Technologies from the Past: The Teletype — Understanding the Mechanism and Resurrecting an Old Machine

A teletype (TTY) is a start-stop receive-transmit telegraph apparatus with a keyboard similar to a typewriter. Unlike standard telegraph systems that use dots and dashes, teletypes transmit text messages via keyboard input and print received messages on paper. These devices served as terminal equipment in early computing and left lasting artifacts in IT history, including computer terminals, Linux tty terminals, terminal commands, and the UART interface.

Previous articles covered teletype technology broadly but omitted the technical details. In this article, I examine the T-63 teletype apparatus during its restoration, detailing its mechanical operation from the inside.

Baudot Code Fundamentals

The Baudot code is a synchronous five-bit binary code for telegraph transmission. The original Baudot code was standardized as ITA1; the modified Baudot-Murray code became ITA2. Soviet systems used MTK-2 (International Telegraph Code No. 2), an asynchronous or "start-stop" variant.

MTK-2 contains 86 symbols (85 distinct): 26 Latin letters, 31 Cyrillic letters (excluding Yo and the hard sign), 10 digits, 11 punctuation and mathematical marks, and 8 control symbols including space. Symbols with printable representations are divided into three registers — Latin, Cyrillic, and digits/marks — while non-printing symbols are register-independent. Register switching uses dedicated control symbols.

The MTK-2 phonetic correspondence between Latin and Cyrillic alphabets was reflected in Soviet teletype keyboards, which combined the JCUKEN and standard Russian layouts. This later influenced computer encodings like KOI-7 and KOI-8 and their Cyrillic letter ordering.

Transmission Mechanics

Baudot code transmits sequentially and asynchronously. Each symbol begins with a start bit, followed by five information bits and a stop bit. No modulation is used — the presence of current represents a logical 1, and its absence represents a logical 0. This closely resembles modern UART operation, except UART typically uses 8 bits per symbol and variable stop bits.

Two teletypes connect in a manner similar to telephones: the transmitter's normally closed contacts replace the microphone, while the receiver electromagnet replaces the speaker. A current of approximately 40 mA flows continuously during standby; transmitted symbols modulate this current. Both devices receive identical text — if both transmit simultaneously, garbled output results, requiring an operator restart.

Communication sessions typically began with "Who is there?" commands. Some teletypes could automatically respond with pre-programmed identification sequences. International three-letter Q-codes were widely used for standardized communications.

Receiver and transmitter synchronize via start bits but require synchronous operation. Since this is a fully electromechanical system, identical motor speeds ensure synchronization. The Telex network, similar to telephone infrastructure, carried telegraph signals over telephone lines and continues operating in many locations today.

The T-63 Apparatus

The apparatus I restored is a T-63 teletype manufactured by RFT in Karl-Marx-Stadt, East Germany, in 1983. Long basement storage had caused severe contact oxidation, though the abundant lubricant and accumulated grime had actually prevented rust damage to the mechanical components.

Initial Challenges

The motor's centrifugal speed regulator had lost contact between its brushes and rings, producing sparking and smoke on startup. I bypassed this regulator and controlled speed by varying the supply voltage instead. The receiver required approximately 40 mA of current, which I supplied via a laboratory power supply at about 12V.

The transmitter was completely inoperable due to severe oxidation of its contacts. Attempts at cleaning with fine sandpaper proved ineffective, requiring full disassembly and detailed restoration.

Transmitter Design

The transmitter employs six contact pairs actuated by cam mechanisms. Five pairs form the Baudot code; the sixth generates start and stop impulses. All contacts operate in parallel. In standby, all contacts remain open except the sixth, which closes the circuit.

The camshaft remains locked until a key is pressed. Pressing any key triggers a release mechanism allowing one complete shaft rotation, after which it re-locks. During one rotation, the sixth contact opens first (start bit), then cams sequentially close contacts 1 through 5 (forming the five-bit symbol), and finally the sixth contact closes again (stop bit).

Coil-driven mechanical linkages transfer cam motion to contacts through rocker arms. Five movable bars with stops beneath the rocker arms create 32 possible combinations. When no stop blocks a rocker arm and the cam engages, the arm "falls" through, closing the contact (logical 1). Stops prevent this fall, leaving contacts open (logical 0). After each cycle, all bars return to their initial positions.

A "combinatorial roller" — a barrel with metal spikes — at the top of the transmitter encodes the automatic response to "Who is there?" queries, storing approximately 20 symbols. Spikes are removed by breaking them off: the presence of a spike creates a logical 0, its absence a logical 1. Reading this message reveals the apparatus's original organization.

