Teletype Model 33 | 1963

Teletype Model 33 

 The Teletype Model 33 is an electromechanical teleprinter designed for light office use. It was less durable and cheaper than previous models. Released in 1963 by Teletype Corporation, the Model 33 was originally designed for the U.S. Navy. It was produced in three versions: the Model 33 ASR (Automatic Send/Receive), which includes a paper tape reader and punch; the Model 33 KSR (Keyboard Send/Receive), which lacks a paper tape reader and punch; and the Model 33 RO (Receive Only), which has no keyboard or reader/punch.

The Model 33 was one of the first products to adopt the newly standardized ASCII character encoding system in 1963, differentiating it from earlier models like the Teletype Model 32, which used the 5-bit Baudot code. Affordable and highly compatible with ASCII, the Model 33 was widely used with early minicomputers and had a significant impact on the development of several informal standards in the 1960s and 1970s.

An advertisement from 1974 emphasized the extensive and long-term use of the Model 33. After its release in 1963, the Model 33 became one of the most popular terminals in the data communications industry, remaining so until the late 1970s. By 1975, over half a million Model 32 and 33 units had been produced, with the 500,000th unit being gilded and specially displayed. In the following 18 months, an additional 100,000 units were produced, and for the U.S. Bicentennial in 1976, the 600,000th unit was painted in red, white, and blue and toured the country for display.

The Model 33 was priced around $1,000 (about $10,000 today), much cheaper than other teleprinters or computer terminals from the mid-1960s, such as the Frieden Flexowriter or IBM 1050. By 1976, the new Model 33 RO printer was priced at about $600 (around $3,000 today).

Teletype Corporation realized the popularity of the Model 33 and began improving the components with issues from earlier models, gradually upgrading it from "light duty" to "standard duty." Advertisements later emphasized this upgrade. The Model 33, with its good durability and competitive pricing, had little competition until the arrival of Digital Equipment Corporation's (DEC) DECwriter series.

The Teletype Company referred to the version of the Model 33 with a paper tape reader and punch as "Model 33 ASR," but many computer users began referring to it as the "ASR-33." This naming convention was first seen in Digital Equipment Corporation (DEC) documents. For example, in a September 1963 brochure for the PDP-4, the Model 28 KSR was referred to as "KSR-28," and DEC extended this naming convention to other Teletype equipment. This naming convention became widely adopted by other computer manufacturers, and the Micro Instrumentation and Telemetry Systems (MITS) sold the Teletype Model 33 ASR as the "Teletype ASR-33."

The abbreviation "TTY" became a widely used informal shorthand for Teletype and was commonly used as a text input/output device in early computer systems. This term is still used today to refer to devices used in amateur radio or assistive technologies for the hearing impaired.

In the early 1970s, early video terminals capable of text output began to emerge, but these early models were extremely expensive, costing around $10,000 (about $100,000 today). However, with the advancement of integrated circuit and semiconductor memory technologies, the prices of video terminals dropped quickly, making them much cheaper than Teletype terminals.

Cheap video terminals, known as "dumb terminals," began to take over the Teletype terminal market. Basic video terminals like the 1975 ADM-3 could sequentially display and scroll text, and were referred to as "glass TTYs," a concept similar to Teletype. High-end video terminals like DEC's VT52 in 1975, the ADM-3A in 1976, and the VT100 in 1978 could process data much faster and support full-screen text editing without the need for paper output. By the mid to late 1970s, Teletype machines were quickly replaced in new installations by dot-matrix printers and video terminals.

With sales declining, Teletype ceased production of the Model 33 in 1981.

The design goal of the Model 33 was to create a machine suitable for small office spaces that would work well with other office equipment of the time and could operate for about two hours a day. Since it was designed for light-duty tasks, Teletype made adjustments using metal bars and levers rather than screws, which were used in previous teleprinter designs. Many of the Model 33's parts were not heat-treated or hardened. The basic structure was made from die-cast metal, but it used its own screws, and parts were assembled using fit-together parts instead of bolts.

