Path: utzoo!news-server.csri.toronto.edu!cs.utexas.edu!usc!rpi!dali.cs.montana.edu!milton!hayes.ims.alaska.edu!floyd From: floyd@ims.alaska.edu (Floyd Davidson) Newsgroups: comp.dcom.modems Subject: Re: comp.dcom.modems lexicon Message-ID: <1991Mar8.082523.25819@ims.alaska.edu> Date: 8 Mar 91 08:25:23 GMT References: <3832.27d4dcf9@hayes.uucp> <1991Mar7.055236.16871@ims.alaska.edu> <19594@cbmvax.commodore.com> Organization: University of Alaska, Institute of Marine Science Lines: 113 In article <19594@cbmvax.commodore.com> grr@cbmvax.commodore.com (George Robbins) writes: >In article <1991Mar7.055236.16871@ims.alaska.edu> floyd@ims.alaska.edu (Floyd Davidson) writes: >> Duplex vs. simplex originally was used to define radio >> communications that could be carried on in both directions at >> the same time (duplex), as opposed to communications that used >> the same spectrum and facilities for both directions, but not at >> the same time (simplex). The terms were applied to two-way >> radios, for instance CB, police FM, etc. Certainly the same usage >> could be applied to a two wire telephone circuit used for data. > >Cough, choke, gasp... I'm not sure what you are choking on there. >The simplex/duplex distinction goes back to telegraphy, like Western >Union, and this usage probably predates the radio orgin you suggest. How is it applied to telegraphy ????? Perhaps exactly as described above... >I don't know when the full/half duplex distinction came in - it could >have been early or could have awaited telephone carrier system to >show up. > >> I'm not so sure that Toby is correct in calling it a misuse of >> the terminology to use full/half duplex to refer to local echo. >> That is exactly how the term was derived as it relates to >> data modems. But it certainly has become ambigiuos, and such >> use should be discouraged. > >It's a misuse when all it refers to is enabling and disabling >local echo, while the modem is actually still modulating in >the full duplex mode. Not so. See below. (By the way, would you be so kind as to define "modulating in the full duplex mode". :-) > Remember that things like teletypes didn't >really generate a "local echo", it's just the transmitter and >receiver were normally wired in series on the same circuit and >couldn't the receiver seeing what was trasmitted, unless you >manually bridged across it. Maybe you will have to blame Ma Bell for that definition. At the other end of that teletype loop there is a modem, though it was called a Teletype Terminal Unit at the time, which had two modes of operation: half duplex and full duplex. In some cases, such as the Western Electric 43A1 it had a switch, in others it was just wired differently. In either case the modem put out an analog transmit signal and received an analog receive signal. In other words the modem was "duplex" on the analog side no matter what was on the digital side. The fd/hd switch or option only affected the digital side. There is a little more to a TTY circuit than what you see at the customer end. Half dux vs. full dux had more to do with the terminal unit than it did with how the machine was wired. The machine in fact was wired in series for half dux. The transmitting distributor and receiver selector magnets were wired to separate loops for full dux. Pretty simple. On the modem it was still pretty simple, but more complex than that. (I am sitting six feet from four each eleven foot bays of Lenkurt telegraph equipment. Some pretty old 25D stuff, there are about 200 modems, all of which have a half/full duplex option.) >> Likewise I don't think a modem that can only transmit or receive >> is a simplex device. It is a "read-only" or "transmit-only" >> device. > >You're on very thin ice here. You want to avoid confusing the >sense of the terms as used to describe radio equipment >from data communications modems and communication channels. My turn to cough and gag. >A simplex modem is one that can only send or receive. A simplex >data terminal is something like a stock ticker. I've been looking at circuit layout cards for a couple decades and don't recall seeing one labeled that way. They are always labeled as read-only or transmit-only. Of course, usually they are in fact simplex because there is only one pair provided and they use the entire bandwidth allowed in only one direction. Usually, but not always, they can in fact be wired up for the other half of the circuit. >I don't have too much trouble with the CCITT trying to define these >terms precisely and non-ambiguously, however that doesn't particularly >effect the traditional definitions of the words, at least outside the >context of the standards community. The problem is the several "traditional" definitions. And as you have demonstrated, most have very foggy derivations, not to mention meanings. I think it is fairly obvious where the modem people (and the computer people) originally got the terms full and half duplex. But they don't seem to know what the telephone company used the terms for. With big changes in technology came a slight change in terminology... And now it is ambigious. It doesn't make the original meaning wrong. >I would be impressed if any committe could elide infamous "baud rate"... The committee has exactly the right idea, never use the term baud. Maybe "baud rate" won't go away, but at least they aren't adding to its popularity. Floyd -- Floyd L. Davidson | floyd@ims.alaska.edu | Alascom, Inc. pays me Salcha, AK 99714 | Univ. of Alaska | but not for opinions.