A PROGRAMMABLE LOGIC CONTROLLER (PLC) is an industrial computer control system that continuously monitors the state of input devices and makes decisions based upon a custom program to control the state of output devices.

Almost any production line, machine function, or process can be greatly enhanced using this type of control system.

However, the biggest benefit in using a PLC is the ability to change and replicate the operation or process while collecting and communicating vital information.

Another advantage of a PLC system is that it is modular. That is, you can mix and match the types of Input and Output devices to best suit your application.

WHAT IS INSIDE A PLC?

The Central Processing Unit, the CPU, contains an internal program that tells the PLC how to perform the following functions:

  • Execute the Control Instructions contained in the User's Programs. This program is stored in "nonvolatile" memory, meaning that the program will not be lost if power is removed
  • Communicate with other devices, which can include I/O Devices, Programming Devices, Networks, and even other PLCs.
  • Perform Housekeeping activities such as Communications, Internal Diagnostics, etc.

WHAT ARE INPUT/OUTPUT DEVICES?

INPUTS
OUTPUTS
– Switches and Pushbuttons
– Sensing Devices
  • Limit Switches
• Photoelectric Sensors 
• Proximity Sensors
– Valves
– Motor Starters
– Solenoids
– Actuators


– Condition Sensors
– Encoders
  • Pressure Switches
• Level Switches
• Temperature Switches
• Vacuum Switches
• Float Switches
– Horns and Alarms
– Stack lights
– Control Relays
– Counter/Totalizer 
– Pumps 
– Printers
– Fans


PLC SELECTION CRITERIA

There are several requirements to keep in mind when choosing PLCs. Is the proposed system new or an existing one? Either way, ensure the controller works with mating hardware.

Environmental conditions will affect PLC performance. Typical controllers operate in temperatures from 0 to 55°C (32°-130°F). The number of discrete devices (On/Off logic devices) and analog devices determines the number of I/O connections the PLC will need. If the discrete devices are ac or dc, determine if the PLC can support the required signal.

Determining CPU requirements is important for calculating the amount of RAM needed for data manipulation and storage. Counters and timers use RAM to store set points, current values, and other internal flags. If data must be stored over a long period of time, CPU memory must be sized appropriately.

Program memory or ROM stores program instructions. Analog devices usually require 25 words of memory per device. Examples of analog devices are voltage, current, and temperature meters or sensors. Simple and sequential applications typically require five words of memory per I/O device. Complex applications are not as predictable and need more program memory space.

Serial and Ethernet connection-based I/O hardware are typical choices for remote connections. Remote devices are needed when the PLC is located separately. Serial connections have a max distance of 50 feet while Ethernet connections can go to a max of 328 feet. These remote devices are referred to as distributed I/O. Finally, be sure the PLC understands program instructions. Some PLCs come with proportional integral derivative functions that eliminate the need for technicians to write specific code for closed-loop process control. 

Rievtech Micro PLC is a highly reliable, easy-to-use and widerange product. It will become an indispensable assistant for those who want to optimize and automate their production or system.


Rievtech Micro PLC is versatile! They are a high quality product! As the manufacturer, we provide excellent technical support and help!


Choosing our brand of Micro PLC will provide you with confdence in your choice!