【正文】 電壓。如果我們有一個第二個電源，我們還需要連接的中性，使這兩個中立國將連接到同樣普遍。常見的是一個參考，或基準電壓，用于為0V使用，但地面是用于防止沖擊和損害到設(shè)備。這是連接到建筑物的電氣系統(tǒng)，對電源插座，在電氣設(shè)備的金屬案件有關(guān)。不幸的是許多工程師，制造混淆的地面和共同的。最后一個概念，初學者往往陷阱是每個輸入卡是孤立的。當發(fā)生這種情況的其他卡將無法正常工作。 輸出模塊正如輸入模塊，輸出模塊很少提供任何權(quán)力，而是作為開關(guān)。典型的輸出電壓下面列出，并大致排序受歡迎。一種常見的選擇，采購卡的繼電器輸出，晶體管或可控硅。他們有能力開關(guān)交流和直流輸出。繼電器輸出通常被稱為干觸點。晶體管和可控硅輸出稱為切換輸出。這使得混合電壓（AC或DC和電壓水平，直至最高），以及絕緣輸出以保護其他產(chǎn)出和PLC。此方法是最不敏感的電壓變化和尖峰。晶體管輸出NPN或使用PNP晶體管高達1A典型。Automating Manufacturing Systems with PLCsControl engineering has evolved over time. In the past humans were the mainmethod for controlling a system. More recently electricity has been used for control andearly electrical control was based on relays. These relays allow power to be switched onand off without a mechanical switch. It is mon to use relays to make simple logicalcontrol decisions. The development of low cost puter has brought the most recent revolution,the Programmable Logic Controller (PLC). The advent of the PLC began in the1970s, and has bee the most mon choice for manufacturing have been gaining popularity on the factory floor and will probably remainpredominant for some time to e. Most of this is because of the advantages they offer. Ladder logic Ladder logic is the main programming method used for PLCs. As mentioned before, ladder logic has been developed to mimic relay logic. logic diagrams was a strategic one. By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and tradespeople was greatly reduced.Modern control systems still include relays, but these are rarely used for logic. A relay is a simple device that uses a magnetic field to control a switch, as pictured in Figure . When a voltage is applied to the input coil, the resulting current creates a magnetic field. The magnetic field pulls a metal switch (or reed) towards it and the contacts touch, closing the switch. Figure Simple Relay Layouts and SchematicsRelays are used to let one power source close a switch for another (often high current) power source, while keeping them isolated. An example of a relay in a simple control application is shown in Figure . In this system the first relay on the left is used as normally closed, and will allow current to flow until a voltage is applied to the input A. The second relay is normally open and will not allow current to flow until a voltage is applied to the input B. If current is flowing through the first two relays then current will flow through the coil in the third relay, and close the switch for output C. This circuit would normally be drawn in the ladder logic form. This can be read logically as C will be on if A is off and B is on.Figure A Simple Relay ControllerThe example in Figure does not show the entire control system, but only the logic. When we consider a PLC there are inputs, outputs, and the logic. Figure shows a more plete representation of the PLC. Here there are two inputs from push can imagine the inputs as activating 24V DC relay coils in the PLC. This in turn drives an output relay that switches 115V AC, that will turn on a light. Note, in actual PLCs inputs are never relays, but outputs are often relays. The ladder logic in the PLC is actually a puter program that the user can enter and change. Notice that both of the input push buttons are normally open, but the ladder logic inside the PLC has one normally open contact, and one normally closed contact. Do not think that the ladder logic in the PLC need so match the inputs or outputs. Many beginners will get caught trying to make the ladder logic match the input types.Figure A PLC Illustrated With RelaysMany relays also have multiple outputs (throws) and this allows an output relay to also be an input simultaneously. The circuit shown in Figure is an example of this, it is called a seal in circuit. In this circuit the current can flow through either branch of the circuit, through the contacts labelled A or B. The input B will only be on when the output B is on. If B is off, and A is energized, then B will turn on. If B turns on then the input B will turn on, and keep output B on even if input A goes off. After B is turned on the output B will not turn off.Figure A Sealin Circuit ProgrammingThe first PLCs were programmed with a technique that was based on relay logic wiring schematics. This eliminated the need to teach the electricians, technicians and engineers how to program a puter but, this method has stuck and it is the most mon technique for programming PLCs today. An example of ladder logic can be seen in Figure . To interpret this diagram imagine that the power is on the vertical line on the left hand side, we call this the hot rail. On the right hand side is the neutral rail. In the figure there are two rungs, and on each rung there are binations of inputs (two vertical lines) and outputs (circles). If the inputs are opened or closed in the right bination the power can flow from the hot rail, through the inputs, to power the outputs, and finally to the neutral rail. An input can e from a sensor, switch, or any other type of sensor. An output will be some device outside the PLC that is switched on or off, such as lights or motors. In the top rung the contacts are nor