larry@uunet.uu.net (Larry Lippman) (09/23/89)
In article <telecom-v09i0394m02@vector.dallas.tx.us> npl@mozart.att.com (Nickolas Landsberg) writes: > In the above mentioned article, you mentioned that the speed > was around 60 baud. Well, it was actually something called "75-speed" > which indicated a maximum speed of 75 WPM, transmitted in 5-level > "baudot" encoding. After doing all the mumbo-jumbo af adding start/stop > bits, etc. this translates to an equivalent bits-per-second of 56.83 > or thereabouts. Actually, there were about half a dozen different versions of 5-level Baudot teletype systems which had baud rates that varied from a low of 45.5 at a nominal 60 wpm to a high of 75 at a nominal 100 wpm. What confuses the issue is that while 8-level ASCII has the same duration for start, information and stop bits, some of the 5-level Baudot systems used a LONGER stop bit duration which was a non-integer multiple of the start and information bits. For example, the earliest Baudot systems ran at 45.5 baud with a start/information bit duration of around 22 ms, and a stop bit duration of around 31.2 ms; this provided a nominal 60 wpm. The 75 wpm system in Nickolas Landsberg's example is 56.9 baud with a start/information bit duration of around 17.6 ms and a stop bit duration of around 25 ms. Even though there were timing differences, from a practical standpoint many of the 5-level Baudot systems did in fact communicate with each other. The Teletype Corp. Model 28 apparatus could run at either 60 or 75 wpm, and the Model 32 apparatus could run at a maximum of 100 wpm, if it felt in the mood. :-) The Model 32 may have been faster, but it was a LOT less rugged than the slower Model 28! > A system I worked on once had to monitor transmissions > from a telco switch which also used this. An interesting sidelight is > that they could transmit the full upper-case character set, the numerics > and a goodly supply of punctuation using just 5 bits. (Actually, they > used a "switch" character to flip-flop between meanings of particular > bit patterns. The "alternate" set would continue to be used until > the "switch" character was seen again.) All 5-level Baudot machines have a "Letters" mode and a "Figures" mode in order to fit the various alpha characters, numbers, punctuation symbols and control characters into the coding scheme. These modes were set by corresponding control characters, 11011 for FIGURES and 11111 for LETTERS. The most unusual device I have ever seen which generated 5-level Baudot to drive a Model 28RO page printer was the WECO 520 Emergency Reporting PBX. This system, which was designed during the 1950's, was an early PBX used by police and fire departments to handle call box telephones used by police and the public. A teletype printer was used to log the box number of a reporting telephone and the date/time. The PBX used wire-spring relays and crossbar switches. The interesting part is that WECO used a sequential series of standard 22-position stepping switches to actually encode and generate the properly-timed 5-level Baudot characters to drive the teletype printer! One could look at the bank wiring on the step switches and actually SEE the character encoding for <CR>, <LF>, the digits, etc. <> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp. <> UUCP {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> TEL 716/688-1231 | 716/773-1700 {hplabs|utzoo|uunet}!/ \uniquex!larry <> FAX 716/741-9635 | 716/773-2488 "Have you hugged your cat today?" [Moderator's Note: A curiosity when I was employed by the University of Chicago Telecommunications Department (then, in 1959-61 they just called it the 'telephone room') were the two TWX machines. One functioned normally and brought us messages from everywhere, but the other was on a special circuit to the local telco central office, known as the 'Kenwood Bell' due to its location on the corner of 61st Street and Kenwood Avenue, just south of the campus. All our long distance service went through there. To place a long distance call then, most people dialed 211 and passed the call to the operator. We plugged in on our board to tie-lines which went straight to certain positions at telco. They knew it was UC calling, and all we had to pass was the caller's extension number; then we could cut out and go on to other calls. Once an hour, the aforementioned TWX would come to life and a message would print out listing 'time and charges by extension'. We took this and matched it with the toll tickets we started when first putting the call through. All this machine was ever used for was to bring us the time and charges, for billing purposes to the campus extensions. IBT billed the campus master account; we re-billed each department. PT]