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Teletypewriters, peripherals and related equipment
This part of the Crypto Museum website deals with automatic
(analogue or digital)
telegraphy by means of typewriter-style devices, using a binary code
such as the common 5-bit Baudot code,
or the 7/8-bit ASCII standard.
Such systems are generally known as
teletype machines (after the Teletype brand),
teleprinters,
telex (short for Teleprinter Exchange),
teletypewriters
or by the abbreviation TTY.
On these pages we will use telex or teleprinter to identify such systems.
Teleprinters are operated over land lines (TTY)
or via radio (RTTY), mainly using the standard speed of 45.45 baud (USA)
or 50 baud (Europe). The use over radio links is also known as
Telex Over Radio (TOR).
Punched paper tape was commonly used with telex
equipment for storing and (re)transmitting messages.
Although telex has been superceeded by modern computers, some computer
terminal sessions are still called TTY, for example on UNIX-like
operating systems.
Although teleprinters are not cryptographic devices,
many of them were used in combination with external
cipher machines and some even had built-in cryptographic capabilities. For that reason, several machines and peripherals are described
on this website. For a more complete overview of telex machines,
please check out the links at the bottom of this page.
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Teleprinters on this website
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Telex manufacturers on this website
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For many years, Telex was the de-facto standard for communication
with the Armies world-wide.
It was introduced long before WWII (in 1849) and lasted until the 1990s.
It was also used by press agencies, governments, large corporations and
by the police. Telex can be employed reliably over (fixed) land lines
as well as over radio (HF). In the past, there was an international
Telex network, consisting of decicated land lines and special exchanges,
but towards the 2000s most of them were gradually phased out.
Telex is still used today by amateur radio operators (HAMs).
Although most telex systems use the 5-bit digital
ITA2 code, generally
known the Baudot code,
there are systems that use a less-common standards,
such as the multi-tone COQUELET code,
often used in France,
and the 14-bit ETK standard
that was introduced by Gretag
in the 1950s.
Such systems were considered more fault-tolerant but never met
wide-spread acceptance.
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Telex, short for Teleprinter Exchange,
is a world-wide communications system
of the past. It was once one of the most important methods of communication
between companies, embassies, governmental bodies, public services,
etc., but lost its popularity in the 1980s, when they were gradually
replaced by fax machines (which have meanwhile been superseded by modern
computers).
Nevertheless, they are marvels of electromechanical and electronic engineering,
and many collectors and museums make an effort to preserve them for the future
and keep them running whenever possible. This can be done by wire (TTY) or via
radio (RTTY). There are several initiatives that allow (private) collectors
and museums to take part in a modern world-wide network of teleprinters,
such as TelePhone
and i-Telex project listed below.
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TelexPhone is an old project that allows vintage teleprinters to
connect to a worldwide network of (bobbyist) teleprinters, via a
regular analogue dial-up telephone line (POTS).
With the demise of the old analogue networks, the project has
meanwhile been superceeded by the i-Telex project (below).
Our TelexPhone page is kept for reference only.
➤ More information
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As analogue telephone lines are gradually disappearing, and telecom
providers are discouraging the use of such lines, the above TelexPhone idea
has been ported to the internet.
By using a simple interface, it is now
possible to run teleprinters on a world-wide virtual telex network
through the internet [6].
➤ More information
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Telex machines were developed and produced world-wide by a variety
of manufacturers. Most of these machines were compatible in one way
or another. The initial machines worked at the rather low baud-rate
of 45.45 baud or 50 baud, but later machines were capable of running
at 75, 100 and even speeds up to 150 baud as well. The following manufacturers produced
telex equipment:
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Alternative devices, which resemble a teleprinter but do not follow
the Baudot/ITA-2 standard, were developed by:
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- Teletypewriter
- Teleprinter
- Teletype
- TTY (TeleType)
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- RTTY (Radio TeleType)
- Telex (Teleprinter Exchange)
- Fernschreiber (German)
- Verreschrijver (Dutch)
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There are two methods for transporting the data.
The first methods transports the digital data bits are currents (either
on/off or +/-) through two or wire wires. The second method uses
Frequency-Shift Keying (FSK) to transfer them as tones.
The methods are named as follows:
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TW39 Single/double-current line, 2-wire or 4-wireED1000 FSK (virtual double-current)
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When used on dial-up telex lines, the teleprinter setup must be capable of
dialling a subscriber number. It must also be able to answer an incoming call.
This is known as signalling.
Generally speaking, there are two methods for
dialling a subscriber number:
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- By means of a rotary dial
- Via the teleprinter keyboard
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Both dialling methods are supported by i-Telex. In addition, different
expansion cards are available for the TW39 and ED1000 standards.
Which card you need, depends on the type of teleprinter you want to
connect.
