16 Essential PLC Knowledge

Anyone engaged in electrical work knows that industrial production and technological development cannot be separated from the automation control provided by PLCs. PLCs can be broadly understood as centralized relay extension control cabinets. In practical production applications, PLCs greatly reduce the cost of industrial control and enhance centralized management and automatic control of equipment. To master PLCs well, a solid foundation in PLC basics is essential.

From the composition of PLCs, apart from the CPU, memory, and communication interfaces, what other interfaces are directly related to the industrial site? Please explain their main functions.

(1) Input Interface: Accepts signals from 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.

What are the basic units of a PLC? What is the function of each?

(1) CPU: The core component of the PLC, directing the PLC to perform various tasks. This includes 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 accept signals from the controlled devices and output the execution results of the program;

(4) Communication Interface: Facilitates information exchange with other devices such as monitors and printers through the communication interface;

(5) Power Supply.

What types of PLC digital output interfaces are there? What are their characteristics?

Thyristor output type: Generally, can only drive AC loads, fast response speed, high action frequency;

Transistor output type: Generally, can only drive DC loads, fast response speed, high action frequency;

Relay output type: Generally, can drive both AC and DC loads, but has a longer response time and lower action frequency.

What types of PLCs are there based on structural types? What are their characteristics?

(1) Integrated type: All components including CPU, power supply, and I/O parts are concentrated in one chassis, compact structure, low price, generally used for small PLCs;

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

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

What is the PLC scanning cycle? What mainly affects it?

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

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

How does a PLC execute user programs? What stages are included in the execution process?

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

What advantages does a PLC control system have compared to a relay control system?

(1) In terms of control methods: PLCs implement control through programming, making it easy to change or add control requirements, and the contacts of a PLC are virtually unlimited;

(2) In terms of working methods: PLCs use a serial working method, improving the system’s anti-interference capability;

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

(4) In terms of timing and counting: PLCs use semiconductor integrated circuits as timers, with clock pulses provided by crystal oscillators, ensuring high delay precision and a wide range. PLCs also have counting functions that relay systems do not possess;

(5) In terms of reliability and maintainability: PLCs utilize microelectronic technology, offering high reliability, and their self-checking functions can promptly identify faults, while monitoring functions facilitate debugging and maintenance.

Why does a PLC exhibit output response lag? How can I/O response speed be improved?

Because PLCs use a centralized sampling and centralized output cyclic scanning working method, the state of the input can only be read during the input sampling phase of each scanning cycle, while the execution results of the program are only sent out during the output refresh phase. Additionally, delays in PLC input and output, as well as the length of the user program, can cause output response lag.

To improve I/O response speed, one can use direct input sampling and output refresh, or implement interrupt input/output and intelligent I/O interfaces among other methods.

What types of internal soft relays are there in the FX0N series PLC?

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

How to select a PLC?

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 count and user storage capacity;

3) I/O module selection: Includes selecting switch and analog I/O modules, as well as special function modules;

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

Briefly describe the characteristics of the centralized sampling and centralized output working method of PLCs. What are the advantages and disadvantages of using this working method?

Centralized sampling: Within a scanning cycle, input state sampling occurs only during the input sampling phase. Once the program execution phase begins, the input end is locked.

Centralized output: Within a scanning cycle, the output state is only refreshed during the output refresh phase when the output image register is transferred to the output latch. During other phases, the output state remains in the output image register. This working method can improve the system’s anti-interference ability and enhance reliability, but it can cause lag in PLC input/output response.

What working method does a PLC adopt? What are its characteristics?

A PLC adopts a working method of centralized sampling, centralized output, and cyclic scanning.

Characteristics: Centralized sampling means that within a scanning cycle, the PLC samples the input state only during the input sampling phase. Once the program execution phase begins, the input end will be locked.

Centralized output means that within a scanning cycle, the PLC only transfers the states related to the output from the output image register to the output latch during the output refresh phase, refreshing the output interface. During other phases, the output state remains in the output image register.

Cyclic scanning means that within a scanning cycle, the PLC needs to execute multiple operations, using time-sharing scanning to execute them sequentially, repeatedly running in cycles.

What are the main components of an electromagnetic contactor? Briefly describe the working principle of an electromagnetic contactor.

An electromagnetic contactor generally consists of an electromagnetic mechanism, contacts, an arc extinguishing device, a releasing spring mechanism, a bracket, and a base. The contactor operates based on electromagnetic principles: when the electromagnetic coil is energized, the current generates a magnetic field, attracting the armature, which drives the contacts to operate, causing the normally closed contacts to open and the normally open contacts to close; these actions are linked. When the coil is de-energized, the electromagnetic force disappears, and the armature falls under the action of the releasing spring, restoring the contacts, meaning that the normally open contacts open and the normally closed contacts close.

Briefly describe the definition of a programmable logic controller (PLC).

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 associated peripheral devices should be designed to easily integrate into industrial control systems and facilitate the expansion of their functions.

Briefly describe the differences in working principles between a PLC system and a relay contactor system.

Different components;

Different numbers of contacts;

Different methods of implementing control;

Different working methods.

What are the characteristics of the STL stepping ladder instructions in the Mitsubishi FX 2N series PLC?

(1) The transfer source automatically resets;

(2) Allows dual outputs;

(3) Main control function.