By far, the most popular method of communicating with external devices is by using the "RS-232" communications method. Communication with external devices is viewed by many plc programmers to be difficult if not "all but impossible" to understand. This is far from true! It's not "black art", "witchcraft" or "weird science". Read on...
All plc communication systems have their roots in the old telegraph we may have seen in the old movies. (Remember the guy working at the train station with the arm band and plastic visor?) Early attempts to communicate electronically over long distances began as early as the late 1700's. In 1810 a German man (von Soemmering) was using a device with 26 wires (1 for each letter of the alphabet) attached to the bottom of an aquarium. When current passed through the wires, electrolytic action produced small bubbles. By choosing the appropriate wires to energize, he was able to send encoded messages "via bubbles". (It's true...really) This then caught the attention of the military and the race to find a system was on.
In 1839, 2 Englishmen, Cooke and Wheatstone, had a 13 mile telegraph in use by a British railroad. Their device had 5 wires powering small electromagnets which deflected low-mass needles. By applying current to different combinations of 2 wires at a time the needles were deflected so that they pointed to letters of the alphabet arranged in a matrix. This "2 of 5" code only allowed 20 combinations so the letters "z,v,u,q,j and c" were omitted. This telegraph was a big step for the time, but the code was not binary (on/off) but rather it was trinary (the needle moved left,right,or not at all).
The biggest problems with these devices was the fact that they were parallel (required multiple wires). Cooke and Wheatstone eventually made a two wire device but the first practical fully serial binary system generally gets credited to S.F.B. Morse. In Morse code, characters are symbolized by dots and dashes(binary- 1's and 0's). Morse's first system isn't like we see today in the movies. (It's on display at the Smithsonian in DC if you want to see it) It actually had a needle contacting a rotating drum of paper that made a continuous mark. By energizing an electromagnet the needle would "bounce" away from the paper creating a space. Very soon telegraph operators noticed that they didn't have to look at the paper to read the code but they could interpret the code by the sound the needle made when scratching the paper. So this device was replaced by a sounder that produced click sounds instead of paper etchings. Teleprinters came later, and todays serial communications devices are more closely related to them. The rest is history... extinct, but history anyway!
Incidentally, the terms MARK and SPACE (we'll see them later) originated from Morse's original device. When the needle contacted the paper we called this a MARK and when the needle bounced it was called a SPACE. His device only produced UPPERCASE letters which wasn't a big problem though. Further, the Titanic sinking "standardized" the code of "SOS" which means "Save Our Ship" or if you were ever in the US military you might know it better as "S*%$ On a Shingle" which was chipped beef on bread.
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