Programmable Logic Controllers (PLC) is an electronic computer that is easy to use control functions for various types and levels of various difficulty.

Definition Programmable Logic Controller according Capiel (1982) are:

electronic systems that operate digitally and is designed for use in industrial environments, where the system uses a programmable memory for internal storage of instructions that implement specific functions such as logic, sequence, enumeration and arithmetic operations to control the engine or process through the modules I / O digital and analog.

Based on his concept of the PLC are as follows:

1. Programmable

demonstrate ability in terms of memory to store programs that have been made that easily altered function or usefulness.

2. Logic

demonstrate the ability to process input in the arithmetic and logic, which compares the operation, add, multiply, divide, subtract, negation, AND, OR, and so forth.

3. Controller

demonstrate the ability to control and manage the process to produce the desired output.

PLC is designed to replace a circuit relay sequensial in a control system. In addition to programmable, this tool can also be controlled, and operated by people who do not have knowledge in computer operation in particular. This PLC has a programming language easily understandable and can be operated when the program was created by using the software according to the type of PLC used is entered.

It works based on the inputs available and depending on the circumstances at a particular time which will then ON or OFF to right outputs. 1 shows that the expected conditions are met, while 0 means the expected condition is not fulfilled. The PLC can also be applied to control systems that have a lot of output.

PLC function and use very broad. In practice, the PLC can be divided in general and in particular .

In general, the PLC functions are as follows:

1. Sequential Control

PLC processing binary input signals into outputs that are used for the purposes of processing techniques sequentially (sequential), here PLCs keep all the step or steps in a sequential process going in the right order.

2. Plant Monitoring

The PLC continuously monitors the status of a system (eg temperature, pressure, level elevation) and take appropriate action in relation to a controlled process (for example, already exceeds the limit value) or display the message on the operator.

While specific PLC function is able to provide input to the CNC (Computerized Numerical Control). Some of the PLC can provide input to the CNC for further processing purposes. CNC when compared with the PLC has a higher precision and more expensive. CNC is usually used for the finishing process, forming a workpiece, molding and so on.

Working principle of a PLC is to receive input signals and controlled process to a series of logical instructions to the input signal is in accordance with programs stored in memory and then generate output signals to control actuators or other equipment.

Advantages and disadvantages of PLC

In industries that exist today, the presence of the PLC is needed primarily to replace the wiring or cabling system that previously was used in controlling a system. By using the PLC will get many benefits include the following:


In the past, each different electronic devices controlled by each controller. For example ten machines need ten controllers, but now only one tenth PLC machine can be run with each program.

Changes and easier system error

If one system will be modified or corrected is done only at programs contained in the computer, in a relatively short time, then downloaded to the PLC.

If not using a PLC, such as relays the changes done by changing wearing. This method must take a long time.

The number of contacts that many

The number of contacts that are owned by the PLC on each coil more than contacts that are owned by a relay.

The price is cheaper

PLC is capable of simplifying a lot of cabling compared to a relay. So the price of a PLC is cheaper than the price of a few relays that can do the wiring with an amount equal to a PLC. PLC include relays, timers, counters, sequencers, and various other functions.

Pilot running

The PLC is programmed to run and evaluated in advance at the office or laboratory. The program can be written, tested, and modified if diobserbvasi is needed and this saves time when compared with conventional relay system is tested with the best results at the factory.

Observation of visual

During the program starts, the operation of the PLC can be viewed on CRT screens. Errors of operations can be observed even when there is.

Speed of operation

PLC operating speeds faster than the relay. Speed is determined by the PLC scan time in units of milliseconds.

Ladder Programming Method or Boolean

PLC programming can be expressed with ladder programming for technicians, or Boolean algebra to the programmers who worked on the digital control systems, or Boolean.

its time-tested

Solid state devices are more resistant than the relay test and mechanical or electrical timers. The PLC is a solid state device that is more resistant to the test.

Simplify the components of control systems

In the PLC also counters, relays and other components, so that does not require these components in addition. Counter requires the use of relays, timers or other components as additional equipment.


Printout of the PLC can be directly obtained and do not need to see the blueprint of his circuit. Unlike the printout relay circuits can not be obtained.