Most older electromechanical teleprinters, such as the
Siemens T-37 and the immensely popular T-100,
follow the TW39 standard, whilst later electronic machines, such as the
Siemens T-1000 and the TeKaDe FS-200,
usually follow the ED1000 standard.
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Teletypewriters can be interconnected in a variety of ways, depending on
the local and remote situation, the distance and the transport medium
(wire-line or radio). Below are several popular configurations. For a good
understanding it is important to use unambiguous terminology. In the following
text, it is assumed that 5-bit digital telegraphy is used, according to the
ITA2 (CCITT-2) standard. Each data word consists of
7½ timing units: 1 start bit, 5 data bits and 1½ stop bit.
In TTY terminology, the logic states '1' and '0' are known as 'MARK' and 'SPACE'
respectively. In a single-current configuration, a MARK is represented by a
negative current (-40 mA) whilst SPACE is a currentless state.
In a double-current configuration, a SPACE is represented by a +40 mA current.
Historically, the five data bits are numbered channel 1 to 5, with channel 1
representing the least-significant bit (LSB).
In computer terminology however, these bits are commonly known as
b0 to b4 respectively.
As an example, the diagram below shows the timing of the letter 'Y',
which has the bit pattern (10101).
In rest, the machine constantly sends a MARK signal (logic '1').
When sending a character, the machine always begins with a start-pulse (ST),
which is always a logic '0' (SPACE). Next it sends the 5 databits (b0-b4),
followed by a stop-pulse (Logic '1') which has a length of 1½ unit.
The receiver is then ready to accept the next character.
➤ More
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2-wire single-current
half-duplex
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In the simplest configuration, the TX and RX circuits of both machines are
connected in series. This situation, which is known as 2-wire single-current,
is shown in the diagram below. As all circuits are connected in series, only
a single current source is needed. Anything typed on the local machine, will
appear at both sides. This configuration is also known as half-duplex,
as each party has to wait until the other side has finised its transmission,
before sending a message.
This configuration is very popular for creating a small demonstration network.
It can be implemented with just two electromechanical teleprinters, such as
the Siemens T-100, and a simple connection device like the
Siemens Anschlussgerät 13. As this is an ON/OFF configuration
(current/no-current), it is only suitable for short distances.
The diagram below shows how a teleprinter is connected to a nearby exchange,
via a TCD and a 2-wire single-current line.
In practice, this configuration was not very popular for permanent
(leased line) connections, as the distance between the two endpoints would
probably be too large. It was suitable however for connection to a nearby
exchange, which could be up to several kilometres away. This configuration
is also suitable for use in combination with a modern internet interface,
such as i-Telex:
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2-wire double-current
half-duplex
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The diagram below shows a 2-wire double-current configuration, in which
the MARK and SPACE signals are represented by -60V and +60V respectively.
In this configuration, the line is never currentless; it is either +40 mA or
-40 mA, as a result of which it can be used for long distance telegraphy.
This is by far the most common configuration for long distance telegraphy
over hundreds of kilometres. This diagram below shows a 2-wire double-current
line to an exchange.
In this mode, the TCD takes care of the conversion from the teleprinter's
4-wire connection to the 2-wire line and vice versa. The 2-wire double-current
mode was also popular as part of more complex routed telegraphy connections,
that ran over several long and short distance telegraphy paths, as illustrated
below. In this example the signal first runs over a long distance
2-wire path, is then converted to a 4-wire line, and is then multiplexed
over another long distance path.
The 2-wire double-current configuration is also extremely useful for
use on long distance leased lines, as illustrated below.
It allows two teleprinters, each with a suitable
teleprinter connection device (TCD),
to be linked directly over long distances (> 100km)
with just a single wire pair.
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4-wire single-current
full-duplex
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When the transmit and receive circuits are wired separately, four wires are
needed. In that case, full-duplex communication is possible. The diagram
below shows a 4-wire single-current configuration, in which each side has
its own current source that drives the telegraph relay at the other end.
Although technically viable, this configuration was hardly ever used in
practice.
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4-wire double-current
duplex
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The diagram below shows a 4-wire double-current configuration, in which
the MARK and SPACE signals are represented by -60V and +60V respectively.
This is by far the most common configuration for long distance telegraphy.
It is compatible with most popular teletypewriters and cipher machines, such
as the Siemens T-52 Geheimschreiber.
Send and receive circuits are fully isolated.
The arrows show the direction of the 40 mA current when both sides are at rest
(i.e. MARK).
In this configuration, the line is never currentless; it is either +40 mA or
-40 mA. When switching from MARK to SPACE, the current is simply reversed.
At the receiving end, this forces the polar relay
to its alternative position.
As a result, this configuration is suitable for long distance lines.
The diagram above shows how this configuration could be used for local setups
and demonstrations, in which no signalling is required.
