A programmable logic controller (PLC) consists of a set of modules or cards (printed circuit boards) that house the electronic components enabling its operation. Each card performs a specific function. Some PLCs have a cover or housing, commonly called a "rack," which is a frame where the cards are neatly arranged and generally interconnected.
The programmable controller has a structure very similar to programming systems, such as the computer, whose physical structure (hardware) consists of:
- Power supply.
- Central processing unit (CPU).
- Input/output (I/O) modules or interfaces.
- Memory modules.
- Programming Unit.
In some cases, when the work that the controller must perform is more demanding, the following are included:
The following figure shows the block diagram of an automated system controlled by a PLC, and below, each of the parts of the programmable controller is described in more detail.
Block diagram of a PLC controlling a process
POWER SUPPLY
The function of the power supply in a controller is to provide electrical power to the CPU and other cards according to the PLC configuration.
In the internal circuits of a power supply, the alternating current voltage from the mains is transformed into direct current voltage, at levels that guarantee the operation of the PLC hardware.
The power supply is also known as the power source.
Power supply for a Simatic S5 modular PLC (Courtesy of Siemens)
All power supplies are protected against short circuits by fuses, which can be easily replaced in case of a failure.
CENTRAL PROCESSING UNIT (CPU)
It is the most complex and essential part of the programmable controller; in other words, it could be considered the brain of the controller.
The central unit is designed using microprocessors and memory. The memory is of the ROM and RAM types.
ROM (Read Only Memory): is a read memory, which remains fixed in the CPU, contains the operating system with which the controller operates, IT IS NOT ERASED.
RAM (Random Access Memory): random access memory, is volatile memory and easy to modify.
The following are located in RAM:
- The user's memory.
- Timers.
- The accountants.
- The bits or internal memories.
- Database.
Central Processing Units: Telemecanique (TSX 87-40 and TSX 107-40) / (Courtesy of Telemecanique)
CPUs, like computers, can be classified according to their memory capacity, the functions they can perform, and their processing speed. Program read time depends on the number and type of instructions and is typically on the order of milliseconds. This extremely short time means that any change in the state of an input signal almost instantaneously modifies the state of an output signal.
Help us grow
Finally, if you are determined to learn Industrial Automation and advance your professional career, you may be interested in our Premium courses:
– Click to see all our courses at MyAutomationClass.com
- Course: «"Become a PLC expert in 60 days, from basic to pro"»
- Course: «"PID Controllers Master Class – 100% practical"»
- Course: «"How to create SCADA/HMI projects from scratch with InTouch"»
- Course: «"PID Tuning Ultimate: The Definitive Course for Tuning"»
- Course: «"Practical Process Modeling: Definitive Steps to Becoming an Expert"»
- Program: «"The Ultimate Process Control Formula" (All-inclusive: PID Control, Tuning, and Modeling)
- Course: How to Learn Industrial Instrumentation, Easily (Complete)
If you have any questions, contact us via Facebook Messenger
Until next time. COMMENT AND SHARE! I'll be happy to answer your questions.
Spanish 
English