Programmable controllers have grown throughout industrial control applications because of the ease they bring to creating a controller: ease of programming, ease of wiring, ease of installation, and ease of changing. PLCs span a wide range of sizes, but all contain six basic components:
- processor or central processing unit (CPU);
- rack or mounting;
- input assembly;
- output assembly;
- power supply;
- programming unit, device, or PC/software
We will start with explaining the physical components you see when looking at a PLC system – and then explore what goes on inside each part, and how the components relate to each other.
Most medium to large PLC systems are assembled such that the individual components – CPU, Input/Output, Power Supply – are modules that are held together within a rack.
In smaller PLC systems – all of these components may be contained in a single housing or “brick” – these smaller systems are sometimes referred to as “bricks” or “shoebox” PLCs.
The power supply provides power for the PLC system. The power supply provides internal DC current to operate the processor logic circuitry and input/output assemblies. Common power levels used are 24V DC or 120 VAC.
The processor, central processing unit, or CPU is the “brain” of the PLC. The size and type of CPU will determine things like: the programming functions available, size of the application logic available, amount of memory available, and processing speed. Understanding the CPU can be a complex subject and we will tackle that in other articles.
Inputs carry signals from the process into the controller, they can be input switches, pressure sensors, operator inputs, etc. These are like the senses and sensors of the PLC.
Outputs are the devices that the PLC uses to send changes out to the world. These are the actuator the PLC can change to adjust or control the process – motors, lights, relays, pumps, etc.
Many types of inputs and outputs can be connected to a PLC, and they can all be divided into two large groups – analog and digital. Digital inputs and outputs are those that operate due to a discrete or binary change – on/off, yes/no. Analog inputs and outputs change continuously over a variable range – pressure, temperature, potentiometer.
The PLC is programmed using a specialty programmer or software on a computer that can load and change the logic inside. Most modern PLCs are programmed using software on a PC or laptop computer. Older systems used a custom programming device.