16 Basic PLC Knowledge Points You Should Learn

(1) Input Interface: Accepts signals from the controlled devices and drives the internal circuit to connect or disconnect through optocoupler devices and input circuits.

(2) Output Interface: The execution results of the program are output through the optocoupler devices and output components (relays, thyristors, transistors) of the output interface, controlling the connection or disconnection of external loads.

(1) CPU: The core component of the PLC, directing the PLC to perform various tasks, such as accepting user programs and data, diagnostics, and executing programs;

(2) Memory: Stores the system and user programs and data;

(3) I/O Interface: The connection component between the PLC and the controlled objects in the industrial production site, used to receive signals from controlled devices and output the execution results of the program;

(4) Communication Interface: Exchanges information with other devices like monitors and printers through the communication interface;

(5) Power Supply.

Transistor Output Type: Generally can only carry DC loads, fast response speed, high action frequency;

Relay Output Type: Generally can carry both AC and DC loads, but has a longer response time and lower action frequency.

(1) Integrated Type: Combines the CPU, power supply, and I/O components all in one chassis, compact structure, low price, generally used in small PLCs;

(2) Modular Type: Divides the PLC into several separate modules, allowing for flexible configuration and easy expansion and maintenance, generally used in medium and large PLCs. Modular PLCs consist of a frame or baseboard and various modules, installed in sockets on the frame or baseboard.

(3) Stacked Type: Combines the characteristics of integrated and modular types, where the CPU, power supply, and I/O interfaces are also independent modules connected by cables, making the system flexible in configuration and compact in size.

The scanning process of a PLC includes five stages: internal processing, communication service, input processing, program execution, and output processing. The time required to scan through these five stages once is called the scanning cycle.

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

The PLC executes user programs in a cyclic scanning manner. The execution process of the user program includes the input sampling stage, program execution stage, and output refresh stage.

(1) In terms of control method: PLC uses programming to achieve control, making it easy to change or add control requirements, and PLC contacts are unlimited;

(2) In terms of working mode: PLC adopts a serial working mode, improving the system’s anti-interference ability;

(3) In terms of control speed: PLC contacts are essentially triggers, with instruction execution times in the microsecond range;

(4) In terms of timing and counting: PLC uses semiconductor integrated circuits as timers, with clock pulses provided by crystal oscillators, offering high delay accuracy and wide range. PLC has counting functions that relay systems do not possess;

(5) In terms of reliability and maintainability: PLC uses microelectronic technology, has high reliability, and its self-checking functions can promptly identify faults, making monitoring convenient for debugging and maintenance.

Because PLC adopts a centralized sampling and centralized output cyclic scanning working mode, the input state can only be read during the input sampling stage of each scanning cycle, while the execution results of the program are only sent out during the output refresh stage; additionally, delays in PLC input and output, the length of the user program, etc., can all cause output response lag.

To improve I/O response speed, one can employ various methods such as direct input sampling, output refreshing, interrupt input/output, and intelligent I/O interfaces.

Input relays, output relays, auxiliary relays, status registers, timers, counters, data registers.

1) Model selection: should consider structural form, installation method, functional requirements, response speed, reliability requirements, and model uniformity;

2) Capacity selection: should consider I/O point numbers and user storage capacity;

3) I/O module selection: includes selection of digital and analog I/O modules, as well as special function modules;

4) Selection of power supply modules and programming devices, among other equipment.

Centralized sampling: In a scanning cycle, sampling of input states occurs only during the input sampling stage. Once in the program execution stage, the input terminals are blocked.

Centralized output: In a scanning cycle, only during the output refresh stage will the states related to the output from the output image register be transferred to the output latch for refreshing the output interface. During other stages, the output states remain in the output image register. This working mode can enhance the system’s anti-interference ability and reliability but may cause response lag in PLC input/output.

The PLC adopts a centralized sampling, centralized output, and cyclic scanning working mode.

Characteristics: Centralized sampling means that in a scanning cycle, PLC samples input states only during the input sampling stage, blocking the input terminals during the program execution stage.

Centralized output means that in a scanning cycle, PLC only transfers the states related to the output from the output image register to the output latch for refreshing during the output refresh stage, while during other stages, the output states remain in the output image register.

Cyclic scanning means that PLC needs to execute multiple operations within a scanning cycle, adopting a time-sharing scanning method to execute them sequentially, repeating the process.

Electromagnetic contactors generally consist of an electromagnetic mechanism, contacts, an arc extinguishing device, a release spring mechanism, a bracket, and a base. The contactor operates based on electromagnetic principles: when the electromagnetic coil is energized, the coil current generates a magnetic field that attracts the armature, causing the contacts to act, opening the normally closed contacts and closing the normally open contacts, which are linked. When the coil is de-energized, the electromagnetic force disappears, and the armature is released by the spring, restoring the contacts to their original state, closing the normally open contacts and opening the normally closed contacts.

A programmable logic controller (PLC) is an electronic device designed for digital operation in industrial environments. It uses programmable memory to store instructions for executing logical operations, sequential operations, timing, counting, and arithmetic operations, and can control various types of machinery or production processes through digital or analog inputs and outputs.

PLCs and their related peripheral devices should be designed to easily integrate into an industrial control system and facilitate the expansion of their functions.

Different component devices;

Different numbers of contacts;

Different methods of implementing control;

Different working modes.

(1) The transfer source automatically resets;

(2) Allows dual output;

(3) Master control function.