Communications History / 通讯的变迁
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...
到目前为止，与外部设备通信的最流行的方法是使用“RS-232”通信方法。许多PLC编程人员认为，与外部设备通信，即使不是“完全不可能”，也很难理解。这根本不是真的！它不是“黑色艺术”、也不是“巫术”或“怪异科学”。继续阅读吧……
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通信系统都起源于我们可能在老电影中看到的旧电报机（还记得那个在火车站工作，带着护腕和塑料遮阳板的家伙吗?）。早在18世纪晚期，人们就开始尝试远距离电子通信。1810年，一个德国人（Von Soemmering）正在使用一种连接着26根电线（1根代表字母表中的每个字母）的附着在水族缸底部的装置。当电流通过导线时，电解作用产生小气泡。通过选择适当的电线来通电，他能够“通过气泡”发送加密信息（这是真的……真的）。这引起了军方的注意，于是开始了寻找系统方法的竞赛。
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，两个英国人，库克和惠斯通，在英国铁路上使用了13英里长的电报实验。他们的设备有5根电线来驱动小电磁铁，从而使小质量的指针发生偏转。通过将电流同时施加到两根不同的导线组合上，指针就会发生偏转，从而指向排列在矩阵中的字母。这个“2 / 5”的编码只允许20个组合，所以字母“z,v,u,q,j和c”被省略了。这封电报是当时的一大进步，但它的编码不是二元的（On/Off），而是三元的（指针向左、向右或完全不动）。
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!
这些设备最大的问题是它们是并行的(需要多根电线)。库克和惠斯顿最终制造了一个双线装置，但第一个实用的全串行二进制系统通常归功于S.F.B.莫尔斯。在摩尔斯电码中，字符用点和横杠（二进制的1和0）来表示。莫尔斯的第一个系统和我们今天在电影里看到的不一样（它在华盛顿特区的史密森尼博物馆展出，如果你想看的话）。它实际上有一根指针与一个纸转筒相接触，纸转筒上有一个连续的记号。通过给电磁铁通电，指针就会从纸上“弹”开，形成一个空间。电报员很快就注意到，他们不必看着这张纸来读纸上的代码，他们可以通过指针在纸上划动时发出的声音来破译代码。所以这个装置被一个发声装置取代，它能发出咔嗒声，而不是纸被刮擦的声音。电传打字机是后来才出现的，而今天的串行通信设备与电传打字机的关系更为密切。剩下的就是历史了……绝了，但不管怎样，这就是历史!
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.
顺便说一句，术语MARK和SPACE（我们稍后会看到它们）起源于莫尔斯的原始设备。当指针与纸接触时，我们称之为MARK，当指针弹起时，我们称之为SPACE。他的设备只显示大写字母，这并不是什么大问题。此外，泰坦尼克号的沉没“标准化”了“SOS”的代码，这代码意味着“Save Our Ship”（拯救我们的船）。
