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What are teletypes and why were they used with computers?



A woman who used a telephone type in the late 1960s.
Systems Engineering Laboratories

For several decades, many computer system administrators used devices called teletypes to communicate with computers using a typewriter-style keyboard and output printed on spools of paper. This is why.

What is a teletype?

A teletype (or more accurately, a teleprinter) is a communication device that allows operators to send and receive text-based messages using a typewriter-style keyboard and print on paper.

The term “teletype”

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

Extract from a 1929 Teletype ad
An excerpt from a 1929 Teletype ad. Teletype Corporation

To understand the basic principle behind teleprinters, imagine two electric typewriters connected by wires (or a wireless radio link). Whatever you type on one typewriter will automatically print on the other. Now imagine these two typewriters could be at any distance thanks to wired networks or radio transmissions, and you will understand what a revolution in communication they represented in the early 20th century.

Primitive teleprinters emerged as far back as the 1840s and offered an advantage over Morse code operations using a telegraph key, as the output of a teleprinter could be read immediately by humans without the need for special training. In the early 1900s, teleprinters became more reliable and easier to use, with a familiar QWERTY keyboard and the ability to record messages on paper tape for repeated transmission. A single teletype operator familiar with typewriter operation could replace two trained telegraph operators, and news could be immediately sent around the world to receiving teletypes that did not require a keyboard.

Why did people use teletypes with computers?

To imagine why a teletype would be useful with a computer, remember those two remotely linked typewriters from the last example and replace one with an interactive computer system. Rather than communicating with a remote teleprinter, you send and receive human readable text to and from a computer. The computer can be in the same room, in a different part of a building, or even halfway around the world if it is connected through a telephone network.

Many early large computer systems (especially those sold by IBM) worked in batches, which meant that one program would be typed on punch cards, the punch cards would be fed into the machine with other programs (in a batch), and then the results would be on pile of punched cards. The output stack would then be fed into a tabulation 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

In addition to batch computing in the mid-1950s, engineers began experimenting with interactive computing, where a computer operator could provide input and get the results back in near real time in a kind of interactive ‘conversation’ with the machine. Many of these computers, such as the Bendix G-15 (1956) and the IBM 610 (1954), used modified electrical typewriters as input or output devices, but not necessarily commercial teleprinters.

The invention of time-sharing in 1959 allowed multiple users to simultaneously share an interactive computer system, making low-cost, single personal terminals such as teletypes desirable for computing. As time-sharing became more common in the 1960s, organizations with mainframe computers began to purchase standard commercial teletype machines that they could use as terminals more often.

Enter the Teletype Model 33

One of the main reasons the term “teletype” became so strongly associated with computers was the Teletype Corporation Model 33 (also referred to as the “ASR 33”), which was first introduced in 1963. Unlike most others teleprinters at the time, the Model 33 was able to 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 store and transmit letters and numbers, allowing many different brands of computers to easily communicate with each other.

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

Popular minicomputers of the late 1960s and early 1970s, such as the PDP-8, PDP-11, and the Data General Nova, supported ASCII encoding, making the Model 33 an ideal (relatively) low-cost input / output (I / O). ) terminal for them. DEC’s PDP series in particular were influential machines, and if you look up historical photos of them, you’ll almost always see a Teletype Model 33 in use next to them.

If you were using a teletype with a mainframe computer like this one, you would see your own local input on paper as you type, and then you would get a response from the computer printed beneath it as the teletype being printed to a continuous supply of paper stored in the device.

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

RELATED: What is a TTY on Linux? (and how to use the tty command)

The Age of Teletype Games

A teletype print 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 spawned some classic text-only games that went on to impact the video and computer game industry. Well-known examples include Zork, Lunar lander, Hunt the Wumpus, Star Trek, and The Oregon Trail. All of these games were originally played as text-only games with messages input and output printed on teletype paper.

Why did people stop using teletypes with computers?

While teletypes were popular for a time, they had some significant drawbacks as computer terminals. They were very noisy due to the mechanical action of the impact printhead hitting the paper quickly. 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 screens instead of paper for output. These early “glass teletypes” sought to provide faster interaction rates and save money on paper waste. Still, many computer administrators in the 1970s often stuck with teletypes because of their lower costs.

While at least three manufacturers were producing video terminals in 1970, each cost significantly more than a Teletype Model 33. In 1974, Hewlett-Packard sold a new version of the groundbreaking Datapoint 3300 video terminal, the HP2600A, for $ 4,250. Around the same time, a Teletype Model 33 cost about $ 755 to $ 1,220, depending on the options installed, which is a significant savings. But video terminal prices fell dramatically in the 1970s, dropping to about $ 800 per unit in 1980, depending on capabilities. (Around that time, the revered DEC VT-100 terminal typically sold for around $ 1,550).

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

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

In the mid-1970s, personal computers such as 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 television (with an RF modulator) as a display device, eliminating any type of external terminal – teletype or otherwise.

So the next time you sit in front of your PC with a high-speed, high-resolution bitmap screen that’s completely quiet and pushing for power, be thankful you don’t have to read How-To Geek through a printed input machine – shoot away at 10 characters per second. But then again, it might be fun.

RELATED: How to write an Apple II BASIC program in your web browser




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