Programmable logic controllers (PLCs)

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Learn more about PLCs

Our Automation Compass platform provides all the details on PLC control methodologies available and how they work.

Programmable logic  controllers (PLCs) are solid-state, electronic devices that control the operation of a machine or process.

They use logic functions, that are programmed into their memory via programming software. In simple terms, a PLC is the “brains” behind an automated process.  

PLCs are designed to monitor inputs from sources like pressure sensors, temperature sensors, limit switches, auxiliary contacts, and pilot devices.

Based on the status of these inputs (on/off, voltage value between O and 10 V or amperage value between) and 24 mA, the PLC runs these inputs through its programming. The PLC will execute the programming by updating its outputs such as starting a motor, turning on a pilot light, or changing the speed of a conveyor. PLCs are designed for multiple input/output (I/O) arrangements.  

PLCs are general-purpose controllers that can control and monitor any industrial machine or process.

How does a PLC work?

PLCs work by continually scanning a program. Think of this scan cycle as consisting of the following three important steps. 

Step 1 – Reading Input status
The PLC takes a snapshot of each input to determine whether it's on or off and writes it into memory. 

Step 2 – Execute the program 
The PLC executes your program one instruction at a time using the input states in memory. 

Step 3 – Updating output 
Finally, the PLC updates the status of the outputs based on which inputs were on during the first step and the results of executing your program during the second step.

Brick PLC

A Brick PLC has an integrated CPU, an input/output and a power supply. Inputs are hardware to devices monitored by the PLC. Outputs are hardwired to devices controlled by the PLC. 

Modular PLC

A Modular PLC has a separate CPU module, input/output (I/O) modules and a power supply. Mix and match I/O modules as needed. Inputs are hardwired to devices monitored by the PLC. Outputs are hardwired to devices controlled by the PLC.

1. What is the number and type of inputs/outputs needed?
   
   
2. What are the networking requirements?
   
   
3. Are there high-speed input or output requirements?
   
   
4. Are there any special environmental requirements?
   

1. Understand how the program and input/output (I/O) scan works
   
   
2. Understand how the I/O is addressed
   
   
3. Understand the internal memory addressing
   
   
4. Familiarity with the instruction set (ladder diagram)
   
   
5. Familiarity with the programming software (create a project, add logic, download to the controller, monitor online and online editing)