6ES7321-1BH02-0AA0 Siemens SIMATIC S7-300, digital input SM 321

SIMATIC S7-300, digital input SM 321, potential isolated, 16 digital inputs, 24V DC, 1 20 pin

S7-300 is one of the programmable logic controller (PLC) series products produced by Siemens in Germany. Its modular structure, easy distributed configuration, high cost performance, strong electromagnetic compatibility, and good shock resistance make it an economical and practical solution in a wide range of industrial control fields.

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 indicators, mode switches, 24V power terminals, battery boxes, and memory module boxes 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 that require strict time requirements and high memory capacity.

-Counting: Counter module

-Positioning: fast/slow feed drive position control module, electronic cam controller module, stepper motor positioning module, servo motor positioning module, etc

Siemens PLC S7-300 Selection

1、 Selection of model

（1） Types of Siemens PLC S7-300

Siemens PLC is divided into two types based on its structure: integrated and modular, and on-site installation and control room installation according to the application environment; According to CPU word length, it can be divided into 1 bit, 4 bit, 8 bit, 16 bit, 32 bit, 64 bit, etc. From an application perspective, selection can usually be based on control functions or input/output points.

The I/O points of the integrated Siemens PLC are fixed, so the user has less choice and is used for small control systems; Modular PLC provides multiple I/O cards or cards, so users can reasonably select and configure the I/O points of the control system. Its function expansion is convenient and flexible, and it is generally used in 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 method should be considered. For output modules, the type of output module to be selected should be considered. Generally, relay output modules have characteristics such as low price, wide voltage range, short lifespan, and long response time; The thyristor output module is suitable for frequent switching and low inductance power factor load situations, but it is expensive and has poor overload capacity. The output module also has DC output, AC output, and analog output, which should be consistent with the application requirements.

According to application requirements, intelligent input and output modules can be reasonably selected to improve control level and reduce application costs.

Consider whether expansion racks or remote I/O racks are needed.

（3） Memory selection

Due to the development of computer integrated chip technology, the price of memory has decreased. Therefore, to ensure the normal operation of application projects, it is generally required to select at least 8K memory based on 256 I/O points for PLC memory capacity. When complex control functions are required, memory with larger capacity and higher level should be selected.

（4） Selection of redundant functions

1. Redundancy of control units

(1) Important process units: CPU (including memory) and power supply should be 1B1 redundant.

(2) When necessary, a hot standby redundant system composed of PLC hardware and hot standby software, as well as a redundant error tolerance system with 2-fold or 3-fold redundancy, can also be selected.

2. Redundancy of I/O interface units

(1) The multipoint I/O cards of the control circuit should be configured with redundancy.

(2) Multi point I/O cards for important detection points can be configured with redundancy. 3) According to the needs of important I/O signals, I/O interface units with 2-fold or 3-fold can be selected.

2、 Selection of control functions

This selection includes the selection of features such as calculation function, control function, communication function, programming function, diagnostic function, and processing speed.

To reduce CPU communication tasks, communication processors with different communication functions (such as point-to-point, fieldbus, industrial Ethernet) should be selected based on the actual needs of network composition.

3、 Estimation of input/output (I/O) points

When estimating I/O points, appropriate margin should be considered. Usually, based on the input and output points counted, an additional 10% to 20% of the expandable margin should be added as the input and output point estimation data. When placing an actual order, it is also necessary to round off the input and output points based on the product characteristics of the manufacturer's PLC.

4、 Estimation of memory capacity

Memory capacity is the size of the hardware storage unit that the programmable controller itself can provide, while program capacity is the size of the storage unit used by user application projects in memory, so program capacity is smaller than memory capacity. During the design phase, as the user application program has not yet been developed, the program capacity is unknown during the design phase and needs to be determined after program debugging. In order to have a certain estimation of program capacity during design and selection, memory capacity estimation is usually used instead.

There is no fixed formula for estimating the memory capacity of memory. Many literature sources provide different formulas, which are generally based on 10-15 times the number of digital I/O points, plus 100 times the number of analog I/O points. This number is used as the total number of words in memory (16 bits are a word), and a margin of 25% of this number is considered.

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