For a long time, industrial control has been the main application area of Siemens PLCs. With technological advancements, current Siemens PLC products utilize high-performance microprocessors such as 16-bit and 32-bit processors. Improvements in communication technology have further expanded the applications of Siemens PLCs. Today, programmable logic controller (PLC) technology is very mature. The overall development trend of PLCs is towards high integration, small size, large capacity, high speed, ease of use, and high performance. In the early 1980s, small Siemens PLCs were more expensive than relay control devices used in small systems. With continuous development, PLCs now boast compact structures, are very easy to operate, and have continuously increased functionality. Large and medium-sized PLCs have been adapted to smaller PLCs, making them an irreplaceable control device in modern electrical control systems. Simultaneously, Siemens PLCs are beginning to be applied in various aspects of daily life.
For example, consider automatic doors, a common sight in daily life. Traditional automatic door systems typically used logic control modules, but these were gradually phased out due to high failure rates, low reliability, and inconvenient maintenance. In a modern automatic door control system, a PLC is used as the controller. An ultrasonic transmitter and a photoelectric receiver serve as input devices. Two different sets of sensors input control signals to the PLC. The PLC then controls the system's encoding table, I/O allocation table, and the automatic door's movement, thereby controlling the door's opening or closing (rising or falling).
Siemens PLC products have gained widespread recognition from users due to their reliability and high performance through continuous innovation. PLC products are widely used in low-voltage power supply switches, high-voltage power distribution equipment, combination switches, load centers, and other products, gradually replacing single-chip microcomputer protectors and ordinary PLC products. The availability of Siemens mining PLCs is becoming a selling point and advantage for some mining equipment companies.
In traditional relay control systems, achieving a specific control over a target in a given operation involves connecting low-voltage electrical appliances according to a defined logical relationship. In other words, to fulfill a control requirement, we use a specific control circuit composed of low-voltage electrical appliances.
For example, the full-voltage starting control of electric motors that we are familiar with uses contactors, push-button switches, fuses, and thermal relays, which are connected together according to logic control requirements.
A PLC (Programmable Logic Controller) uses software to program and implement control requirements. Programming involves various programming elements, which are analogous to the electrical components in a relay control circuit. These programming elements include input registers, output registers, bit memories, timers, counters, general-purpose registers, data registers, and special-function memories.
The internal memories of a PLC function very similarly to the relays used in a relay contact control system, also having "coils" and "contacts," with unrestricted contact usage. However, they are not "hard" relays, but rather storage units within the PLC's memory. When the logic state written to this unit is "1," it indicates that the corresponding relay coil is energized, its normally open contact closes, and its normally closed contact opens. Therefore, these internal relays are called "soft relays." We refer to these PLC storage units and "soft relays" as programming elements.
Specifically, there are: 1. Input Relays (represented by the symbol I): These store the results of the CPU sampling the input terminals during the input scanning phase. 2. Output Relays (represented by the symbol Q). 3. Intermediate Relays (represented by the symbol M): Located in the bit storage area of the PLC memory, these relays function similarly to intermediate relays in a relay control system, storing intermediate states of the control logic and other control information. 4. Variable Memory (represented by the symbol V): Primarily used to store intermediate results of control logic operations during user program execution, and can also be used to store other data related to processes or tasks. 5. Special Flag Relays (represented by the symbol SM). 6. Timers (represented by the symbol T): Timers are important programming elements in PLCs, serving as internal devices for accumulating time increments. 7. Counters (represented by the symbol C): Primarily used to accumulate the number of input pulses. PLC programming elements are mainly accessed and used through instructions.
PLC instructions are the instruction set of a programmable logic controller (PLC), also known as PLC programming instructions. These instructions are the basic building blocks used to write and control PLC programs, and they include input and output instructions, logic instructions, data processing instructions, timer and counter instructions, numerical comparison instructions, and so on. PLC instructions can be used to implement complex control logic, such as controlling machines and equipment on a production line in an industrial automation system. PLC instruction sets vary depending on the PLC brand, but they generally conform to the standards of the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC).
There are three basic types of PLC instructions:
1. Input commands (I commands): Used to read input signals from external devices, such as sensor signals or switch states, to determine or control program execution. For example, I0.0 represents reading the state of input port 0.
2. Output Commands (O Commands): Used to control the output signals of external devices, such as motors, lights, and solenoid valves. Output commands can control the switching on and off of these devices. For example, O1.2 means controlling the second bit of output port 1 to be on.
3. Intermediate instructions (M instructions): These are used to process numerical values, logical operations, and function control, enabling relatively complex control functions. For example, M10 represents an intermediate address, which can be used to store intermediate values or perform logical operations.
PLC (Programmable Logic Controller) instructions are used to write programs that control the operations performed within those programs. Here are some common PLC instructions:
1. LD (or LDI): stands for "Load/Load Immediate Value", used to read input from the input signal.
2. OUT: Indicates "output", used to output the output signal to the specified output device.
3. AND: means "and", used to perform logical AND operations.
4. OR: stands for "or", and is used to perform a logical OR operation.
5. ADD: means "addition", used to perform addition operations.
6. SUB: means "subtraction" and is used to perform subtraction operations.
7. MOV: stands for "move", used to move data from one location to another.
8. JMP: stands for "jump", used to jump to a specified program address.
9. RET: means "return", used to return from a subroutine to the main program.
10. CALL: means "call", used to call a subroutine.
11. COM: stands for "two's complement invert", used to perform the two's complement invert operation.
12. NOP: stands for "no operation", used to occupy program space when there is no operation.
These instructions are only a part of the PLC instructions; there are actually many other instructions. Different PLC brands and models may have different instruction sets.
