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TELETYPE SYSTEMS Standard Teletype systems evolved as an automated Telegraphy system called Telex . Originally, a rotating mechanical commutator (a rotating switch) was started by a "start bit". The commutator would distribute the other bits to set Relay s that would pull on Solenoid s which would cause the mechanism to print a figure on paper. The routing was automated with rotary electromechanical dialing systems like those used in early telephone systems. When Computer s became commonplace, these serial communication systems were adapted using I/O devices called Serial Port s that used UART s. The development of communications hardware had a deep continuing impact on the nature of software and operating systems, both of which usually arrange data as sequences of characters. SERIAL BUSES Integrated Circuit s are more expensive when they have more pins. To reduce the pins, many ICs use a Serial Bus to transfer data when speed is not important. Some examples of such low-cost serial buses include SPI , I&2C , and 1-Wire . SERIAL VERSUS PARALLEL The communications links across which computers—or parts of computers—talk to one another may be either serial or parallel. A parallel link transmits several streams of data (perhaps representing particular bits of a stream of bytes) along multiple channels (wires, printed circuit tracks, optical fibres, etc.); a serial link transmits a single stream of data. At first sight it would seem that a serial link must be inferior to a parallel one, because it can transmit less data on each clock tick. However, it is often the case that serial links can be clocked considerably faster than parallel links, and achieve a higher data rate. A number of factors allow serial to be clocked at a greater rate:
In many cases, serial is a better option because it is cheaper to implement. Many ICs have serial interfaces, as opposed to parallel ones, so that they have fewer pins and are therefore cheaper. EXAMPLES OF SERIAL COMMUNICATION ARCHITECTURES
(see additional examples in Computer Bus ) There are a number of serial communication standards which, while significantly at variance with the RS-232 standard, are casually and incorrectly called "RS-232". SEE ALSO
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