In practice, 4-wire double-current was sometimes used for the connection
to a local or remote concentrator, after the data was multiplexed with
other lines on a long distance route to the exchange. This situation
is illustrated in the diagram below.
In the same vein it is possible to connect the teleprinter directly to a
remote teleprinter via a fixed or leased line. Like in the configuration above,
the four wires may run directly to the other end, but will in most cases
be multiplexed with other leased lines. This situation is shown here:
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Teleprinter connection device
TCD
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In order to connect two teleprinters together, or to connect a teleprinter
to a telex subscriber line or a leased line, a Teleprinter Connection Device (TCD)
or a Line Terminating Device (LTD) is required.
It forms the interface between the line and the teleprinter.
Some TCDs provide signalling, which is needed for dial-up lines.
Others are 'dumb' devices that simply connect to a leased line.
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An important feature of a TCD is the use of hybrid circuits.
A hybrid circuit, also known as a fork circuit, can combine and split
two individual signals without one 'seeing' the other. This is done
to avoid delayed echos on long haul telecommunications lines. It fully
separates the two-way traffic, in the same way as the hybrid circuit in a
telephone set is used to cancel voice echo.
Contrary to a telephone however, in which the hybrid is usually
implemented as a special transformer, the hybrid of a TCD is
less obvious. It is often implemented with one or more
polar relays with multiple coils each.
Several sub-circuits in the TCD contribute to the currents through
these coils, as a result of which they are not easily recognised
in the circuit diagram.
➤ More about hybrid circuits
➤ More about polar relays
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In the German language a TCD is – confusingly – known under various names,
including:
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FAG Fernschreiber-AnschlussgerätFAG Fernschreiber-AnschaltgerätFSG FernschaltgerätFES Fernschreib-EndsatzAG AnschlussgerätAG AnschaltgerätEG Endgerät
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Teleprinter connection devices on this website
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When bringing old teleprinter equipment back to life, it may be useful
to know how existing telephone connectors were wired at the time for use
in combination with telex equipment. Note that the wiring of a telex machine
can be very different from a standard telephone set and that many different
configurations are possible. Also note that a current-loop system is used.
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One some older teleprinters, in particular in the Netherlands, a large
6-pin circular connector was often used. These connectors were also used on
early cipher machines like the
Philips Ecolex II
and Ecolex IV.
In later years, the 6-pin connectors and sockets were often replaced
by the circular Walzenstecker (ADoS)
or the more popular ADo 8 (see below).
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- TX relay A (common)
- TX relay Z (mark)
- 120Ω RX relay (with 6)
- TX relay T (space)
- Alarm contact (with 4)
- 120Ω RX relay (with 3)
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ADo4, ADoS, Walzenstecker
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The eldest connector used for teleprinters in Europe is the so-called
Walzenstecker (wheel plug); a circular plug with four contacts
that was originally used for the connection of telephone equipment,
wired as (1) a, (2) b, (3) bell and (4) ground, but redefined to accomodate
the TX and RX contacts of a teleprinter. The pin-out for use in combination
with telex equipment is given below.
The official designator for this plug is Anschlußdosenstöpsel ZB 27,
or ADoS ZB 27. It is sometimes erroneously called ADo4 – even in official
documentation – indicating that it has 4 contacts.
The mating socket is known as ADo ZB 50. It has an extra
contact (a1) which is shorted to (a) when no plug is inserted.
Below is the pinout for telephone and teleprinter equipment.
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| | Telephone | | Teletype (telex) |
| | Name | Colour | Description | | Name | Colour | Description | |
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| 1 | a | white | Line a (La) | 1 | a1 | white | TX a |
| 2 | b | brown | Line b (Lb) | 2 | b1 | brown | TX b |
| 3 | W2 | green | Extra bell | 3 | a2 | green | RX a |
| 4 | c | yellow | Ground (hold) | 4 | b2 | yellow | RX b |
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The Walzenstecker shown above was succeeded by the more
versatile 8-pin connector known as ADo 8. This connector was used for
a variety of equipment and had two 'keyed' guide pins at the centre,
to ensure that the plug is entered into the socket with the right side
up, and to avoid the wrong plug being inserted into the wrong equipment.
The sockets are 'programmable'.
Below is the pinout of the socket – with the correct teleprinter keying –
when looking into the socket.
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a1 White TX ab1 Brown TX ba2 Yellow RX ab2 Green RX bs Red Shorting bridge to 6 1s Blue Shorting bridge to 5 1n.c. Pink unusedn.c. Grey unused
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The orientation of two small discs inside the socket can be altered in
order to configure it for various applications.
The image above shows the correct configuration for use with
a teleprinter. Pins 1-4 are used for the same signals as on the earlier
Walzenstecker. The bridge between pins 5 and 6 is optional 1 and is
used on newer equipment to signal that the teleprinter is connected.
