S7-300 is one of the programmable logic controller (PLC) series products produced by Siemens AG in Germany. Its modular structure, easy implementation of distributed configuration, high cost-effectiveness, strong electromagnetic compatibility, and good resistance to vibration and impact make it an economical and practical solution in a wide range of industrial control fields.
Product Features
For modular small and medium-sized control systems with low performance requirements, different grades of CPUs can be equipped, and different types of expansion modules can be selected. Up to 32 modules can be expanded, and the modules are integrated with backplane buses, network connections - Multipoint Interface (MPI), - PROFIBUS or - Industrial Ethernet. All modules can be accessed through the programmer PG without slot restrictions.
Product Features

Short cycle time and high processing speed; The instruction set is powerful (containing over 350 instructions) and can be used for complex functions; The product design is compact and can be used in situations with limited space; Modular structure, more flexible design; There are different performance levels of CPU modules available for selection; Functional modules and I/O modules can be selected; There are module types that can be used in harsh outdoor conditions.

CPU module
Various CPUs have different performance, for example, some CPUs integrate digital and analog input/output points, while others integrate communication interfaces such as PROFIBUS-DP. There are status fault indicator lights, mode switches, 24V power terminals, battery box, and memory module box on the front panel of the CPU (some CPUs do not have them)
Signal Module (SM)
Digital input module: 24V DC, 120/230V AC
Digital output module: 24V DC, relay
Analog input module: voltage, current, resistance, thermocouple
Analog output module: voltage, current
Functional Module (FM)
The functional module is mainly used for process signal processing tasks with strict time requirements and large memory capacity requirements.
-Counting: Counter module
-Positioning: fast/slow feed drive position control module, electronic cam controller module, stepper motor positioning module, servo motor positioning module, etc
Selection of Siemens PLC S7-300
1、 Model selection
（1） Types of Siemens PLC S7-300
Siemens PLCs are divided into two types based on their structure: integral type and modular type. They are also classified into on-site installation and control room installation based on their application environment; Divided by CPU word length into 1-bit, 4-bit, 8-bit, 16 bit, 32-bit, 64 bit, etc. From an application perspective, the selection can usually be based on control functions or input/output points.
The I/O points of the integral Siemens PLC are fixed, so users have limited options and are used for small control systems; Modular PLCs offer a variety of I/O cards or cards, allowing users to select and configure the number of I/O points for the control system in a reasonable manner. They are convenient and flexible in function expansion and are generally used for large and medium-sized control systems.
（2） Selection of Input/Output Modules
The selection of input and output modules should consider consistency with application requirements. For example, for input modules, application requirements such as signal level, signal transmission distance, signal isolation, and signal power supply mode should be considered. For output modules, the type of output module to be selected should be considered. Generally, relay output modules have the characteristics of low price, wide voltage range, short lifespan, and long response time; The thyristor output module is suitable for frequent switching, low power factor inductive loads, but it is expensive and has poor overload capacity. The output module also includes DC output, AC output, and analog output, which should be consistent with the application requirements.
Intelligent input and output modules can be selected reasonably according to application requirements to improve control level and reduce application costs.
Consider whether an expansion rack or remote I/O rack is needed.
（3） Selection of Memory
Due to the development of computer integrated chip technology, the price of memory has decreased. Therefore, in order to ensure the normal operation of application projects, it is generally required to select at least 8K memory capacity for PLC based on 256 I/O points. When complex control functions are required, larger capacity and higher grade memory should be selected.
（4） Selection of redundant functions
1. Redundancy of control units
(1) Important process units: The CPU (including memory) and power supply should have 1B1 redundancy.
(2) When needed, a hot standby redundant system composed of PLC hardware and hot standby software, as well as a 2nd or 3rd redundancy fault-tolerant system, can also be selected.
2. Redundancy of I/O interface units
(1) The multi-point I/O cards of the control circuit should be redundantly configured.
(2) Multi point I/O cards for important detection points can be redundantly configured. 3) According to the needs of important I/O signals, 2-fold or 3-fold I/O interface units can be selected.
2、 Selection of control functions
This selection includes the selection of features such as computational function, control function, communication function, programming function, diagnostic function, and processing speed.
To alleviate CPU communication tasks, communication processors with different communication functions (such as point-to-point, fieldbus, industrial Ethernet) should be selected according to the actual needs of network composition.
3、 Estimation of Input/Output (I/O) Points
When estimating the number of I/O points, appropriate margin should be considered. Usually, based on the counted input and output points, an additional 10% to 20% expandable margin is added as the input and output point estimation data. When placing an actual order, it is necessary to round the input and output points according to the product characteristics of the manufacturer's PLC.
4、 Estimation of Memory Capacity
Memory capacity refers to the size of hardware storage units that can be provided by the programmable controller itself, while program capacity refers to the size of storage units used by user application projects in memory. Therefore, program capacity is smaller than memory capacity. During the design phase, as the user application has not yet been developed, the program capacity is unknown and will only be known after program debugging. In order to estimate the program capacity when designing and selecting, the estimation of memory capacity is usually used instead.
There is no fixed formula for estimating the memory capacity of a storage device. Many literature sources provide different formulas, generally based on 10-15 times the number of digital I/O points and 100 times the number of analog I/O points. This number is used as the total word count of the memory (16 bits per word), and the remaining 25% is considered.