Communications History / 通訊的變遷
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...
到目前為止，與外部設備通信的最流行的方法是使用“RS-232”通信方法。許多PLC編程人員認為，與外部設備通信，即使不是“完全不可能”，也很難理解。這根本不是真的！它不是“黑色藝術”、也不是“巫術”或“怪異科學”。繼續閱讀吧……
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.
所有的PLC通信系統都起源於我們可能在老電影中看到的舊電報機（還記得那個在火車站工作，帶著護腕和塑料遮陽板的傢伙嗎?）。早在18世紀晚期，人們就開始嘗試遠距離電子通信。1810年，一個德國人（Von Soemmering）正在使用一種連接著26根電線（1根代表字母表中的每個字母）的附著在水族缸底部的裝置。當電流通過導線時，電解作用產生小氣泡。通過選擇適當的電線來通電，他能夠“通過氣泡”發送加密信息（這是真的……真的）。這引起了軍方的注意，於是開始了尋找系統方法的競賽。
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).
1839年，兩個英國人，庫克和惠斯通，在英國鐵路上使用了13英里長的電報實驗。他們的設備有5根電線來驅動小電磁鐵，從而使小質量的指針發生偏轉。通過將電流同時施加到兩根不同的導線組合上，指針就會發生偏轉，從而指向排列在矩陣中的字母。這個“2 / 5”的編碼只允許20個組合，所以字母“z,v,u,q,j和c”被省略了。這封電報是當時的一大進步，但它的編碼不是二元的（On/Off），而是三元的（指針向左、向右或完全不動）。
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!
這些設備最大的問題是它們是並行的(需要多根電線)。庫克和惠斯頓最終制造了一個雙線裝置，但第一個實用的全串行二進制系統通常歸功於S.F.B.莫爾斯。在摩爾斯電碼中，字符用點和橫槓（二進制的1和0）來表示。莫爾斯的第一個系統和我們今天在電影裡看到的不一樣（它在華盛頓特區的史密森尼博物館展出，如果你想看的話）。它實際上有一根指針與一個紙轉筒相接觸，紙轉筒上有一個連續的記號。通過給電磁鐵通電，指針就會從紙上“彈”開，形成一個空間。電報員很快就注意到，他們不必看著這張紙來讀紙上的代碼，他們可以通過指針在紙上划動時發出的聲音來破譯代碼。所以這個裝置被一個發聲裝置取代，它能發出咔嗒聲，而不是紙被刮擦的聲音。電傳打字機是後來才出現的，而今天的串行通信設備與電傳打字機的關係更為密切。剩下的就是歷史了……絕了，但不管怎樣，這就是歷史!
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.
順便說一句，術語MARK和SPACE（我們稍後會看到它們）起源於莫爾斯的原始設備。當指針與紙接觸時，我們稱之為MARK，當指針彈起時，我們稱之為SPACE。他的設備只顯示大寫字母，這並不是什麼大問題。此外，泰坦尼克號的沉沒“標準化”了“SOS”的代碼，這代碼意味著“Save Our Ship”（拯救我們的船）。