The entire mechanical operation is driven by a single electric motor located at the back of the machine. The motor runs continuously when powered on, producing the familiar buzzing and slight vibration noise. The noise is significantly louder when the printing or paper tape mechanism operates. This noise became an iconic sound associated with newswires and computer terminals. Additionally, the machine features a mechanical bell activated by Control-G (BEL) code to grab attention.

The Model 33, including its stand, measures 34 inches (860mm) high, 22 inches (560mm) wide, and 18.5 inches (470mm) deep (excluding the paper holder). The entire machine, including the stand and paper, weighs 75 pounds (34kg). It operates on 115VAC 60Hz, requiring less than 4 amperes. The recommended operating environment ranges from 40 to 110°F (4 to 43°C) in temperature, 2% to 95% relative humidity, and altitudes from 0 to 10,000 feet (0 to 3,048 meters). The printing paper is a 8.44×4.5-inch (214×114mm) roll, and paper tape comes in 1-inch (25mm) wide rolls of 1,000 feet (300m). The nylon ink ribbon is 0.5 inches (13mm) wide and 60 yards (55m) long, with a plastic spool and eyelet for automatic ribbon feed direction switching.

The Model 33 ASR requires periodic lubrication on approximately 500 points.

For cost-saving reasons, the optional paper tape mechanism relied on the keyboard and page printer. The interface between the paper tape reader and the rest of the terminal was entirely mechanical, with power, clock, and 8 data bits (referred to as "intelligence" in Teletype) transmitted in parallel via metal levers. User-configurable options like parity were set through mechanical clips that pressed or released various levers. The paper tape reader detected holes mechanically using metal pins to check for the presence or absence of holes. The paper tape reader and punch could process 8-bit data and were efficiently used for downloading or uploading binary data from computers.

In previous Teletype designs like the Model 28 , users could punch tape while independently sending already-punched tapes or outputting other tasks. However, in the Model 33 ASR, independent use of the paper tape punch and reader was not possible.

The tape punch required lubricated paper tape to ensure smooth operation. Additionally, the tape punch had a transparent, detachable chip tray underneath that needed periodic emptying.

The printing mechanism was designed to operate at a typical speed of up to 10 characters per second (100 words per minute, or wpm), with slower speeds like 60 wpm, 66 wpm, 68.2 wpm, and 75 wpm also available. The machine offered various typing element options. A Teletype service bulletin listed 69 factory-installed typewheel options for the Model 33 (frequent changes were inefficient). The typewheel was cylindrical, with 16 characters arranged in four layers, allowing the printing of 64 characters. To print, the typewheel was rotated clockwise or counterclockwise, raised or lowered, and struck by a padded hammer to print the character on the ink ribbon and paper.

The Model 33 printed on 8.5-inch (220mm) wide paper, supplied in rolls of 100 feet (30m) long and 5-inch (130mm) diameter. The paper was fed by friction instead of a tractor feed. The Model 33 had a fixed 10 characters per inch typing width, supporting lines up to 74 characters long (commonly referred to as 72 characters).

The Model 33 ASR keyboard supported only uppercase ASCII characters, generating 7-bit ASCII code, specifically CCITT international telegraph alphabet number 5, with one parity bit (even parity) and two stop bits. The transmission speed was set to 110 baud (110 bits per second), but this code only supported uppercase letters and did not include lowercase letters or special characters like , {, |, }, ~.

The Model 33 could operate in both half-duplex and full-duplex modes. In half-duplex, signals typed on the keyboard are sent to the printing mechanism and output immediately (local echo). In full-duplex, signals typed on the keyboard are sent only to the transmission line, and the receiver must send them back to the Model 33 for output (remote echo). The default setting is half-duplex, but users could switch to full-duplex.

The Model 33 generally included a response mechanism for dial-up networks (such as teletype exchanges). At the start of a message, the transmitting machine could send a WRU ("Who Are You?") signal, which would trigger the receiving machine to respond.

Teletype printed the Model 33 with its standard name and classification.