Teletype Model 33

While the manufacturer called the Model 33 teleprinter with a tape punch and tape reader a "Model 33 ASR", many computer users used the shorter term "ASR-33". The earliest known source for this equipment naming discrepancy comes from Digital Equipment Corporation (DEC) documentation,{{cite web |url=http://www.bitsavers.org/pdf/dec/pdp4/F-41D_PDP4brochure_Sep63.pdf |title=F-41D PDP-4 Brochure September 1963 |page=5}} where the September 1963 PDP-4 brochure calls the Teletype Model 28 KSR a "KSR-28" in the paragraph titled "Printer-Keyboard and Control Type 65". This naming convention was extended from the Teletype Model 28 to other Teletype equipment in later DEC documentation, consistent with DEC's practice of designating equipment using letters followed by numerals. For example, the DEC PDP-15 price list from April 1970 lists a number of Teletype Corporation teleprinters using this alternative naming convention.{{cite web |url=http://www.bitsavers.org/pdf/dec/pdp15/1970_PDP15PriceList.pdf |title=Digital Equipment Corporation pdp15 Price List | page=2}} This practice was widely adopted as other computer manufacturers published their documentation. For example, Micro Instrumentation and Telemetry Systems marketed the Teletype Model 33 ASR as "Teletype ASR-33".

The trigram "{{mono|tty}}" 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 radio amateurs ("ham radio") and in the hearing-impaired community, to refer to text input and output assistive devices.{{Cite web |date=September 2013 |title=Text Telephone Devices (TTY or TDD) |url=https://www.ncdhhs.gov/documents/files/text-telephone-devices/open |publisher=North Carolina Department of Health and Human Services}}

Early video terminals, such as the Tektronix 4010, did not become available until 1970, and initially cost around $10,000 (equivalent to ${{inflation|US|10000|1963|r=-3|fmt=c}} today).{{Cite journal |year=1973 |title=Tektronix Products 1973 |url=http://bitsavers.informatik.uni-stuttgart.de/pdf/tektronix/catalog/Tektronix_Catalog_1973.pdf |journal=Tektronix Products |pages=275}} However, the introduction of integrated circuits and semiconductor memory later that decade allowed the price of cathode-ray tube-based terminals to rapidly fall below the price of a Teletype teleprinter.

"Dumb terminals", such as the low-cost ADM-3 (1975) began to undercut the market for Teletype terminals.{{Cite news |date=December 20, 1976 |title=ComData advertisement |url=https://books.google.com/books?id=Zab9x-fFopsC&q=adm-3+33-ksr&pg=PA15 |periodical=Computerworld |page=15 |volume=10 |issue=51 |issn=0010-4841}} 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.{{Cite book |url=https://archive.org/details/dictionaryofcomp0000unse_e6z7/page/220/mode/2up?view=theater |title=A Dictionary of Computing |publisher=Oxford University Press |year=2008 |edition=5th |pages=220 |language=en}} 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.{{cite book |url=http://www.vt100.net/docs/vt100-tm/ek-vt100-tm-002.pdf |title=VT100 Series Technical Manual |date=1979 |publisher=Digital Equipment Corporation |page=2{{hyp}}14 |access-date= |archive-url=https://web.archive.org/web/20200101071544/https://vt100.net/docs/vt100-tm/ek-vt100-tm-002.pdf |archive-date=2020-01-01 |url-status=live}} Teletype machines were gradually replaced in new installations by much faster dot-matrix printers and video terminals in the middle-to-late 1970s.

Because of falling sales, Teletype Corporation shut down Model 33 production in 1981.{{Cite magazine |last=Libes |first=Sol |date=December 1980 |title=Bytelines |url=https://archive.org/details/byte-magazine-1980-12/page/n215/mode/1up?view=theater |magazine=BYTE |pages=214–218 |volume=5 |issue=12}}

File:Teletype-IMG 7299.jpg

The design objective for the Model 33 was a machine that would fit into a small office space, match with other office equipment of the time and operate up to two hours per day on average. Since this machine was designed for light duty use, adjustments that Teletype made in previous teleprinters by turning screws were made by bending metal bars and levers. Many Model 33 parts were not heat-treated and hardened. The base is die-cast metal, but self-tapping screws were used, along with parts that snapped together without bolting.{{Cite web |title=Teletype ASR 33 |url=https://www.curiousmarc.com/mechanical/teletype-asr-33 |access-date=2024-05-28 |website=www.curiousmarc.com |language=en-US}}

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.{{cite web |last=Gesswein |first=David |title=ASR 33 Teletype Information |url=https://www.pdp8online.com/asr33/asr33.shtml |website=pdp8online |access-date=2022-01-24}} 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 BEL), to draw special attention when needed.

