I have read that the early terminals were Teletypes, for example:


So you send your input to the (large) computer, and then you receive the result which gets printed on paper I suppose.

But could you actually see what you were typing? Does a Teletype have a small screen or something that allows you to see what you are typing?

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    That's exactly like the teletype my school had before they threw it out in favour of an RM 380Z. My memory might be wrong, but I think the TTY in my school had an acoustic coupler where the blanking plate on the right of the one in the photograph is.
    – JeremyP
    Commented Apr 19, 2017 at 12:09
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    @JeremyP - your memory is correct! That's what went on the right side. 110 baud! Woohoo! I remember when Harvey Mudd upgraded to ASR-43s! God were those fast! 300 baud!
    – davidbak
    Commented Apr 19, 2017 at 18:03
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    From memory, Microsoft BASIC for CP/M is particularly hard to use because it doesn't assume a virtual teletype like most software from just a year or so later. Pressing backspace adds a backslash and then a fresh copy of the letter removed. Then the next letter removed, etc, followed by an additional backslash once the list of characters you've deleted has ended. So e.g. a correction might appear as PROM\MO\INT A.
    – Tommy
    Commented Apr 19, 2017 at 18:28
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    @cbmeeks Those "holes in the front" are not holes at all, they are the letters T E L E T Y P E.
    – user4511
    Commented Apr 20, 2017 at 2:08
  • That's actually a fairly late model -- the ASR-33. But to my knowledge most models worked similarly. While it was possible to disconnect the keyboard from the printer, they were normally connected together, either directly or by being "echoed" through the attached computer or whatever.
    – Hot Licks
    Commented Apr 20, 2017 at 19:29

6 Answers 6


You could see what you were typing because it appeared on the paper in front of you. A teletype is just an electric typewriter at heart.

There was no screen, the picture is just as the system was.

The teletype was the console input to the computer. Once logged in, you had the standard command prompt. You entered your command which was sent to the mainframe character by character as you typed it. Then the response was presented on the paper. This was replaced by the console, or shell, display on a video monitor when such things came along later.

As shown in your picture, there was a punched tape reader / writer on the left hand side, so you could also "type" onto punched tape, or read the contents of tape into the computer or onto the roll of paper.

I spent my formative years programming on these. They were very slow to use because the keys had to be depressed a long way before contact was made and the keystroke detected. This made it impossible to touch-type at any sort of respectable speed. They were also incredibly noisy. The sound of the clattering carriages in a room full of a couple of dozen of these could be deafening.

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    "They were very slow to use" - this is the main reason behind the oldest UNIX programs being so cryptically named - ls, rm, cp, etc. Only having to type two characters rather than four on one of these things is a significant saving!
    – Muzer
    Commented Apr 19, 2017 at 9:06
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    @Muzer: Indeed so, and also why on games like Adventure, you entered single character commands.
    – Chenmunka
    Commented Apr 19, 2017 at 9:23
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    However, it's probably important to remember that since the output was printed, backspace simply moved the location of the printhead and didn't (and couldn't) actually erase the character.
    – Edward
    Commented Apr 19, 2017 at 11:43
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    That's also why you have back space and delete instead of just a single key that erases the last character typed. Back space moved the type head back one and also the paper tape. Delete sends a delete character to the computer (if connected) and punches a line of holes in the paper tape which would obliterate any existing character there. This is why ASCII DEL is 0x7f
    – JeremyP
    Commented Apr 19, 2017 at 12:03
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    @TripeHound This is an ASR-33 or something very similar - produced by the Teletype Corporation. The type head was a cylinder. The golf ball/typeball was an IBM invention and available from IBM only.
    – davidbak
    Commented Apr 19, 2017 at 17:55

Many Typetype-style devices (I'm not positive about the popular Teletype® brand ASR-33 itself) could be switched to use "full-duplex", "local", or "half-duplex" mode. In full-duplex mode, characters which were typed would be transmitted to a remote machine but would not be processed locally. In local mode, they would be processed locally but not transmitted. In "half-duplex" mode (so named because it was suitable for use on "half-duplex" lines that could either transmit or receive, but could not do both simultaneously), characters that were typed would be both transmitted and processed locally.

In cases where such usage would be practical, the optimal behavior is often to have characters that are typed be transmitted without being processed locally, but then have the remote system echo back characters as they are processed. This can provide the user with clean feedback as to when and how characters are being processed, but requires that characters be processed as soon as possible with minimal buffering delays. Single-user computers seem to favor such an approach since a computer that is doing nothing but awaiting input will be able to respond to it quickly. Today, Unix systems seem to use a hybrid approach where the terminal itself is not expected to echo characters locally, but the characters get echoed by the system automatically before the application layer requests them.

