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Short notes on the ideas and machines that grew up around the five-unit code.

Start-stop transmission

Line signal for the letter Y: start element, five code elements and a stop element — The letter Y on a start-stop line. The line rests in the Z condition; a start element (A) wakes the receiver, the five code elements follow, and a stop element returns the line to rest.

The receiver only measures time from the start element to the end of the character, so a slightly fast or slow clock is corrected at every new character.

Paper tape

Five-track tape let operators prepare a message offline and send it at full line speed. It could be copied, stored and fed back into a transmitter, an early form of store-and-forward messaging.

Radioteletype

On radio the two conditions are sent as two nearby frequencies. Amateur radio operators still use ITA2 this way, most often at 45.45 baud with a 170 Hz shift.

From ITA2 to ASCII

Seven-bit codes removed the need for shift signals and added lower-case letters, but kept carriage return and line feed, which still end lines of text today.