What is a PLC ?

PLC Programmable Logic Controller

PLC block diagram

At the center of most industrial automated machine control is a PLC (Programmable Logic Controller) with an industrial grade processor. The PLC has the capabilities of reading the status of inputs as well as controlling the state of outputs. These input devices include switches, contacts and temperature sensors. In addition, the PLC performs logic based on the status of the input signals and as a result, turns on the appropriate output. PLC programming is accomplished using an application software developed for that particular manufactures PLC and should follow the IEC 61131-3 specification. Furthermore, this specification defines several different types of programming languages. The different programming types are Ladder Logic, Structured Text, Function Block and Sequential Flow Chart. The system integrator or automation service PLC programmer typically uses a combination of different programming languages in a project.

What you should know before selecting a PLC:

What is a PLC ? Programmable Logic Controllers are microprocessor based industrial grade component hardware. They typically have a proprietary operating system with Input and Output control. The algorithm is created using an application software created from the manufacture of the PLC.

Questions you need to answer before selecting a PLC

  • How many Inputs / Outputs are required and what type are needed ?
  • What are the processor Cycle Times are required ?
  • What type of external interfaces are needed (Ethernet, Serial, Ethernet IP) ?
  • Do you plan on using an external data base for data collection ?

Mitsubishi Electrics Current PLC Offerings

The most common and inexpensive type of PLC is the shoe box type. These units come with a preconfigured number of inputs and outputs and have limited expansion capabilities. The input and output type and quantity will vary based on the model of the chosen unit. They are usually found on stand alone machines with lower processor speed requirements and a minimum amount of communications capabilities needed.

Mitsubishi FXU5

Another popular style is the rack mount type. These units have a parallel backplane where the power supply and CPU along with the input and output modules plug into the rack. The CPU can communicate to the module through the backplane which reduces overall processing speed. They typically have several selections of processor speed and memory configurations. They also have many types of input and output configurations along with communication options and built in safety system control.

i-QR Mitsushi PLC