Wondering why this chapter is here? It's to explain a few things so you'll understand more about what I talk about in the next chapter when I explain my "telex hacks".
A "primer" on the operation of Telex.
From firstname.lastname@example.org Tue Sep 23 14:28:06 1997 Date: Tue, 23 Sep 1997 07:38:15 -0400 (EDT) From: The Cheshire CatalystHi Colin,
To: "Colin R. Y. Gallagher" Subject: Re: Telex info On Tue, 23 Sep 1997, Colin R. Y. Gallagher wrote: > How does telex work? Is it a digital signal? Is it transmitted over > phone lines or dedicated wires? Is it packet switched or circuit > switched.
Telex worked on old, electro-mechanical teletype machines (as you know from the machine at your father's business). It used a dedicated line from the Western Union Telegraph Company (though it was later sold to AT&T), and sent voltages through the line. It used a 5-bit code called Baudot, named for Emile Baudot of France.
The Baudot code has 2 to the 5th bits, and can therefore represent only 32 different characters. Crafty old Emile, however, took two of those codes, and called them "Figures Shift" and "Letters Shift". The letters shift characters were the following:
The blank character was all Spaces, and the Letters character was all Marks (representations used before the Digital Computer era when we started referring to Zeros & Ones). The digits were represented on the Telex machine where qwertyuiop (the top row of the Typewriter keyboard) was equal to 1234567890, but only AFTER receiving the Figures Shift character.
When "cutting a tape" (preparing a message on paper tape for transmission at full operating speed), and a typo was made (and caught on the spot), the telex operator could back up the paper tape, hit the LTRS key, and fill over the mis-typed characters with Letters Shift (the character made up of all Marks - a hole in all 5 tape positions), and then continue typing into the paper tape. When sent, there would be a couple of character spaces where the type head just stood still, but it was still faster than retyping a whole new tape, since it only "cost" a couple of characters of time.
Some TTYs (engineering shorthand for TeleTYpe machines) had two "cages" with letter slugs in them, and others had cylinders with letters on them. As each character came in, the box would shift to the proper letter, and a hammer would strike the back of the slug/cylinder to print the letter. If a Figures Shift was received, the type boxes were physically shifted so the second type box was in front of the striker, or the cylinder was turned 180 degrees, so that the back of the cylinder was used to hit the figures.
When the Letters Shift character was received, the type head shifted back to the original position. Sometimes, however, the Letters Shift would be garbled and not received, or the original Figures Shift character was missing, and the text of the message would garble, since characters were sent in the "wrong shift". Good telex operators could recognize this, and usually get the message back.
There were a number of different kinds of "Figures Shift" type faces floating about. The Weather Service had their standard with their special weather symbols, and stock brokers used their own with 1/4 and 3/8 characters set up. Usually only the Period, Comma and maybe the per cent sign were pretty much standard throughout the world. Finally the ITU (International Telecommunications Union) set up the International Telegraph Alphabet #2 for the Baudot code.
By then, ITA #5 was coming in (the ASCII code - American Standard Code for Information Interchange), and TWX (TeletypeWriter eXchange) teletypes with the 7-bit (with 1 bit for Parity) ASCII code was making inroads, not only on the telex circuits, but in computer usage as well.
Because fractions were not a regular part of the Telex character set, the printing of fractions was covered in the Telegraph Regulations on how telegrams were to be transmitted. A number like "one and a half" would be sent like this: 1-1/2. Remember that before teletype machines, telegrams were sent by hand using ITA #1. Of course, International Telegraph Alphabet #1 was the Morse Code. While most people using computerized word processing today leave a blank space between the whole number and the numerator (as in 1 1/2), a telegraph operator needed to have an actual character in there or the recipient might get confused as to what number was being transmitted. The "hyphen" character (known to telegraphers as the "double dash") was the quick fix.
Telex communications was always "circuit switched", similar to telephone calls. Once a call was established, either by hand using telephone like cord-switchboards in the early days, or by using "Strowger Switches" where rotary telephone dials were used to "dial" telex numbers, the circuit established for the communication was maintained until the call was completed, and disconnected by either party.
Keep Smiling, Cheshire Richard Cheshire http://CheshireCatalyst.Com/ The Cheshire Catalyst mailto:email@example.com "I *must* be a 'Patron Saint' of hackers. Wired Magazine said so!" - The Cheshire Catalyst http://CheshireCatalyst.Com/beyondhope/#wired --- Florida has a State Sponsored Religion. It's main sects are the Seminoles & the Gators.