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The Teletype Model 33 is an electromechanical teleprinter designed for light-duty office use. Teletype Corporation's Model 33 terminal, introduced in 1963, was one of the most popular terminals in the data communications industry until the late 1970s. Over a half-million 33s were made by 1975, and the 500,000th was plated with gold and placed on special exhibit.Telephone Engineer & Management, Volume 79, Harcourt Brace Jovanovich Publications, 1975 Another 100,000 were made in the next 18 months, and serial number 600,000, manufactured in the United States Bicentennial, was painted red, white and blue, and shown around the country.
The Model 33 was one of the first products to employ the newly standardized ASCII character encoding method, which was first published in 1963. A companion Teletype Model 32 used the older, established five-bit Baudot code. Because of its low price and ASCII compatibility, the Model 33 was widely used, and the large quantity of teleprinters sold strongly influenced several de facto standards that developed during the 1960s.
The Model 33 originally cost about $1000 (equivalent to $ today), much less than other teleprinters and computer terminals in the mid-1960s, such as the Friden Flexowriter and the IBM 1050. In 1976, a new Model 33 RO printer cost about $600 (equivalent to $ today).
As Teletype Corporation realized the growing popularity of the Model 33, it began improving its most failure-prone components, gradually upgrading the original design from "light duty" to "standard duty", as promoted in its later advertising (see advertisement). The machines had good durability and faced little competition in their price class, until the appearance of Digital Equipment Corporation's DECwriter series of teleprinters.
It is less rugged and cost less than earlier Teletype models. The Teletype Corporation introduced the Model 33 as a commercial product in 1963,"Auerbach Guide to Alphanumeric Display Terminals", Auerbach Publishers, 1975 after it had originally been designed for the United States Navy. The Model 33 was produced in three versions:
The trigram "" became widely used as an informal abbreviation for "Teletype", often used to designate the main text input and output device on many early computer systems. The abbreviation remains in use by ("ham radio") and in the hearing-impaired community, to refer to text input and output assistive devices.
"Dumb terminals", such as the low-cost ADM-3 (1975) began to undercut the market for Teletype terminals. Such basic video terminals, which could only sequentially display lines of text and scroll them, were often called Glass teletypes ("glass TTYs") analogous to the Teletype printers. More-advanced video terminals, such as the Digital Equipment Corporation VT52 (1975), the ADM-3A (1976), and the VT100 (1978), could communicate much faster than electromechanical printers, and could support use of a full-screen text editor program without generating large amounts of paper printouts. Teletype machines were gradually replaced in new installations by much faster dot-matrix printers and video terminals in the middle-to-late 1970s. (A malfunctioning Teletype served as a plot point in the 1971 science fiction film The Andromeda Strain).
Because of falling sales, Teletype Corporation shut down Model 33 production in 1981. Some remained in working use, as with a scientific project in Australia, in which one was connected to a phase ionosonde (to investigate the ionosphere) until 1984.
Everything is mechanically-powered by a single electric motor, located at the rear of the mechanism. The motor runs continuously as long as power is on, generating a familiar humming and slight rattle from its vibration. The noise level increases considerably whenever the printing or paper tape mechanisms are operating. Similar noises became iconic for the sounds of an active newswire or computer terminal. There is a mechanical bell, activated by code 07 (Control-G, also known as Bell character), to draw special attention when needed.
The Teletype Model 33, including the stand, stands high, wide and deep, not including the paper holder. The machine weighs on the stand, including paper. It requires less than 4 at 115 VAC and 60 Hz. The recommended operating environment is a temperature of , a relative humidity of between 2 and 95 percent, and an altitude of . The printing paper is an diameter roll, and the paper tape is a roll of wide tape. Nylon fabric are wide by long, with plastic spools and eyelets to trigger automatic reversal of the ribbon feed direction.
The entire Model 33 ASR mechanism requires periodic application of grease and oil in approximately 500 locations.
