't agree on technical details. A final draft of the committee report was distributed to participants, and the committee died. You can probably get a copy of that draft from the EIA. The staff person was Peter Bennett; The EIA Committee was TR-41. EIA's address is: Electronic Industries Assn. 2001 Eye Street NW Washington DC 20006 If EIA can't help you let me know; I've still got my copy of the draft around here someplace - God knows where - and with luck I might even be able to find it.