The Teletype Model 33, including the stand, stands {{convert|34|inch}} high, {{convert|22|inch}} wide and {{convert|18.5|inch}} deep, not including the paper holder. The machine weighs {{convert|75|lb}} on the stand, including paper. It requires less than 4 amperes at 115 VAC{{Clarification needed|date=May 2025}} and 60 Hz. The recommended operating environment is a temperature of {{cvt|40 to 110|°F}}, a relative humidity of between 2 and 95 percent, and an altitude of {{convert|0 to 10,000|feet}}. The printing paper is an {{convert|8.44 by 4.5|inch|adj=on}} diameter roll, and the paper tape is a {{convert|1000|foot|adj=on}} roll of {{convert|1|inch|adj=on}} wide tape. Nylon fabric ink ribbons are {{convert|0.5|inch|adj=on}} wide by {{convert|60|yard|adj=on}} long, with plastic spools and eyelets to trigger automatic reversal of the ribbon feed direction.{{Cite journal |date=May 1974 |title=Teletype Model 33 Vol. 1: Technical Manual |url=https://drive.google.com/file/d/1mri9Mbln4le_bVx22ORpn8q9drf4UhVu/view |journal=Technical Manual: 33 Teletypewriter Sets |publisher=Teletype Corporation |volume=1 |pages=}}{{Rp|page=16}}

The entire Model 33 ASR mechanism requires periodic application of grease and oil in approximately 500 locations.

File:ASR 33 three quarter interior.jpg | Paper tape punch and reader visible in foreground

File:ASR-33 backside.jpg | Rear of unit, with bell and answer back drum to left of motor

File:ASR-33 3.jpg | Fully-exposed mechanism

File:TTY-ASR-33-Clutch.jpg|Mechanism which translated serial electrical pulses onto a parallel mechanical data bus

File:Teletype Model 33 ASR Carriage (1968) (14560514420).png|Closeup of a well-worn type element

File:TTY33ASR.jpg

As a cost-saving measure, the optional paper tape mechanisms were dependent on the keyboard and page printer mechanisms. The interface between the paper tape reader and the rest of the terminal is completely mechanical, with power, clock, and eight data bits (which Teletype called "intelligence") all transmitted in parallel through metal levers. Configuration of user-selectable options (such as parity) is done with mechanical clips that depress or release various levers. Sensing of punched holes by the paper tape reader is done by using metal pins which mechanically probe for their presence or absence. The paper tape reader and punch can handle eight-bit data, allowing the devices to be efficiently used to download or upload binary data for computers.

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.{{cite web |url=http://www.baudot.net/docs/haynes--notes.pdf |author=Jim Haynes |title=Some Notes on Teletype Corporation}}{{cite book |title=TECHNICAL MANUAL :33 TELETYPEWRITER SETS RECEIVE-ONLY (RO) KEYBOARD.SEND-RECEIVE (KSR) AUTOMATIC SEND-RECEIVE (ASR) |date=1968 |publisher=Teletype Corporation |location=Skokie, IL |edition=Bulletin 310B |chapter=SECTION 574-124-100TC 33 TAPE READER GENERAL DESCRIPTION AND PRINCIPLES OF OPERATION}}

The tape punch required oiled paper tape to keep its mechanism lubricated. There is a transparent, removable chad receptacle beneath the tape punch, which required periodic emptying.{{Rp|page=33}}

