Page "Universal asynchronous receiver/transmitter" Paragraph 18

Some early telegraph schemes used variable-length pulses ( as in Morse code ) and rotating clockwork mechanisms to transmit alphabetic characters.

The first UART-like devices ( with fixed-length pulses ) were rotating mechanical switches ( commutators ).

These sent 5-bit Baudot codes for mechanical teletypewriters, and replaced morse code.

Later, 6-bit codes became common to avoid the figures vs. character errors that could occur when Baudot's shift character was corrupted.

ASCII was adopted by the U. S. so that the military services would all use the same teletypewriter codes.

ASCII added upper and lower case, with enough punctuation to print most documents.

This required a seven bit code.

When U. S. telephone systems became digital in the 1960s, they used an 8-bit data sample size to digitize voice.

As a result, most data transmission and computers shifted to 8-bit character sizes to use the lowest cost data transmission system.

For example, IBM built computers in the early 1960s with 8-bit characters, and an " upper half " of Latin I characters became common in many data networks that had formerly used 7-bit ASCII.

The economic effect of the telephone system is large: It effectively forced character systems with more than 8-bits ( e. g. Unicode ) back into an 8-bit form ( e. g. UTF-8 ), and most commercially important computers for the last forty years have used internal word sizes that are multiples of 8 bits.