Bringing A Teletype Into The 21st Century

Before modern CRTs with ancient VGA connectors, and before fancy video terminals, the display for computers large and slightly smaller was the Teletype. While many of these Teletypes were connected directly, they were designed to be a remote terminal, connected through Ma Bell’s network. [NeXT] over on the Vintage Computer Forums is bringing the iconic ASR33 Teletype into the 21st century by giving this old display a modern way to connect to the outside world.

If you ever see a Teletype in action, it will be connected to a local machine. This was certainly not always the case. The Teletype was designed to connect to remote systems. [NeXT]’s Teletype came with a Call Control Unit designed for Telex lines, which do not exist anymore. Modems for the ASR33 existed, but good luck finding one. Lucky for [NeXT], nearly every modem ever made is backwards-compatible with the Bell Dataphone, one of the standard ways of plugging a Teletype into a phone line. All [NeXT] had to do was put a modem inside this Teletype.

With relays, transistors, LEDs, and a lot of perfboard, [NeXT] successfully built a circuit that would interface the Teletype’s Call Control Unit to a Hayes Smartmodem tucked away inside the stand. Believe it or not, this is an exceptionally useful build; if you ever find a Teletype tucked away in the back of an old office, in a surplus shop, or on Craigslist, odds are it won’t be compatible with any modern electronics. That’s not to say land lines are particularly modern, but since there’s a microcontroller included in the new circuitry, it’s relatively easy to add a USB port to this ancient terminal.

7 thoughts on “Bringing A Teletype Into The 21st Century

  1. The ASR-33 is arguably the coolest TeleType.

    Here is more information about connecting RS232 serial to the ASR-33:
    http://retrowagon.org/wiki/index.php/20ma_current_loop_to_RS-232_conversion

    How about a schematic? That might be useful too:
    https://ia600401.us.archive.org/8/items/bitsavers_teletype3375_5720393/Model_33_Schematic_Jun75.pdf

    Some articles describe that the I/O interface is different between teletypes, based on serial number (when it was made). That complicates it some.

    1. This gets repeated a lot, but none of the actual adjustments I’ve had to make on a number of Teletype Model 33s required bending. I did have to unbend bars in the keyboard matrix on a unit that was dropped on its face, but it’s not like the actual Teletype manual is full of “grab it with pliers and bend until working.” I think the misconception comes from things like the the type drum height adjustment, where you loosen a locking screw, insert a screwdriver into a slot, and lever the two pieces of the linkage apart or together until it’s where you want.

      There are plenty of adjustments in the Model 33 that specify torques and spring scale loads, and some things like the type drum height or dashpot orifice that are more an “adjust by feel,” but I don’t remember ever coming across a “bend it until it works” adjustment.

      1. I actually have to counter that. With this machine when I got it a period (.) would generate a form feed because somewhere a single bit was being flipped when it should of been. One of the codebars was ever so slightly bent out of alignment and had to be bent back before the machine would differentiate between the two different keypresses.

  2. These teletype machines are still quite usable on the amateur radio (Ham) bands where RTTY (FSK with 5-bit Baudot character encoding) is still used today with an audio Terminal Unit (TU) which converts the audio FSK to a serial 20 mA current loop that feeds a teletype device like the ASR33. Wired-line subscriber support for Telex (e.g., TWX) is now essentially deprecated. A traditional RTTY setup included a TU with an X-Y oscilloscope display showing a Lissajous pattern of two crossed ellipsoids to help the radio operator tune-in the received FSK teletype signal. Today, while RTTY is still used on the Ham bands, usually all the audio modulation/demodulation of the audio FSK signals are done via a computer sound card in a dedicated application. And then there’s the subject of punched paper tape to send pre-formed messages. Messages to be sent often were punched to paper tape then looped so they can be sent repeatedly when initiated by the operator. Think of the looped paper tape as a form of write-once, read-many memory.

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