5-BIT BAUDOT TELETYPES

Introduction to Baudot Teletypes and Documentation Archive

The 5-bit Teletypes restored here were introduced in 1925 (Model 14), 1930 (Model 15) and 1940 (Model 19), and are all very closely related, sharing similar mechanisms. They do not use the modern 8-bit ASCII code, but instead they use the 5-bit Baudot code that preceded it. As such, they make very poor machines to connect to computers. However they are incredibly robust text transmission systems. These units are military grade, and were workhorses in the Army and Navy through the second world war and the Vietnam war.  They found extended civilian use in the Telex system, and were used in newsrooms until the mid 1980's. 

I have uploaded a large set of documentation that we have used throughout the restoration:

• Teletype Model 14, Model 15 and Model 19 Documentation.zip

The essential individual documents in this set are linked below in context.

Episode 1-3: Main Documents

Here are the main reference documents to the Model 15/19 Teletypes:

Navy Guide to Teletypes

This manual is the best introduction to 5-bit Teletypes I have found. It gives a great introduction to the basic principles and describes various machines and use cases.

Model 15 Printer

This is the main manual for the printer (receiver) section of the Teletype (both for the Model 15 and Model 19)

Model 19 Table

The devilish wiring of the Model 19 table got us stumped for at first. The above collection of schematics helped us disentangle it.

Model 15 Keyboard and Punch

This is the manual for the keyboard and punch section.

Model 19 Base

This manual has the complex wiring for the base that holds the keyboard, printer, motor, polar relay and external interconnect block.

Episode 2: 60 mA Loop Interfaces, TTYLoopDriver and BaudotRSS

John Nagel TTYLoopDriver

A 60 mA Baudot USB adapter for working with the BaudotRSS software below. Derives its own high voltage supply from the USB power, which is very neat. It is good hardware, but the description and documentation of the project is poor. It says it does not need drivers, but it sure does, and they are hard to find. Carl managed to build one.

John Nagel BaudotRSS

Software to use with the above hardware interface. Can be used to print rss news feeds. Similar to the above, nicely done, but difficult to get going. We eventually managed to make it work when we discovered it only works with Python v2.7. It requires a whole bunch of extra packages. COM ports numbers are off by one (use 0 to talk to COM1!). It eventually worked.

For those that want a ready to go solution, there is an excellent and very inexpensive interface board designed by Eric Volpe :

• Volpe Board overview

• Volpe Board by Jordan Cuningham

My own interface, based in part on Eric Volpe's excellent design, is discussed further below in the documentation for the Linux episode.

Chart of Baudot Codes

Baudot codes use only 5 data bits. However, the 5 data bits are preceded by a start bit, always a "space", which is represented by an open line, and followed by a stop bit, always a "mark", which is represented by a 60 mA current (Note: this is for neutral mode line wiring. In polar mode, space is -30mA and mark is +30mA). The stop bit is longer than all other bits, in theory 1.42 bit period but in practice 1.5. This is done to leave enough time for the mechanical receiver to sync on the next start bit. Since it does not hurt to make the stop bit slightly longer (except for the slightly lower effective data rate), it is often made two 2 bits long on electronic computers. On your computer serial port, that's the "2 stop bits" setting.  So in total, counting the start and stop framing bits, a single Baudot character is sent using 7.5 bits (1 start, 5 data, 1.5 stops). 

The Model 19 uses a transmission rate of 45.5 bauds, so each bit is 1/45.5 seconds, or 22 ms per bit. Many other Baudot rates have been used, notably 50 Bauds in Europe, and faster 56, 75 and 100 Bauds. These speeds are sometimes quoted in the confusing "Word Per Minute" metric, which oddly assumes that a word is 6 characters long. I guess they were not writing in German! So a 60 WPM machine is 45.5 Bauds, 66 WPM is 50 Bauds, 75 WPM is 56 Bauds, 100 WPM is 75 Bauds, and 133 WPM is 100 Bauds. For example, the later Teletype Model 28 can work at 45.5, 50, 56 or 75 Bauds. The change in rate is accomplished by changing a set of gears in the machine.

When there is no other character to send, the stop bit prolongs indefinitely. Which means that an idle Baudot line is the "mark" condition, which is a closed 60 mA line. This is done so that you can immediately spot if a line is working even in idle (or conversely, if a line is broken), a practice that dates back from the first Morse code telegraph lines. This is still with us today, where an idle serial line is in the logic high state, say +5V, and not at logic low as you'd expect. You can blame it on the Morse code telegraph.

Episode 4: Power Supply

Inside the door schematics

This complete schematic was glued inside the power supply door. I took a picture of the damaged original and cleaned it up so we could reprint it.

