A teleprinter sends typed text as a fixed-length code over a line and prints it at the far end, with no Morse skill needed by either operator. Baudot’s five-bit code gives only 32 symbols, which cannot hold 26 letters and 10 digits at once, so machines shift between a letters set and a figures set. Each character is framed by a start and a stop element, which lets two cheap clocks stay in step provided they agree to within about 7.7% in the ideal case.
Part of Telecommunications
Machines that send typed text as five-bit coded pulses over a telegraph or telephone line, with punched paper tape as the storage medium.
A Fixed-Length Code Replaces The Operator’s Ear
Morse code needs a trained operator at each end (Morse Code System). A teleprinter does the coding itself: a typed key becomes a fixed pattern of pulses, the line carries them, and a printer at the far end turns them back into letters. Code Structure discusses how code design sets speed and noise resistance; a fixed-length code is the easiest for a machine to read.
Five Bits Give 32 Codes, So The Machine Must Shift
A group of n two-state elements has 2ⁿ combinations. For five:
- 2⁵ = 32 codes
The corpus confirms it for 5-hole tape: “32 possible characters” (Punched Tape). But 26 letters plus 10 digits is already 36, which is more than 32 before a single punctuation mark or control. The usual answer, from general knowledge rather than a corpus figure, is two sets selected by shift codes: one code means “now letters”, another “now figures”, and the same pulse pattern prints a letter or a figure according to the last shift. The cost is that a lost shift code garbles everything after it until the next one.
For comparison, the 7-hole tape in the same Punched Tape article carries 6 data holes and a parity hole for 2⁶ = 64 characters, and 8-hole ASCII tape has 256 codes.
A Start And A Stop Element Frame Each Character
Teleprinters do not share a clock. Each character is preceded by a start element that wakes the receiver and followed by a stop element that returns the line to rest, as in Framing. Assume one start and one stop element per five data bits:
- Frame length: 1 + 5 + 1 = 7 elements
- Efficiency: 5 / 7 = 71% (the 8-bit case in Framing is 8 / 10 = 80%)
One start and one stop element is an assumption for the arithmetic; a real machine’s stop element may be longer.
Worked Example: How Far Can The Two Clocks Differ?
The receiver starts its clock at the start edge and samples the middle of each element. Element k of the five data elements is sampled at (k + 0.5) element-times after the start edge, and the stop element at 6.5.
- Last data element, k = 5: sampled at 5.5 element-times. A clock error of ε moves the sample by 5.5 × ε element-times. The sample stays inside the element while that is below 0.5: ε < 0.5 / 5.5 = 9.1%
- Stop element at 6.5: ε < 0.5 / 6.5 = 7.7%
That is the combined sender-plus-receiver error, ideal sampling and no noise. The tolerance is generous because the clock only has to hold for one character; the next start edge resets it. For 8N1 at 9,600 baud the same method gives 0.5 / 8.5 = 5.9% and 0.5 / 9.5 = 5.3% (Serial Communication). Real margins must be smaller to allow for noise and edge distortion.
Paper Tape Lets A Slow Typist Use A Fast Line
Punched Tape describes 5-hole tape 17.53 mm wide, with five data holes and a sprocket hole in each row. A hand punch makes 1 to 5 characters per second. Suppose the line rate is 50 baud and a character is 7 elements (an assumption for the arithmetic):
- Line speed: 50 / 7 = 7.1 characters per second
- A 1,000-character message by hand punch at 1 to 5 characters per second: 1,000 / 5 = 200 s to 1,000 / 1 = 1,000 s of typing
- The same message from tape at line speed: 1,000 / 7.1 = 140 s of line time
Preparing messages on tape and sending them at full line speed uses the line for 140 s instead of 200 to 1,000 s. The tape can also be fed into another line at a relay point.
What You Need
- A keyboard or hand punch that makes the five-hole pattern
- A reader: five phototransistors plus one for the sprocket row, as in the 8-phototransistor reader of Tape Reader
- A line: telegraph wire or a modem over a telephone pair (Modem Concepts)
- A printing mechanism. The corpus has no teleprinter printing mechanism design. A printed output may be the hardest part, and a rebuilt system could start by reading tape by eye or at a screen.
Common Mistakes
- Dropping a shift code. The following characters print in the wrong set.
- Assuming 5 bits are enough for text. They are enough for capital letters, digits and little else.
- Matching speed by ear. Measure both clocks; keep inside the tolerance above.
- Sending without a check. Teleprinter lines are noisy; add an acknowledgment convention (Acknowledgments).
Safety
Machinery and line voltage
Tape punches and printing mechanisms have moving parts that pinch and cut; keep fingers clear while they run. A telegraph sounder circuit runs on a 6 to 12 V battery (see Telegraph), and a long line is a lightning antenna (Lightning Protection).
What’s Next
- Learn next: Modem Concepts, sending these pulses over a voice line
- Combine with: Data Communication for framing and error checking