PLC interface and its functions

(1) Input interface

Accepts the signal of the controlled device and drives the internal circuit to turn on or off through the coupler device and the input circuit.

(2) Output interface

The execution result of the program is output through the coupler device and output components (relay, thyristor, transistor) of the output interface to control the state of the external load.

The basic units of PLC and their functions

(1) CPU

The core component of the PLC directs the PLC to perform various tasks. Such as accepting user programs and data, diagnosing, executing executive programs, etc.

(2) Memory

Store system and user programs and data

(3) I/О interface

The connection part between PLC and the controlled object in the industrial production site, used to receive the signal of the controlled device and output the execution result of the program

(4) Communication interface

Exchange information with other devices such as monitors, printers, etc. through communication interfaces

(5) Power supply

PLC switch output interface type and characteristics

(1) Thyristor output type

Under normal circumstances, it can only carry AC load, with fast response speed and high operating frequency.

(2) Transistor output type

Under normal circumstances, it can only carry DC load, with fast response speed and high operating frequency.

(3) Relay output type

Under normal circumstances, it can carry AC and DC loads, but its response time is long and the operating frequency is low.

PLC structure type and characteristics

(1)) Monolithic

The CPU, power supply, and I/О components are all concentrated in one chassis, with compact structure and low price. Generally, small PLCs adopt this structure.

(2) Modular

Each part of the PLC is divided into several separate modules, and different modules can be selected to form a system according to the needs. It has the characteristics of flexible configuration, convenient expansion, and maintenance. Generally, medium, and large PLCs adopt this structure. The modular PLC consists of a frame or base plate and various modules, and the modules are mounted on the sockets of the frame or base plate.

(3) Stacked

Combining the characteristics of integral and modular, the CPU, power supply, L/О interface, etc. of the stacked PLC are also independent modules, but they are connected by cables, which makes the system not only flexible in configuration but also small in size.

Scan cycle of PLC

The scanning process of PLC includes five stages: internal processing, communication service, input processing, program execution, and output processing. The time required for one scan of these five stages is called scanning cycle.

The scan cycle is related to the running speed of the CPU, the hardware configuration of the PLC and the length of the user program.

PLC execution method and process

PLC executes user program by cyclic scanning.

The execution process of user program includes input sampling stage, program execution stage and output refresh stage.

Comparison between PLC control system and relay control system

(1) Control method

PLC adopts program method to realize control, it is easy to change or increase control requirements, and PLC has unlimited contacts

(2) Working method

PLC adopts serial working mode to improve the anti-interference ability of the system.

(3) Control speed

The contacts of the PLC are triggers, and the execution time of the instruction is in microseconds.

(4) timing and counting

PLC uses semiconductor integrated circuits as timers, clock pulses are provided by crystal oscillators, with high delay accuracy and wide range. It has the counting function that the relay system does not have.

(5) Power supply

PLC adopts microelectronic technology and has high reliability. Its self-checking function can detect its own fault in time, and its monitoring function is convenient for debugging and maintenance.