Episode 5-6: Model 15 Keyboard and Punch

Annotated punch schematics and actual wiring diagrams

The original schematics and the actual wiring diagrams are very hard to comprehend. These annotated versions developed during our reverse engineering should help greatly.

Episode 7: Blue Boxes

Episode 9: Model 15 Printer Section

Model 15 Printer Adjustments

Model 15 Physical Wiring Diagram

Annotated wiring diagrams

The wiring diagrams for the printer base are very hard to comprehend, particularly the optional neutral/polar relay wiring and the wiring posts. These annotated versions developed during our reverse engineering should help. Note that our Model 15 base was wired  slightly differently from our Model 19 base.

Episode 11: Visiting Mr RTTY

Episode 12: Model 14 T/D (Transmitter/Distributor)

Model 14 T/D Main Manual

Model 14 T/D Adjustments

Model 14 T/D Wiring Diagram

Episode 13: Model 19 Table

Episode 14: Adding the Motor Stop Feature

Episode 15: Model 14 Reperforator

Model 14 Reperf Manual 1

Model 14 Reperf Manual 2

Model 14 Reperf Adjustments

Model 14 Reperf Wiring

Teletype 15 and 19 Demo

TTY as Linux Terminal

Finished TTY interface box

60 mA loop analog board

View of the box internal wiring

Arduino Mega board on the left, analog converter board on the right.

Getting a TTY Linux login prompt on a serial port

You have to jump through a few hoops to get a login prompt on the serial port, and it's even worse if you want to do that through USB (see below).

 My old machine had a real serial port COM1 serial appeared was /dev/ttyS0 on linux, as reported by

dmseg | grep tty

 But acces to it is denied by default. For getting access, one method is to change the permissons temporarily, but it gets quickly annoying as you have to redo it after each login:

sudo chmod 666 /dev/ttyS0

A more permanent fix is to create then add yourself to the dialout group :

sudo newgrp dialout

sudo gpasswd -add ${USER} dialout

Then you want to set the port for 9600 bauds and Xon/Xoff using the following command:

sudo setty -F /dev/ttyS0 9600 ixon

Only then will agetty be able to do its thing and prompt a login on the TTY attached to the serial port. The tty33 terminfo is the oldest TTY description I could find, but was not very effective in modern Linux from preventing the pervasive use of color codes and escape sequences.

sudo /sbin/agetty -L 9600 /dev/ttyS0 tty33

Getting a TTY Linux login prompt on a USB port

If you don't have a computer with a hardware serial interface, it's even crazier to get that to work when connected via the USB interface. The Arduino USB port should simply be accessible through /dev/ttyACM0. But even adding yourself to the dialout group won't work to access this one (it's part of the tty group apparently). And worse, the 'agetty' command does not work as documented, being silently prevented on USB by some overactive security watchdog (see https://bugzilla.redhat.com/show_bug.cgi?id=1514235). I had to resort to launching a service through a script, which feels like and unnecessary and complicated workaround such a simple thing. 

My ttystart shell script:

#!/bin/bash

sudo chmod 666  /dev/ttyACM0

sudo -F stty /dev/ttyACM0 9600 ixon

sudo systemctl enable serial-getty@ttyACM0.service

sudo systemctl start serial-getty@ttyACM0.service

To stop the console, a similar script can be used.

My ttystop shell script:

#!/bin/bash

sudo systemctl stop serial-getty@ttyACM0.service

I am still not sure how to specify a terminfo option through these commands.

CuriousMarc's Corrected Teletype Jingle Bells

----------

F stands for FIGS

X stands dor LTRS

Z stands for Space

Cr is carriage return

Lf is linefeed

Don’t type the actual spaces, they are just for visual aid

----------

Cr Lf

FSFSFSXMFS FSFSXEFSFS FSFFSSXRRY

ZFFSFSFSFF SSFSFSFSSS

FSFSFSFSXC FSXHFSFSFS XRFSFSFSXI 

FSFSFSFFSS XSTMASFSFS 

FSFFSSFSFS FSSSFSFSFS FSXZANFSSS 

FSFSFSFSXD ZAFSSSFSFS

FSFSXZHAFS SSFSFSFSFS FSFSFSSSFS

FSFSFSXP FFFSSS

 FSFSFSFSFF FFFSSSFSFS

FSFSFFFFFS SSFSFSFSFS FSFSFSSSFS

FSFSFSFFFS XPFSFSFSXY

FSFSFSXZFS FSFSFFSSXN EWZYFSFSFS

FFSSFSFSFS SSFSFSFSFS

XEFSXAFSFS FSXRFSFSFS XZFSFSFSFF

SSXTOZALFS FSFSFFSSFS

FSFSSSFS FSFSFSXL

Cr Lf