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    The knob on the front is for selecting "Line" (to the left), "Local" (to the right), and "Off" (straight up). Local was used for punching the tape, which you could then feed back to the reader in Line mode so that your "connection time" was reduced. Many remote locations paid for CPU time and Line time (kind of like minutes on the cell phone now), and if your access was over a toll line, spending time typing in a program, line by line, got very expensive. Duplex settings were part of the communications unit. In Line mode nothing was printed until it was received, or "echoed".
    – user4511
    Commented Apr 20, 2017 at 2:58
  • @GypsySpellweaver: When I used an ASR-33 hooked up to an Altair 680 it didn't seem like there was a 200ms lag between keystrokes and echo. Would the Altair have to have been hardware-echoing characters as they were typed (rather than receiving and retransmitting) or did the ASR-33 have a way of doing that automatically, or perhaps whatever interfaced the ASR-33 to the Altair did the echo?
    – supercat
    Commented Apr 20, 2017 at 14:31
  • Several options depending on the setup. If the communications unit was installed and set to half duplex it could do the echo. If the ARS/KRS was connected to an external modem/device, same thing. It could also be directly connected to twx which would be a fast echo. If the it has a full duplex connection and the receiving end had a comma interface unit, typical setup, then the echo is fast since it'S not delayed by the processor.
    – user4511
    Commented Apr 20, 2017 at 16:15
  • @GypsySpellweaver: I'm not familiar with "comma" interface; does that echo the state of the receive wire during transmission? I wouldn't expect a CPU to take very long to echo a character once it's received, but most kinds of circuitry wouldn't start processing a character until it was fully received--86ms after transmission had commenced.
    – supercat
    Commented Apr 20, 2017 at 16:20
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    @HotLicks: What I find interesting is that minicomputers developed a very different I/O philosophy from microcomputers. On a lot of microcomputers, even ones with typeahead buffers, characters that would end up getting consumed by a "read line with echo" system call would not echo until the call is performed, while minicomputers like to echo characters when typed unless echo gets turned off before they're typed. Advocates of the latter design seem incredulous at the idea that it would be possible to disable echo of characters that have already been typed.
    – supercat
    Commented Apr 20, 2017 at 20:19

I own a DEC pdp8/e and two ASR-33 Teletypes.

The ASR-33 can operate in "Line" or "Local" mode. In Local mode, characters are printed as they are typed. This mode is used for reading and punching paper tapes.

In Line mode, typed characters are sent down the line to whatever host you're communicating with. Typing makes a "Bzzt" noise for each character as the mechanical parallel->serial converter spins but nothing is printed. It is the responsibility of the Host to echo back the characters being typed.


Absolutely stone age stuff. We had these in 1970 when I worked at Hawker Siddeley Woodford. We had amazingly fast leased telephone lines that would handle 128 bps (not kilobits or megabits) connecting to the mainframe at Hatfield. The main point of the paper tape reader was that you could have your bulk input punched up in advance offline in the "punch room" and then read it in far faster than you could type it yourself on the teletype (normally without printing it at the same time). We shared the terminal among the programmers in the office and I got half an hour three times a week, so every second counted.

P.S. Thanks Lana for the punching, 50 years ago but not forgotten !


You could see what you were typing, as others have said, because the print head (which was cylindrical) rose up and rotated until the right character was in place, then slammed forward to smack the (simultaneously raised) tape and make a mark on the paper, and then dropped back down. The raise-spin-slam-unspin-and-lower sequence was rather noisy -- and not just the "slam" part.

On a distantly-related note, the data-transmission mechanism for directly-connected model 33s was the "20 mA current loop" -- a loop of wire that went from A to B and back. Machine A would send pulses into this loop by raising the voltage on one end until a current of 20mA flowed, hold it there for a short time, and then release. That was a "1" bit, I believe. A similar stretch of time with no current was a "0" bit.

The cool thing about the 20mA design was that it really didn't matter how long the wire was, or how resistive (up to a point!): if 20mA is flowing HERE, then 20mA must be flowing THERE, and be enough to activate a solenoid or whatever at the remote location. For long wires and higher speeds, things look less and less like switched-DC, and more and more like high-frequency analog signals, and things like inductance and capacitance make the whole plan less workable. But it sure was a cute first draft!


Yes, you could... The usual way of sending messages was to punch a paper tape first, and then run it through the tape reader, but you could also type directly, and even communicate as a 'chat', in real time and with the two texts clearly differentiated by the usual two-color tape, red and black...

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    Despite having user online teletypes extensively (ASR 33, KSR 33), on several different computer systems, I never saw a teletype with two-colour tape. Flexowriter yes, Teletype no. What model was this?
    – dave
    Commented Sep 19, 2020 at 11:48

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