The structural variety of programmable controllers, but the General principles of its composition are basically the same, are based on the microprocessor architecture. Usually consists of a central processing unit (CPU), memory (RAM and the ROM), input/output (I/O), power supply and programmer in several parts.
1. The central processing unit (CPU)
CPU as the core of PLC, plays the Commander-in-chief role. CPU control circuit, ALU and registers. These circuits are usually encapsulated in an integrated circuit chip. CPU address bus and data bus, control bus and storage units, input–output interface circuit connections. CPU function are as follows: read instructions from memory, execute instructions, remove an instruction, and handle interrupts.
2. Memory (RAM and the ROM)
Memory to store system procedures, user programs and data. Storage system called the system software memory for program memory; memory storage application software known as user program memory, stored data memory called data memory. Commonly used memory RAM, EPROM and EEPROM. RAM is random access memory that can be read and write stored user programs to generate user data area, user programs stored in RAM can easily be modified. RAM memory is a high density, low power consumption, low cost semiconductor memory free lithium battery backup power. Power down, you can effectively keep information that is stored. EPROM, EEPROM read–only memory. Managing programs with these types of storage systems and applications.
3. Input/output modules (I/O modules)
I/O unit actually is PLC controlled objects passed between input and output signal interface unit. Good isolation and filtering of I/O units. Received a PLC input interface input devices are a variety of switches, buttons and sensors. PLC outputs control devices are solenoid valves, contactors, relays, relays with AC and DC type, high voltage and low voltage, current type and voltage.
4. Power supply
PLC power supply unit includes the power supply and the battery, the power supply unit‘s role is to convert external power internal operating voltage. PLC has a regulated power supply for PLC CPU module and I/O module power supply.
Programming PLC is the most important peripheral devices. With the program the user program into the PLC memory, you can also programmatically check program, modify the program to monitor the work status of PLC. In addition, add the appropriate hardware interface and software package on a personal computer, you can use the PC to the PLC programming. Using computer as a programmer, you can directly compile and display the ladder diagram.