Once the event is actually triggered, it turns ON or OFF the external control device called the physical device. A programmable controller will repeatedly “loop” in the program set by the “user” waiting for input signal and output signal at the output at programmed time.
In order to overcome the disadvantages of wired controllers (Relay controllers), the PLC has been manufactured to meet the following requirements:
+ Easy programming, programming language for study.
+ Compact, easy to maintain, repair.
Large memory capacity to store complex programs.
+ Fully reliable in industrial environment.
+ Communicate with other smart devices such as computers, networking, extended modules.
+ Competitive individual price.
The first design was to replace the wiring and timing logic hardware. However, the need to increase the memory capacity and ease of PLC but still ensure processing speed. As well as price … This has caused great interest in the use of PLC in the industry. Scripts quickly go from simple logic commands to count commands, timers, translation registers, and then math functions on large machines. The development of computers leads to PLCs that use the The larger the number, the more I / O.
In the PLC, the hardware and CPU are the basic unit for the process of controlling or processing the system. The function that the controller needs to perform will be determined by a program. This program is preloaded into the memory of the PLC, the PLC will perform the control based on this program. So if you want to change or expand the functionality of the technology process, you just need to change the program inside the PLC memory. Changing or expanding the function will be done easily without any physical interference with the wires or the relay.
Structure, operational principle
All PLCs have the following main components: An internal RAM memory program (which can extend some extra memory outside the EPROM). A microprocessor has a communication port for connecting to the PLC. In / Out modules.
In addition, a full set of PLCs come with an additional programming unit, either manually or by computer. Most simple programming units have enough RAM to hold the program in perfect or complementary form. If the programming unit is a portable unit, RAM is usually a backup battery type CMOS, only when the program is tested and ready to use it transfers to the PLC memory. For large PLCs often programmed on the computer to support writing, reading, and checking programs. Programmable units connected to PLC via RS232, RS422, RS458, …
Principles of PLC operation
The CPU controls operations within the PLC. The processor reads and checks the program stored in memory, then executes the order of the commands in the program, closes or interrupts the output. These output states are transmitted to the associated devices for execution. And all of that execution is dependent on the control program stored in memory.
Bus system is a route used for signal transmission, the system consists of many parallel signal lines:
+ Address Bus: The address bus is used to transmit addresses to different modules.
Data Bus: Bus used to transmit data.
Control bus: The control bus is used for transmitting fixed signals and controlling synchronous operations in the PLC.
In the PLC the data is exchanged between the processor and the I / O modules through the Data Bus. Address Bus and Data Bus consists of 8 lines, at the same time allowing simultaneous or parallel transmission of 8 bits of one byte.
If an input module receives its address on the address bus, it will forward all of its inputs to the Data Bus. If a byte address of the eight outputs appears on the Address Bus, the corresponding output module receives data from the Data Bus. The control bus transfers the control signals into the PLC operating cycle. Addresses and metrics are transferred to the respective bus for a limited time.
The bus system will exchange information between CPU, memory and I / O. On the other hand, the CPU is provided with a clock pulse with a frequency of 1 MHZ. This pulse determines the operating speed of the PLC and provides the clocking and clocking factors of the system.