The programmable logic controller, or PLC, is a special purpose computer. It has no display, no keyboard, no printer, no hard drive, and hides in the control panel out on the factory floor, but it is still a computer.

PLC（可編程邏輯控制器）基礎知識

註譯版文稿如下：

Today we are going to talk about PLC（Programmable Logic Controller，可編程邏輯控制器） basics. The programmable logic controller, or PLC, is a special purpose computer. It has no display, no keyboard, no printer, no hard drive(硬盤驅動器)， and hides in the control panel out on the factory floor, but it is still a computer.

Initially, the PLC was a replacement for panels of relays(繼電器） ，devices that turn on and off. Making a machine run using relays as logic was time-consuming and challenging, and making a change in the logic or the operation of the machine was almost as complicated as starting over.

Relays also fail far more frequently than computer components, so that relay logic required more downtime to keep it running than newer PLC-based controls.

On top of all of that, relays use a lot of electricity, make lots of heat and soot(油灰）, and take up lots of space.

An installed PLC is deceptively(貌似) simple in appearance. You have a CPU module and input/output devices, referred to as I/O. Sometimes the I/O is part of the CPU, and sometimes it is in other modules.

The CPU communicates with the I/O, so in most systems they share a backplane(背板) that physically holds them in place and electronically connects them.

In other equipment, the I/O modules can be miles away from the CPU and connected with data cables(數據傳輸電纜), so that the PLC is not limited to a single cabinet or even the same building. Since the PLC is a computer, there is no need to limit it to only digital inputs and outputs. Over the years, the manufacturers added analog(模擬的) and other numerical inputs and outputs.

To make these numerical devices useful, they included calculation capability in the programming as well, so that we can now calculate, for instance, statistical process control (SPC，過程統計分析) values and put PID(Proportion Integration Differentiation,比例-積分-微分控制器) controllers directly into the PLC program.

Since this is called a programmable logic controller, we need to program it. Most PLCs are programmed using an application running on a standard desktop or laptop PC.

They communicate with the PLC using Ethernet or a proprietary(專有的) communication bus, depending on the manufacturer. Most manufacturers seem to have settled on Ethernet and/or USB in recent years.

Unfortunately, the PLC manufacturers have failed to agree on how we program the control logic. Although most of the manufacturers claim some form of “Ladder Logic(梯形圖，是PLC使用得最多的圖形編程語言，被稱爲PLC的第一編程語言),” the specifics of that language are different for each manufacturer, including different capabilities, different ways of doing the same thing, and even differences in the order in which the CPU processes the various pieces of logic.

One attempt to bring some order and inter-operability to this chaos is the IEC 61131 standard(由IEC-國際電工委員會於1993年12月所制定，用於規範可編程邏輯控制器（PLC），DCS，IPC，CNC和SCADA的編程系統的標準) from the American National Standards Institute (ANSI,美國國家標準協會). The excerpt(摘要) from the ANSI web page gives a flavor of what is there.

The IEC 61131 standard defines several “programming languages” with different strengths and weaknesses. Local programming standards – your boss or the customer – will usually specify what language you use.

That wraps up this short discussion of the structure of PLCs – I hope you enjoyed this little introduction to the underlying architecture(基礎體系結構) of many industrial control systems. Keep watching as we continue our exploration of the basics of industrial controls.

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