Changing the PLC can not be done unless the PLC is not locked and programmed. So no one can change the interest is not the PLC program for the PLC is locked.

It can make changes by reprogramming

Since the PLC can be quickly re-programmed, mixed production process can be completed. For example part B will be executed but the A is still in the process, the process in Part B can be re-programmed in seconds.

Addition faster circuits

Users can add a set of control at any time quickly, without the need for energy and cost a lot like the conventional controller.

Besides the benefits mentioned above, there are losses that are owned by the PLC, namely:

The technology is still new

The transformation of the old control system using relay ladder or a computer to the PLC concept is difficult for some people Bad for application programs that remain

Some applications are applications with a single function. While PLC may include several functions at once. In applications with a single function is rarely performed even change at all, so the use of PLCs in applications with a single function will be a waste (cost).

Environmental Considerations

In a processing environment that may experience high heating, vibration direct contact with electronic devices in the PLC and it if happens continuously, thus disturbing the performance of the PLC is not functioning optimally.

Operation with a fixed set

If the circuit in an operation is not changed then the use of more expensive PLC compared with other control equipment. PLC will be more effective if the process program to be upgraded periodically.

Start-Stop circuit

Many systems have a Master Control Relay system for Safety Shutdown PLC operations. When ON, the safety shutdown allows the PLC to operate. When in-deenergize, then the PLC will not operate.

In the picture above if the Start button in the press (ON) then the MCR coil will occur so that the child Energize relay MCR Energize will occur so that the PLC will also operate. Although the Start button, go back to his original position (OFF), MCR coil remains were Energize because of the child other MCR relay parallel with the Start button. When the Stop button is pressed (OFF), then becomes an open circuit causing no more current flow into the coil MCR, so the coil was not MCR Energize again. Because MCR coil is not too longer Energize relaynya the two children will be OFF so that the PLC OFF.
In the picture above there are also Emergency Stop Pushbutton is used if there is something in the system so that the system must be turned off. In addition there is a limit switch associated with the PLC system in which the door is placed. If the door is opened so that the limit switch OFF MCR coil is not too Energize the PLC will cause the system OFF, when the door closed limit switch will be ON so that the PLC system will be ON also. While Suppressor used to reduce or eliminate the signal interference from outside that can make the program a PLC system to malfunction.

PLC Parts
The PLC system consists of five main parts, namely:
Central processing unit (CPU).
This part of the brain or heart is a PLC, because this section is part of the operation / processing program stored in the PLC. Besides the CPU also oversee the operational work of all the PLC, through an internal transfer of information between the PLC bus, memory and the unit I / O.
Part of this CPU include:
Power Supply, power supply change the input supply electricity into the electricity supply in accordance with the CPU and the entire computer.
Alterable  Memory, composed of many parts, these parts form the core chip whose contents in place on the chip RAM (Random Access Memory), but its contents can be modified and deleted by the user / programmer. If there is no power to CPU supplai then its contents will be lost, and therefore this section is called be volatile, but there are also parts that are not volatile.
Fixed Memory, contains programs that have been set by the PLC manufacturer, made in a special chip called a ROM (Read Only Memory), and can not be changed or deleted during CPU operation, because it is often called the non-volatile memory that will not be deleted it although there is no electricity coming into the CPU. Moreover, it can also add a module EEPROM or Electrically Erasable Programmable Read Only Memory is intended to back up the main program so that the processor RAM processor can be programmed to load the EEPROM to the RAM program if the program in RAM is lost or damaged.
Processor, is a part of that control so that information remains that way from one part to another section, this section contains a series of clock, so that each transfer of information to another place just up in time
Battery Backup, most CPUs have this section. This part is maintaining that no loss program that has been incorporated into the PLC RAM when the PLC power supply was suddenly cut off.

Programmer / monitor (PM).
Programming done via the keyboard so that the tool is called the Programmer. With the monitor it can be seen what is typed or the process being carried out by the PLC. Form AM have a great like a PC, there is also a small hand-Eld is a programmer with a small display window, and there is also a form of laptops. AM associated with the CPU through a cable. After completion programmed the CPU PM no longer used for operating the PLC process, so that this section is only needed one for a lot of CPU.