Contact Restoration

The contact pairs exhibited severe oxidation between the contact element and the tinned contact strips. After thoroughly cleaning all surfaces, the contact pairs functioned perfectly. Gap regulation required adjusting screws on each pair. During transmission of code 11111 (the Latin shift command), gaps were set for minimal overlap between contacts during transitions.

The entire transmitter mechanism was cleaned in solvent and re-lubricated with machine oil. An included maintenance manual specified all lubrication points. The keyboard assembly extracted easily with four screws, was cleaned and re-lubricated, and bent keys were straightened. The keyboard features mode switching (RUS/LAT/DIG) that blocks unavailable keys in each mode.

Motor Restoration

Motor brush oxidation had prevented contact with the slip rings. After cleaning, the motor operated successfully, though it produced noise resembling a washing machine on a spin cycle.

The centrifugal regulator functions simply: internal weights control normally closed contacts. When the motor exceeds nominal speed, the contacts open, slowing the motor; when it falls below nominal speed, they close again, restoring power. This pulsing action regulates speed. A large capacitor and wire-wound resistors form a snubber network that reduces interference from contact oscillation.

Speed adjustment uses regulator wheel screws and stroboscopic marks (which require a tuning fork for calibration). Operating at 50 baud, each symbol takes 20 ms per bit — 120 ms total including start and stop bits. Without a stroboscope, I used an oscilloscope to set symbol duration to approximately 120 ms.

Receiver Operation

The receiver employs a single electromagnet with five armatures, sequentially released by cam mechanisms. When current flows through the coil (logical 1), the corresponding armature remains attracted; without current (logical 0), the armature releases and blocks a mechanism.

These blocking actions control the deflection of five lamellae — slotted bars — via rocker arms, resulting in 32 possible positions after symbol reception. The lamellae guide print selection, choosing specific characters from the type basket.

Besides character selection, the printer executes carriage return and line feed commands. A pneumatic brake prevents crash returns, adjustable via spring tension. The "line feed" command advances the paper one line. The apparatus accepts standard (non-thermal) facsimile paper on rolls. A bell sounds 10 symbols before the rightmost carriage position and upon receiving the bell command.

Register commands shift the carriage vertically among three positions: top (Latin letters), bottom (Cyrillic), and middle (digits and symbols).

After cleaning and reassembly, the apparatus functioned correctly, responding to keyboard input with appropriate character selection and printing.

Perforator and Ribbon

A perforator simultaneously encodes printed text onto paper tape (18 mm width). Pre-prepared perforated tape allowed asynchronous message composition for transmission at maximum speed — essential for time-efficient network use and for distributing identical messages to multiple recipients.

Standard typewriter ribbons served as printing media and could be re-inked as needed.

Computer Interface Integration

Without Telex network access, the apparatus functions as little more than a limited electric typewriter. Connecting it to a computer offers far greater functionality. Since the Baudot code operates similarly to UART, an RS-232 connection seemed logical.

However, teletypes operate via current modulation, not voltage, at 40+ mA — far exceeding the capabilities of a standard COM port. Direct connection proved impossible. A custom interface was developed through five iterative designs.

The first four versions experimented with transistor amplification and various circuit topologies but suffered from voltage level and signal quality problems. The fifth and final version used optical relay current amplification with a single +5V supply. A virtual ground via a resistor divider solved the voltage-level issues. Transmission via the TxD line controls a relay that modulates the teletype's current loop. Reception uses a current-sensing resistor (calibrated for 40 plus or minus 15 mA) driving a transistor stage. Protection diodes guard against voltage spikes, and resistors in the motor circuit accommodate 230V mains power.

Software

Standard terminal software like PuTTY recognizes ASCII but not MTK-2, so a custom terminal application was developed supporting two modes:

• File mode: Translates text files to MTK-2, prints them directly on the teletype, then exits.
• Terminal mode: Interactive bidirectional communication. Characters typed on the computer are translated to MTK-2 and printed by the teletype; teletype keyboard input is displayed on screen.

The application is available on GitHub and enables text communication across distances via telephone networks or radio transceivers — mimicking pre-internet-era communication methods.

Conclusion

The restored T-63 demonstrates remarkable mechanical ingenuity. Everything can be disassembled and reassembled using only threaded connections, enabling complete servicing without specialized tools. The entirely electromechanical design — using no electronics whatsoever for code generation or reception — showcases elegant engineering from an era before digital systems. This restoration project reveals how profoundly early computing depended on telegraph technology, with UART and modern terminal conventions directly inheriting teletype protocols.