History of Serial Communications
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[edit] Early serial communications methods
[edit] Visual
[edit] Smoke signals
A smoke signal is a form of visual communication used over a long distance, developed both in the Americas and in China. By covering a fire with a blanket and quickly removing it, a puff of smoke can be generated. With some training, the sizes, shapes, and timing of these puffs can be controlled. Puffs may be observed from long distance, apparent to anyone within its visual range. With this in mind, signaling stations were often created to maximise the viewable distance. Stone bowls used by Native Americans and the towers of the Great Wall of China are examples of signaling stations.[1]
As the signals were visible to all, unless they had a secretly understood significance they would be conveying the information alike to friend and enemy. There were, however, certain more or less recognized abstract smoke signals, of which the following are a few. One puff meant attention. Two puffs meant all's well. Three puffs of smoke, or three fires in a row, signifies danger, trouble, or a call for help.[2]
[edit] Semaphores
[edit] Wire
[edit] Current loop
There are two types of current loops, analog and digital. An example of analog current loop is 4-20 mA ("four to twenty milliamp current loop") where 4 mA represents 0% and 20 mA represents 100%. In a digital current loop, an absence of current represents high, and the presence of current represents low.
A communications current loop, as used in teletype/Baudot codes was 0-20mA/0-50mA on/off. Cryptographic teletype current loop ( the red side ) was/is 0-80uA due to electromagnetic snooping capabilities circa 1966 timeframe( TEMPEST program ).
The 4-20mA current loop is not a communications current loop but a process control standard for chemical/industrial sensors and actuators. The 4-20mA is analog and linear or square-root(rate of flow). The minimum 4mA is to get the low end above the noise level possible in an industrial enviroment. 4-20mA standard was developed so that the sourcing current had enough power to drive electromechanical devices at the receiving end of the current loop.
The first digital communications serial links required the sending unit to provide all the (current)loop power. The receiving devices were all electro-mechanical with the output a mechanical motion.
[edit] Morse code
[edit] Teletype
[edit] Baudot code
[edit] ASCII code
Work on ASCII began in 1960. The first edition of the standard was published in 1963,[1] a major revision in 1967, and the most recent update in 1986. It currently defines codes for 128 characters: 33 are non-printing, mostly obsolete control characters that affect how text is processed, and 95 are printable characters. (stub)
The non-printing ASCII codes were needed to actuate the teletype's mechanical mechanisms:
- line-feed, carrage-return, upper-lower case, etc.
The ASCII codes were re-coded to Baudot codes used by the mechanical teletype.
The teletype was the only standard I/O device until about 1970. From 1970 on, the teletype was the most economic hobby I/O due to the military surplus of teletypes.
[edit] The development of RS-232
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[edit] The development of EIA-485 (RS-485)
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[edit] The development of IEEE 1394 (FireWire)
[edit] The development of MIDI
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[edit] The development of Universal Serial Bus (USB)
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[edit] The development of Serial ATA (SATA)
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[edit] Recent developments
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[edit] Frontiers
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