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This is a bridge inside the socket (not the plug).
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When used with ED1000-standard teleprinters (FSK) the definitions
of the pins of the ADo8 socket are different. The line (a, b) is
connected to pins 1 and 4, and is used for transmission and reception
simultaneously (full duplex). The shorting bridge between pins 5 and 6
of the socket, is needed by several teleprinters for sensing that it
is connected. Without this shorting bridge, the teleprinter may not work.
Below is the pinout when looking into the socket.
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a White TX/RX an.c. Brown -n.c. Yellow -b Green TX/RX bs Red Shorting bridge to 6 1s Blue Shorting bridge to 5 1n.c. Pink Option: paper-end detectionn.c. Grey Option: paper-end detection
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Shome teleprinters close a contact when it senses the end of the
paper supply. When used, these contacts are available on pins 7 and 8,
so that an external signal (e.g. a bell) can be sounded.
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This is a bridge inside the socket (not the plug).
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Adapter between ADo 8 and Walzenstecker
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The table below shows the wiring for an adapter from ADo-8 to Walzenstecker
or vice versa.
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| Signal | ADo 8 1 | Walze 2 | Colour | Description |
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| a1 | 1 | 1 (a) | white | TX a |
| b1 | 2 | 2 (b) | brown | TX b (no bridge to 3) |
| a2 | 3 | 3 (W2) | yellow | RX a (no bridge to 2) |
| b2 | 4 | 4 (c) | green | RX b |
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Bridge between 5 and 6 in ADo-8 socket only.
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Walzenstecker. Original name when used for telephone shown in brackets.
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Some teleprinters, such as the V.10 version of the
TeKaDe FS-200/FS-220 have a V.10 interface that uses
unbalanced ±6V signalling. Below is the pinout when looking into the
DB25 plug. The pinout is based on the
RS-530 standard, which can be balanced or unbalanded. When used with
balanced signals, the wire it is paired with, is shown in red in square
brackets, like this: [18].
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GND Chassis [18]TXD Transmitted data (out) [14]RXD Received data (in) [16]RTS Request To Send (out) [19]CTS Clear To Send (in) [13]DSR Data Set Ready (in) [22]GND Signal ground [21]DCD Data Carrier Detect (in) [10]/RDS Return RX element timing (in) [17]/DCD Return DCD (in) [8]/TDT Return TX element timing DTE (out) [24]/TDS Return TX element timing DCE (in) [15]/CTS Return CTS (in) [5]/TXD Return TXD (out) [2]TDS TX element timing (in) [12]/RXD Return RXD (in) [3]RDS RX element timing (in) [9]LL Local Loopback (out) [1]/RTS Return RTS [4]DTR Data Terminal Ready (out) [23]RL Remote Loopback (out) [7]/DSR Return DSR (in) [6]/DTR Return DTR (out) [20]TDT TX element timing (out) [11]- Test Mode (in)
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In the case of teleprinters, V.10 is often used in unbalanced configuration,
in which case the return lines (prefixed with '/') should be connected to
ground (pin 7). In practice, only a subset of the wiring above will be
implemented.
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Telex was first developed in Germany between 1926 and 1933 [10]
and was introduced in many countries in the early 1930s.
The diagram below shows the timeline of the telex network in some European
countries, the United States and Canada. In most countries, the Telex service
started in the 1930 and lasted until 2006-2008. In Switzerland, the UK and
Germany, the telex service was taken over by Swiss Telex AG, which closed
its network in 2020.
Telex is still used in some smaller countries, and in countries with an
infrastructure that doesn't support the higher data rates of the internet.
After 2020, the telecom providers kept offering internet based solutions for
connection to telex subscribers is such countries, through services like
iTelegram and EasyLink's Real Time Messenger. As of March 2019, iTelegram
maintains its services in ~180 countries.
Telex is still used in maritime and military environments.
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RTTY
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Radio Teletype
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TOR
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Telex Over Radio
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TTY
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Teletype
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Baud
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Bits per second
Common expression for specifying the transmission speed of a digital
telegraphic data signal, derived from the
Baudot encoding standard,
also known as bits-per-second (bps).
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The following people have contributed to the contents of this page:
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- Heinz Blumberg
- Paul Reuvers
- Marc Simons
- Henning Treumann
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- Dr.-Ing. Fritz Schiweck, Fernschreibtechnik
Lehrbücher der Feinwerktechnik, Band 9.
CFW Leipzig (Germany), 1942.
- Günter Keye, Handbuch für Fernschreiber
Gesellschaft für Sport und Technik. DDR, 1967.
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© Crypto Museum. Created: Saturday 07 April 2012. Last changed: Saturday, 11 January 2025 - 11:28 CET.
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