joel@techunix.BITNET (Yossi (Joel) Hoffman) (05/30/90)
I was wondering: why is it that all the common modem speeds are multiples of 300 baud? Is there anything special about that number? Please reply by e-mail, as I don't often read this group. Thanks. -Joel (joel@techunix.technion.ac.il -or- joel@techunix.BITNET) --
kaufman@Neon.Stanford.EDU (Marc T. Kaufman) (05/31/90)
In article <9686@discus.technion.ac.il> joel%techunix.bitnet@jade.berkeley.edu (Yossi (Joel) Hoffman) writes: >I was wondering: why is it that all the common modem speeds are >multiples of 300 baud? Is there anything special about that number? Why, yes, there is. Long ago in the olden days the Teletype corporation invented a terminal (models 33 and 35) that would wack along at 10 characters per second, which was 110 baud (because there were 2 stop bits those days). Later, Teletype tried to see just how fast a mechanical device could go, so the model 37 was invented - which had upper and LOWER case characters, black and RED ribbon, and in general was always on the hairy edge of working. At that time, they decided that the mechanical clutch used to decode characters would drop out in 1 stop bit time. The mechanism could push 15 characters per second, so it ran at 150 baud. That's probably close to the limit, as IBM only got 14.8 cps on its Selectric terminals (2740 and 2741). The clutch dropout time was the reason old Baudot (5-level) teletypes required a stop bit that was 1.42 data bits long (since rounded to 1.5 in VLSI chips). The Bell 103 modem, which was current in those days, would run reliably only up to 300 baud, because of bandwidth constraints -- so the next logical baud rate was 300. The Bell 212 actually runs at 600 baud, coding 2 bits per symbol. The natural progression was then, 1200, 2400, 4800, 9600, 19200. [Why powers of 2? Because you have to synthesize the baud rate from some standard crystal frequency, and divide-by-2 circuits are easier than other divisors -- before LSI, we had to do it with flip-flops and gates]. ANSI was trying to standardize baud rates during the same time frame, and so there is an ANSI standard somewhere setting standard baud rates at 200, 400, 800, 1600, etc. I actually build an interface to a 1600 baud banking terminal once. Not to be outdone, IBM -- who always want proprietary hardware, started using 1800, 3600, 7200, and 14400 bps as their synchronous bit rates. Tannenbaum was right. The nice thing about standards is that there are so many of them. Marc Kaufman (kaufman@Neon.stanford.edu)
haynes@ucscc.UCSC.EDU (99700000) (05/31/90)
In article <9686@discus.technion.ac.il> joel%techunix.bitnet@jade.berkeley.edu (Yossi (Joel) Hoffman) writes: >I was wondering: why is it that all the common modem speeds are >multiples of 300 baud? Is there anything special about that number? > I'll post a reply here because it might be of more general interest. The formula is actually 75 baud time 2^n (so 75 and 150 are also standard speeds). This was standardized by somebody, probably the military, circa 1960. The origin of 75 is that the standard communication machine of the period was Baudot Teletype at 100 words per minute, using a 7.42-unit start-stop code (which today we call asynchronous). I don't know how 7.42 was arrived at; but it goes way back in history to speeds slower than 100 wpm. At 100 wpm, which is 10 char/sec, the Teletype sends at 74.2 baud. The military, more than anybody else at the time, was interested in synchronous transmission, apparently because cryptographic equipment works that way. So they rounded up to 75 baud from 74.2; and from there it's reasonable to say that every standard speed should be double the next lower speed. Not much point in having a speed increase if you don't double it. Now we get 110 baud, the widely-used speed that doesn't fit the formula, because if you keep the Teletype at 10 char/sec and transmit ASCII in an 11-unit start-stop code that's what you get. Eleven rather than ten because the mechanical equipment needed more than one bit time to get stopped at 10 char/sec. There was also a speed of 1050 baud in use because Teletype made a paper tape punch that was designed for a top speed of 105 char/sec. The short-lived Teletype Model 37 operated at 150 baud, 150 wpm; it was the last gasp of mechanical ingenuity up against electronics rapidly falling in price. haynes@ucscc.ucsc.edu haynes@ucscc.bitnet ..ucbvax!ucscc!haynes "Any clod can have the facts, but having opinions is an Art." Charles McCabe, San Francisco Chronicle
kindred@pyrite.telesci.UUCP (David L Kindred (Dave)) (06/01/90)
In article <1990May31.025608.18545@Neon.Stanford.EDU> kaufman@Neon.Stanford.EDU (Marc T. Kaufman) writes: In article <9686@discus.technion.ac.il> joel%techunix.bitnet@jade.berkeley.edu (Yossi (Joel) Hoffman) writes: >I was wondering: why is it that all the common modem speeds are >multiples of 300 baud? Is there anything special about that number? <lot's deleted> the next logical baud rate was 300. The Bell 212 actually runs at 600 baud, coding 2 bits per symbol. The natural progression was then, 1200, 2400, 4800, 9600, 19200. <more deleted> ANSI was trying to standardize baud rates during the same time frame, and so there is an ANSI standard somewhere setting standard baud rates at 200, 400, 800, 1600, etc. <more deleted> Not to be outdone, IBM -- who always want proprietary hardware, started using 1800, 3600, 7200, and 14400 bps as their synchronous bit rates. <more deleted> Along the lines a strange baud rates, I personally have run into the all time favorite baud rates of 134.5 and 1050. The 134.5 was used in a six bit commodities ticker, but I can not remember what used 1050, I ran into it while working for a company that was doing work for the Associated Press, so I suspect it was used in some of their older network equipment. For really old equipment, 50 and 75 baud were also used. -- EMail: kindred@telesci.UUCP (...!princeton!pyrnj!telesci!kindred) CI$: 72456,3226 (72456.3226@compuserve.com) Phone: +1 609 866 1000 x222 Snail: TeleSciences C O Systems, 351 New Albany Rd, Moorestown, NJ 08057-1177
kaufman@Neon.Stanford.EDU (Marc T. Kaufman) (06/02/90)
In article <KINDRED.90May31185056@pyrite.telesci.UUCP> kindred@telesci.uucp writes: > Along the lines a strange baud rates, I personally have run >into the all time favorite baud rates of 134.5 and 1050. The 134.5 >was used in a six bit commodities ticker,... 134.5 was also the baud rate used by the (half-duplex) IBM 2740 and 2741 Selectric typewriters. This works out to 14.9+ characters per second, since they used a 9-bit code (1 start, 1 stop, 7 data - sent HIGH BIT first). 1050 was what.. anybody.. I seem to remember Kleinschmidt reperforators (tape punches) running at that speed. Marc Kaufman (kaufman@Neon.stanford.edu)