PLCs are often defined as miniature industrial computers that contain hardware and software used to perform control functions. A PLC consists of two basic sections: the central processing unit (CPU) and the input/output interface system. The CPU, which controls all PLC activity, can further be broken down into the processor and memory system. The input/output system is physically connected to field devices (e.g., switches, sensors, etc.) and provides the interface between the CPU and the information providers (inputs) and controllable devices (outputs).
To operate, the CPU "reads" input data from connected field devices through the use of its input interfaces, and then "executes" or performs the control program that has been stored in its memory system. Programs are typically created in ladder logic, a language that closely resembles a relay-based wiring schematic, and are entered into the CPU's memory prior to operation. Finally, based on the program, the PLC "writes" or updates output devices via the output interfaces. This process, also known as scanning, typically continues in the same sequence without interruption, and changes only when a change is made to the control program.
If you would like more information on PLCs and how to use them, check out the free “PLC Handbook” on our library site at:
PLCs are often used to control machines or processes that are sequential in nature, using "discrete" inputs and outputs that have defined states. For example, if a limit switch detects the presence of an object, it provides an "ON" signal to the PLC; if no object is detected, it provides an "OFF" signal. The machine or device typically performs actions based on time or events in a pre-defined order. The expected sequence is typically interrupted only when an abnormal condition occurs.
CLICK is an excellent PLC choice for discrete applications.
Choosing the most effective PLC for your application depends on a number of factors. To begin the selection process, a drawing of the machine or process is a good start. This can help identify field devices and physical requirements for hardware locations. From the drawing, you can determine how many analog and/or discrete devices you will have. Once the field device requirements and hardware locations are defined, you can review PLCs that will meet your requirements.
AutomationDirect offers many affordable PLC options to meet the needs of a variety of customers. If you need assistance in choosing the right PLC, please refer to our PLC Selector Tool found here: PLC Selector Tool
This tool can guide you to the PLC family that is the best fit for your application. However if you need a low cost, practical PLC for small, everyday applications that is reliable and easy to program (with free software) - then CLICK is definitely the one for you!
The #1 value in Automation: low prices, fast shipping, and superior service. AutomationDirect has been in the industrial control market since 1994 (then known as PLCDirect) and today offers tens of thousands of quality industrial automation products with service and support that is unmatched in the industry. Our focus on customer satisfaction has resulted in numerous industry awards for service and with our low prices, FREE tech support, same-day shipping on in-stock products, and FREE shipping on orders over $49 (see store for details), you won't get more for your dollar anywhere else.