③Large quantities of contacts. The PLC has a large number of contacts for each coil available in its programming.suppose that a panel-wired relay has four contacts and all are in use when a design change requiring three more contacts is made. Time would have to be taken to procure and install a new relay or relay contact block. Using a PLC, howere, only three more contacts would be typed in. the three contact would be automatically available in the PLC. Indeed, a hundred contacts can be used from one relay-if sufficient computer memory is available. 

④Lower cost. Increased technology makes it possible to condense more functions into smaller and less expensive packages. Now you can purchase a PLC with numerous relays, timers and counters, a sequencer, and other function for a few hundred dollars.

⑤Pilot Running. A PLC programmed circuit can be evaluated in the office or lab. The program can be typed in, tested, observed, and modified if needed, saving valuable factory time. In contrast, conventional relay systems have been tested on the factory floor, which can be very time consuming.

⑥Visual Observation. A PLC circuit’s operation can be seen during operation directly on a CRT screen. The operation or misoperation of a circuit can be observed as it happens. Logic paths light up on the screen as they are energized. Troubleshooting can be done more quickly during visual observation. In advanced PLC systems, an operator message can be programmed for each possible malfunction.

In advanced PLC systems, an operator message can be programmed for each possible malfunction. The malfunction description appears on the screen when the malfunction is detected by the PLC logic. Advanced PLC systems also may have descriptions of the function of each circuit component. 

⑦Speed of Operation. Relays can take an unacceptable amount of time to actuate. The operational speed for the PLC program is very fast. The speed for the PLC logic operation is determined by scan time, which is a matter of milliseconds.

⑧Ladder or Boolean Programming Method. The PLC programming can be accomplished in the ladder mode by an electrician or technician. Alternatively, a PLC programmer who works in digital or Boolean control systems can also easily perform PLC programming.

 In modern industrial production equipment, and large numbers of simulations of the control devices, such as the cease electrical, electromagnetic devices to turn, the number of products, temperature, Ap power, and set the flow control, the automatic control of the industrial site, If a programmable controller (PLC) to resolve the issue has become a cybernetic one of the most effective tools, this article describes Plc control system design should be noted. Hardware purchase many products currently on the market Plc, with the exception of domestic brands, foreign : Japan Omron, Mitsubishi, FUJJ, ANASONIC, Germany Siemens, Korean LG. In recent years, Plc greater decline in the prices of products, increasing its performance cost rate. This is a technical staff chosen Plc important reasons. Then, how to purchase Plc products?

     First, the system should first determine the size of single control system with Plc or using Plc networking, thus calculating Plc importing, exporting. Few, and in some Plc to be in real need have a certain cushion

     Second, determine the type of load under Plc export-led type of load is the DC or exchange-type, big or small currents, and Plc export points such as the frequency of movements, the use of relays to determine export-export, or transistor output, or goods of export gates. Different load choose different ways to export to the operating system's stability is very important.

     Third, the storage capacity and speed while foreign manufacturers of Plc products essentially the same, but there are some differences. Currently the company has not yet found full compatibility between products. The development of software companies are not the same, and users of storage capacity and procedures to implement the Directive speed are two important indicators. General greater storage capacity, faster speed on the Plc higher prices, but should be based on system size reasonable choice Plc products.