The printing mechanism is usually geared to run at a maximum ten characters per second speed, or 100 words per minute (wpm), but other slower speeds were available: 60 wpm, 66 wpm, 68.2 wpm, and 75 wpm.Teletype Parts Bulletin, No. 1184B, page 35, figure 38 There are also many typefont options. The Teletype Parts BulletinTeletype Parts Bulletin, No. 1184B, pages 27–29, figures 29–31 listed 69 available Model 33 type element factory-installed options (frequent type element changes in the field were impractical). The type element, called a "typewheel" in Teletype's technical manuals, is cylindrical, with characters arranged in four tiers, 16 characters per tier, and thus is capable of printing 64 characters. The character to be printed is selected by rotating the typewheel clockwise or anticlockwise and raising or lowering it, then striking the typewheel with a padded hammer, which impacts the element against the ink ribbon and paper.{{YouTube |id=x_Di7KpHubc |title=Teletype ASR 33 Part 6: Print Head Mechanism}}, October 28, 2019

The Model 33 prints on {{convert|8.5|inch|adj=on}} wide paper, supplied on continuous {{convert|5|inch|adj=on}} diameter rolls approximately {{convert|100|ft}} 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.

File:Mappa Teletype ASR-33.jpg

The Model 33 keyboard generates the seven-bit ASCII code, also known as CCITT International Telegraphic Alphabet No. 5, with one (even) parity bit and two stop bits, with a symbol rate of 110 baud,{{cite web |url=http://www.pdp8.net/asr33/asr33.shtml |title=ASR 33 Teletype Information |access-date=2020-10-22}} but it only supports an upper-case subset of that code; it does not support lower-case letters or the {{mono|`}}, {{mono|{}}, {{mono||}}, {{mono|}}}, and {{mono|~}} characters.

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.{{cite magazine |last=Starr |first=Samuel S. |title=Inside the Amazing ASR 33 |url=https://deramp.com/downloads/mfe_archive/030-Terminals/Teletype/Model%2033/Kilobaud%20July%201977%20Inside%20the%20Amazing%20ASR%2033.pdf |magazine=Kilobaud Microcomputing |date=July 1977 |pages=98–100}}{{cite book |title=Technical Manual, 33 Teletypewriter Sets, Receive-Only (RO), Keyboard Send-Receive (KSR), Automatic Send-Receive (ASR) |url=http://bitsavers.org/communications/teletype/33/310B_Vol_1_33_Teletypewriter_Sets_Technical_Manual_Sep74.pdf |publisher=Teletype Corporation |date=September 1974 |pages=1,6,7}}

File:TTYanswserbackMechanism.jpg

The Teletype Model 33 contains an answer-back mechanism that is generally used in dial-up networks such as the Teletypewriter Exchange Service (TWX). At the beginning of the message, the sending machine can transmit an enquiry character or WRU ("who are you") code, and the recipient machine automatically initiates a response, which is encoded in a rotating drum that had been preprogrammed by breaking off tabs.{{Cite web|url=http://www.pdp8online.com/asr33/pics/main_back.shtml|title=ASR 33 Teletype Rear View of Main Assembly|website=www.pdp8online.com}} The answer-back drum in the recipient machine rotates and sends a unique identifying code to the sender, so that the sender can verify connection to the correct recipient. The WRU code can also be sent at the end of the message. A correct response confirms that the connection had remained unbroken during the message transmission. To conclude the transmission, the sending machine operator presses the disconnect button.{{Rp|page=45}}

The receiving machine can also be set up to not require operator intervention.{{Rp|page=8-9}} 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.

The communications module in the Model 33 is known as a Call Control Unit (CCU), and occupies the space to the right of the keyboard and printer. Various CCU types were available; most of them operated on the telephone network and included the relevant user controls. Variants included rotary dial, DTMF signalling or a mechanical card dialer. An acoustic coupler for a de facto standard-sized and shaped telephone handset was also available.{{Rp|14}}

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.

File:Telex machine ASR-32.jpg service, had a three-row keyboard and narrower, five-hole paper tape.]]

File:LCM - 1960s Teletype computer printer 01.jpg in Seattle]]

{{anchor|Model 32}}

The Teletype Model 32 line used the same mechanism and looked identical, except for having a three-row keyboard and, on the ASR version, a five-hole paper tape reader and punch, both appropriate for Baudot code.

