What is a PLC? What are PLCs and how do they work?
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.
Discrete Applications
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.
Which PLC is right for you?
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 hosts another website called www.aboutPLCs.com with a detailed selection guide to
the entire family of Automation Direct PLCs - but if you are looking for a low cost PLC option for discrete
appllications that is easy to program (with free software) - then look no further - CLICK is for you!
Why should I buy a PLC from Automation Direct?
The #1 value in Automation: low prices, fast shipping, and superior service.
Automation Direct offers over 7000 industrial automation products, including the entire line of Koyo PLC
products at factory-direct low prices. We offer award winning service and support, same-day shipping on
in-stock products, and even free ground shipping on orders over $300. Check out our online store:
www.automationdirect.com and see why we're The #1 value in Automation.
Who is Koyo? You might already know...
In January 1994 PLCDirect (now Automationdirect.com Inc.) was
established as a privately held Georgia corporation that is a
subsidiary of Koyo Electronics. Koyo had designed and manufactured
PLCs for some of the world's largest PLC companies, including
General Electric, Texas Instruments, and Siemens. However, the
future plans of these companies and Koyo were not compatible.
Tim Hohmann, founder and company captain of AutomationDirect,
established the company in 1994 as the first entity within the
Koyo Seiko group to concentrate on the micro PLC market.
Here's a brief history of Koyo Quality in Automation.
