Related topics
Short notes on the ideas and machines that grew up around the five-unit code.
Start-stop transmission
The receiver only needs to measure time from the start element to the end of the character. A slightly fast or slow clock is corrected at every new character, which made simple mechanical teleprinters reliable over long lines.
Paper tape
Five-track tape let operators prepare a message offline and send it at full line speed. A row of small feed holes moves the tape; the larger holes carry the code. Tape could also be copied, stored, and fed back into a transmitter, an early form of store-and-forward messaging.
Telex
Telex connected teleprinters through switched exchanges, much like a telephone network for printed text. Each machine had an answerback code that it sent automatically on request, so a sender could confirm who was at the other end before transmitting.
Radioteletype (RTTY)
On radio, the two conditions are sent as two nearby frequencies, a method called frequency-shift keying. Amateur radio operators still use ITA2 this way, most often at 45.45 baud with a 170 Hz shift.
National variants
Some countries extended the code for their own alphabets. The Soviet MTK-2 code, for example, added a third case for Cyrillic letters on top of the international letter and figure cases.
From ITA2 to ASCII
Seven-bit codes removed the need for shift signals and added lower-case letters. ASCII kept the idea of control characters that Murray had introduced, including carriage return and line feed, which still end lines of text today.
Glossary
- Element (unit)
- One on/off signal of fixed duration; five make an ITA2 character.
- Condition A / Z
- The two states of an element: A is start polarity and no hole, Z is stop polarity and a hole.
- Shift
- A signal that changes how the following combinations are read: letters or figures.
- Answerback
- A teleprinter’s identity string, sent automatically in reply to the WRU signal.
- Baud
- Signal elements per second.