What Are Teletypes, and Why Were They Used with Computers?


Systems Engineering Laboratories

For a few decades, many computer system operators used devices called teletypes to interact with computers using a typewriter-style keyboard and output printed on spools of paper. Here’s why.

What’s a Teletype?

A teletype (or more precisely, a teleprinter) is a communications device that allows operators to send and receive text-based messages using a typewriter-style keyboard and printed paper output.

The term “teletype” originated as a trademarked term for a brand of teleprinters created by the Teletype Corporation in 1928. Teletype Corporation’s products became so ubiquitous that “teletype” evolved into a generic term synonymous with “teleprinter,” especially in the field of computers.

Excerpt from a 1929 Teletype advertisement
An excerpt from a 1929 Teletype advertisement. Teletype Corporation

To understand the basic principle behind teleprinters, imagine two electric typewriters linked together by wires (or a wireless radio link). Whatever you type on one typewriter gets automatically printed out on the other. Now imagine that these two typewriters can be any distance away thanks to wired networks or radio transmissions, and you’ll understand what a revolution in communications they represented in the early 20th century.

Primitive teleprinters first emerged as early as the 1840sand provided an advantage over Morse code operations with a telegraph key, because a teleprinter’s output was instantly human-readable without the need for special training. In the early 1900s, teleprinters became more reliable and easier to use, adding a familiar QWERTY keyboard and the ability to record messages on paper tape for repeated re-transmission. A single teletype operator familiar with operating a typewriter could replace two trained telegraph operators, and news could be dispatched instantly across the world to receiving teletype units that didn’t need to have keyboards.

Why Did People Use Teletypes with Computers?

To imagine why a teletype would be useful with a computer, recall those two remotely linked typewriters from the last example and replace one of them with an interactive computer system. Instead of communicating with a remote teleprinter, you’re sending and receiving human-readable text to and from a computer. The computer could be in the same room, in another part of a building, or even halfway across the world when linked by a telephone network.

Many early large computer systems (especially those sold by IBM) were batch operated, which meant that a program would be typed onto punched cards, the punched cards would be fed into the machine with other programs (in a batch), and then the results would be written onto another stack of punched cards. The output stack would then be fed into a tabulating machine or a printer that would print the results in human-readable form.

An IBM 610 Computer
The IBM 610 (1954) was an early interactive computer that used a modified typewriter for printed output. IBM

Alongside batch computing in the mid-1950s, engineers began to experiment with interactive computing, where a computer operator could provide input and get results back in almost real-time in a sort of interactive “conversation” with the machine. Many of these computers, such as the Bendix G-15 (1956) and the IBM 610 (1954) used modified electric typewriters as either input or output devices, but not necessarily commercial teleprinters.

The invention of time-sharing in 1959 allowed multiple users to share an interactive computer system at the same time, making low-cost, single-personal terminals like teletypes desirable for computer use. As time-sharing became more common in the 1960s, organizations with mainframe computers began to buy off-the-shelf commercial teletype machines to use as terminals more frequently.

Enter the Teletype Model 33

One of the biggest reasons that the term “teletype” became so strongly associated with computing was the Teletype Corporation Model 33 (sometimes called the “ASR 33”), which was first introduced in 1963. Unlike most other teleprinters at the time, the Model 33 could understand the ASCII standard, which the American National Standards Institute had recently developed as a standard code for electronic devices and computers. ASCII provided a common framework for how computers stored and transmitted letters and numbers, allowing many different brands of computers to easily communicate with each other.

A Teletype Model 33 illustration.
A Teletype Model 33-ASR. Teletype Corporation

Popular minicomputers of the late 1960s and early ’70s, such as the PDP-8, PDP-11, and the Data General Nova, supported ASCII encoding, making the Model 33 an ideal low-cost (relatively speaking) input/output (I/O) terminal for them. In particular, the PDP series by DEC were influential machines, and if you look up historical photos of them, you’ll almost always see a Teletype Model 33 in use beside them.

When you used a teletype with a mainframe computer like these, you’d see your own local input on paper as you typed, and then you’d receive a response from the computer printed below it as the teletype printed to a continuous feed of rolled paper stored within the unit.

In 1970, Dennis Ritchie and Ken Thompson developed the UNIX operating system on a PDP-11 using Model 33 teletypes as interfaces, and some of the teletype-related design choices that they made are still with us today. The terms “TTY” on Linux, the Terminal app on Macs, and even, to some extent, the command prompt in Windows 10, all share a lineage with the line-by-line text output that originated on computers with teletype outputs.

RELATED: What is a TTY on Linux? (and How to Use the tty Command)

The Era of Teletype Games

A teletype printout of Hunt the Wumpus.
A teletype printout of Hunt the Wumpus (1972) with handwritten notes. Creative Computing

It’s worth noting that the teletype era produced a number of classic text-only games that went on to influence the video and computer game industries. Notable examples include Zork, Lunar Lander, Hunt the Wumpus, Star Trek, and The Oregon Trail. All these were originally played as text-only games with typed-in messages and output printed on teletype paper.

Why Did People Stop Using Teletypes with Computers?

While popular for a time, Teletypes did have some significant drawbacks as computer terminals. They were very noisy due to the mechanical action of the impact printhead rapidly hitting the paper. They were also slow, often limited to about 10 characters per second. And finally, you had to use a lot of paper.

In the 1960s, companies such as IBM began experimenting with computer terminals that used CRT displays instead of paper for output. These early “glass teletypes” sought to provide faster interaction speeds and save money on paper waste. Still, many computer operators often stuck with teletypes throughout the 1970s due to their lower cost.

While at least three manufacturers produced video terminals by 1970, each cost significantly more than a Teletype Model 33. In 1974, Hewlett-Packard sold a rebranded version of the pioneering Datapoint 3300 video terminal called the HP2600A for $4,250. Around that same time, a Teletype Model 33 cost about $755 to $1,220 depending on which options were installed, representing significant savings. But the price of video terminals dropped dramatically in the 1970s, going down to about $800 per unit by 1980 depending on capability. (Around that time, the well-respected DEC VT-100 terminal typically sold for about $1,550).

The DEC VT-100 Terminal
Video terminals like the DEC VT-100 (1978) made teletypes obsolete as computer I/O devices. DEC

Once video terminals dropped in price and exceeded the capabilities of teletypes, teletypes quickly fell out of favor. Compared to teletypes, video terminals were silent and had no moving parts other than the keyboard, making them more reliable and pleasant to use. Their display speed also wasn’t limited to the mechanical action of a printhead, so they could display more information much faster than a teletype could.

Also, in the mid-1970s, personal computers like the Apple II began to integrate input and output functionality directly into the computer itself. In the case of the Apple II, owners could use a composite video security monitor or a standard TV set (with an RF modulator) as a display device, making any kind of external terminal—teletype or otherwise—unnecessary.

So the next time that you sit down at your PC with a high-speed, high-resolution, bitmapped display that’s completely silent and sips power, be grateful that you don’t have to read How-To Geek through a printed feed machine-gunning away at 10 characters per second. But then again, it might actually be fun.

RELATED: How to Write an Apple II BASIC Program in Your Web Browser





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