{{anchor|TTY35}}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.{{Cite book |url=https://www.navy-radio.com/manuals/tty/35-brochure-1966.pdf |title=Model 35 Equipment |date=1969 |publisher=Teletype Corp. |pages=5 |language=en}}

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, {{IETF RFC|1|link=no}}. The Model 35 was widely used as terminals for the minicomputers 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.{{Cite book |url=http://archive.org/details/TNM_Model_38_wide_platen_data_terminals_build-in__20170923_0064 |title=Model 38 wide platen data terminals, built-in modem - Teletype |year=1971 |language=en}}

More expensive Teletype systems have paper tape readers that used light sensors to detect the presence or absence of punched holes in the tape.{{Cite journal |date=January 1965 |title=Figure 6 - High Speed Tape Punch, Typical Schematic Wiring (Late Design, Five through Eight Levels) |url=https://www.navy-radio.com/manuals/tty/jk/592-803-100-iss2-6501.pdf |journal=Bell System Practices |language=en |issue=2 |pages=6}} 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.

File:ASR 33.jpg

• ASCII was first used commercially during 1963 as a seven-bit teleprinter code for American Telephone and Telegraph's Teletypewriter Exchange Service (TWX) using Teletype Model 33 teleprinters.
• The Teletype Model 33 series was influential in the development and interpretation of ASCII code characters. In particular, the Teletype Model 33 machine assignments for codes 17 (Control-Q, DC1, also known as XON) and 19 (Control-S, DC3, also known as XOFF) became de facto standards.Tomasi, Wayne. "Electronic Communications Systems: fundamentals through advanced", Prentice Hall, 2001, p. 531.
• The programming language BASIC was designed to be written and edited on a low-speed Teletype Model 33. The slow speed of the Teletype Model 33 influenced the user interface of minicomputer operating systems, including UNIX.{{Citation needed|date=May 2025}}
• A Teletype Model 33 provided Bill Gates' first computing experience.{{cite book |last=Manes |first=Stephen |title=Gates: How Microsoft's Mogul Reinvented an Industry and Made Himself the Richest Man in America |publisher=Touchstone Pictures |year=1994 |page=27 |isbn=0-671-88074-8}}
• In 1965, Stanford University psychology professors Patrick Suppes and Richard C. Atkinson, in the pilot program for computer assisted instruction, experimented with using computers to provide arithmetic and spelling drills via Teletypes and acoustic couplers to elementary school students in the Palo Alto Unified School District in California and elsewhere.{{cite journal |last1=Suppes |first1=Patrick |author1-link=Patrick Suppes |last2=Jerman |first2=Max |last3=Groen |first3=Guy |title=Arithmetic drills and review on a computer-based teletype |url=http://suppes-corpus.stanford.edu/articles/comped/54-5.pdf |journal=The Arithmetic Teacher |volume=13 |issue=4 |pages=303–309 |date=April 1966 |doi=10.5951/AT.13.4.0303}}{{cite web |last=Suppes |first=Patrick |title=Computer-Assisted Instruction at Stanford |url=http://suppes-corpus.stanford.edu/techreports/IMSSS_174.pdf |date=May 19, 1971}}
• In 1971, Ray Tomlinson chose the "@" symbol on his Teletype Model 33 ASR keyboard for use in network email addresses.[http://www.shadycharacters.co.uk/2011/07/the-symbol-part-1-of-2/ The @-symbol, part 1 of 2], Shady Characters, July 2011
• The serial ports in Unix-like systems are named {{mono|/dev/tty...}}, which is short for "Teletype".

File:DEC PDP-8 family - PDP-8F (1972), PDP-8 (1965), PDP-8S (c.1966) - and Teletype ASR-33 (1964), on the side of Data General Eclips (1974) & Nova (1969) - Computer History Museum (2007-11-10 23.08.07 by Carlo Nardone).jpg|Teleprinters were standard terminals for DEC and Data General minicomputers

File:Pdp7-oslo-2005.jpeg|PDP-7 computer with teleprinter

File:Altair 8800 and Model 33 ASR Teletype.jpg|The ASR-33 was one of the most affordable terminals for early home computers

• Teletype Model 28

{{reflist}}

• [http://www.columbia.edu/acis/history/teletype.jpg Photo of a Model 33 ASR]
• [http://keyboards.jargon-file.org/#ASR33 Keyboard layout for Windows that simulates the ASR33 keyboard]
• [http://www.pdp8.net/asr33/asr33.shtml ASR 33 Teletype Information] with movies and sound

{{Commons category|ASR-33}}

{{authority control}}

Category:Computer terminals

Category:Telegraphy