Now that we understand how DC inputs work, let's take a close look at AC inputs. An AC voltage is non-polarized. Put simply, this means that there is no positive or negative to "worry about". However, AC voltage can be quite dangerous to work with if we are careless. (Remember when you stuck the knife in the toaster and got a shock? Be careful) Typically, AC input modules are available that will work with 24, 48, 110, and 220 volts. Be sure to purchase the one that fits your needs based upon the input devices (voltage) you will use.

现在我们了解了直流输入的工作原理，让我们来仔细看看交流输入。交流电压是无极的，简单地说，这意味着没有什么正极或负极之分，然而，如果我们不小心，交流电压可能是相当危险的（还记得你把刀插进烤面包机然后被电击的时候吗？注意喔）。通常，交流输入模块可用于24、48、110和220伏，一定要根据你将要使用的输入设备的电压来购买适合你需要的PLC。

OUCH... use care!!

AC input modules are less common these days than DC input modules. The reason being that today's sensors typically have transistor outputs. A transistor will not work with an AC voltage. Most commonly, the AC voltage is being switched through a limit switch or other switch type. If your application is using a sensor it probably is operating on a DC voltage.

目前交流输入模块比直流输入模块少见，原因是现在的传感器通常是晶体管输出，晶体管在交流电压下是无法工作的。最常见的情况是，交流输入模块的交流电压是通过限位开关或其他类型的开关来切入的；如果您的应用项目中使用了传感器，那么它大多数情况下可能是在直流电压下工作的。

We typically connect an AC device to our input module as shown above. Commonly the AC "hot" wire is connected to the switch while the "neutral" goes to the plc common. The AC ground (3rd wire where applicable) should be connected to the frame ground terminal of the plc.(not shown) As is true with DC, AC connections are typically color coded so that the individual wiring the device knows which wire is which. This coding varies from country to country but in the US is commonly white (neutral), black (hot) and green (3rd wire ground when applicable). Outside the US it's commonly coded as brown (hot), blue (neutral) and green with a yellow stripe (3rd wire ground where applicable).

我们通常将交流设备连接到我们的交流输入模块，如上所示，通常交流的“火”线连接到开关，而“零”线去接PLC的公共端，交流电源的接地（如有第三根线）应连接到PLC的机架的接地端子上（上图中没有显示）。与直流接线一样，交流连接线通常也是用彩色编码的，以便设备的各个接线知道哪个是哪个，这种彩色编码因国家而异，但在美国通常是白色（零线）、黑色（火线）和绿色（第三根接地线），在美国以外，彩色编码通常为棕色（火线）、蓝色（零线）和带有黄色条纹的绿色（第三根接地线）。

The PLCs AC input module circuit typically looks like this:

典型的PLC交流输入模块电路是这样的:

The only things accessible to the user are the terminals labeled COMMON, INPUT 0000, INPUTxxxx... The common terminal gets connected to the neutral wire.

用户从外观可以看到的惟一内容是标记为COMMON、INPUT0000、INPUTxxxx……，公共终端连接到电源的零线。

A common switch (i.e. limit switch, pushbutton, toggle, etc.) would be connected to the input terminals directly. One side of the switch would be connected directly to INPUT XXX. The other end goes to the ac hot wire. This assumes the common terminal is connected to neutral. Always check the manufacturers specifications before wiring, to be sure AND SAFE.

一个通用的开关（如限位开关、按钮、拨动开关等）将直接连接到输入端子上，开关的一侧将直接连接到输入端INPUTXXX，另一端接交流电源的火线，这里假设公共终端连接到电源的零线。注意，在布线前一定要检查制造商的说明书，以确保安全。

The photocouplers are used to isolate the PLCs internal circuit from the inputs. This eliminates the chance of any electrical noise entering the internal circuitry. They work by converting the electrical input signal to light and then by converting the light back to an electrical signal to be processed by the internal circuit.

光电耦合器用于将PLC内部电路与输入端隔离，这消除了任何电气噪声进入内部电路的机会，它们的工作原理是把输入的电信号转换成光信号，再把光信号转换回电信号，然后由内部电路进行处理。

One last note, typically an ac input takes longer than a DC input for the plc to see. In most cases it doesn't matter to the programmer because an AC input device is typically a mechanical switch and mechanical devices are slowwwwww. It's quite common for a PLC to require that the input be on for 25 or more milliseconds before it's seen. This delay is required because of the filtering which is needed by the PLC internal circuit. Remember that the PLC internal circuit typically works with 5 or less volts dc.

最后要注意的是，通常交流输入比直流输入需要更长的时间才能被PLC看到。在大多数情况下，这对程序员来说并不重要，因为交流输入设备通常是一个机械开关，而机械设备本来就很慢。对于PLC来说，通常输入打开（ON）25毫秒或更长的时间后PLC才会看到，这种延迟是由于PLC内部电路需要滤波而造成的。请记住，plc内部电路通常工作在5伏或更低的直流电压下。