Earlier Teletype machine designs, such as the Model 28 ASR, had allowed the user to operate the keyboard to punch tape while independently transmitting a previously punched tape, or to punch a tape while printing something else. Independent use of the paper tape punch and reader is not possible with the Model 33 ASR.
The tape punch required Punched tape to keep its mechanism lubricated. There is a transparent, removable chad receptacle beneath the tape punch, which required periodic emptying.
The Model 33 prints on wide paper, supplied on continuous diameter rolls approximately long, and fed via friction instead of a tractor feed. It prints at a fixed pitch of 10 characters per inch, and supported 74-character lines,Teletype Technical Manual Bulletin 273B page 1-15, 1963, Change 2 although 72 characters is often commonly stated.
The keyboard required heavy pressure to operate the keys - on par with a mechanical typewriter - far more than any modern keyboard.
The Model 33 can operate either in half-duplex mode, in which signals from the keyboard are sent to the print mechanism, so that characters are printed as they are typed (local echo), or in full-duplex mode, in which keyboard signals are sent only to the transmission line, and the receiver has to transmit the character back to the Model 33 in order for it to be printed (remote echo). The factory setting is half-duplex, but it can be changed to full-duplex by the user.
The receiving machine can also be set up to not require operator intervention. Since messages were often sent across multiple time zones to their destination, it was common to send a message to a location where the receiving machine was operating in an office that was closed and unstaffed overnight. This also took advantage of lower telecommunication charges for non-urgent messages which were sent at off-peak times.
The sole electric motor in the machine has to be left running continuously whenever unattended operation is expected, and is designed to withstand many hours of idling. The motor displays a "HOT" warning label, clearly visible once the cover is removed.
Another CCU type is called "Computer Control Private Line", which operated on a local 20 mA current loop, the de facto standard serial protocol for computer terminals before the rise of RS-232 signaling. "Private Line" CCUs had a blank panel with no user controls or displays, since the terminal can be semi-permanently hard-wired to the computer or other device at the far end of the communications line.
Teletype also introduced a more-expensive ASCII Model 35 (ASR-35) for heavy-duty use, whose printer mechanism is based on the older, rugged Model 28. The basic Model 35 is mounted in a light gray console that matched the width of the Model 33, while the Model 35 ASR, with eight-hole mechanical tape punch and reader, is installed in a console about twice as wide.
The tape reader is mounted separately from the printer-punch mechanism on the left side of the console, and behind it is a tray for storing a manual, sheets of paper, or other miscellanea. To the right of the keyboard is a panel that can optionally house a rotary dial or DTMF pushbuttons for dialing a connection to a network via telephone lines.
The printer cover in later units also feature soundproofing materials, making the Model 35 somewhat quieter than the Model 33 while printing and punching paper tapes. All versions of the Model 35 have a copy holder on the printer cover, making it more convenient for the operator when transcribing written material.
Teletype Model 35 is mentioned as being used in "Experiment One", in the first RFC, . The Model 35 was widely usedthe Computer History Museum (CHM) in Mountain View, California termed it "ubiquitous" as terminals for the and IMPs to send and receive text messages over the very early ARPANET, which later evolved into the Internet.
The Model 38 (ASR-38) was constructed similar to and has all the typing capabilities of a Model 33 ASR, plus additional features. A two-color inked ribbon and additional ASCII control codes allowed automatic switching between red and black output while printing. An extended keyboard and type element support uppercase and lowercase printing with some additional special characters. A wider pin-feed platen and typing mechanism allowed printing 132 columns on fan-fold paper, making its output similar to the 132-column page size of the then industry-standard IBM 1403 model printers.
More expensive Teletype systems have paper tape readers that used light sensors to detect the presence or absence of punched holes in the tape. These can work at much higher speeds (hundreds of characters per second). More sophisticated punches were also available that could run at somewhat higher speeds; Teletype's DRPE punch can operate at speeds up to 240 characters per second.
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