(1) Computing function
The computing functions of a simple PLC include logic operation, timing and counting functions; The operation functions of ordinary PLC also include data shift, comparison and other calculation functions; More complex arithmetic functions include algebraic operations, data transmission, etc.; Large PLCs also have analog PID calculations and other advanced computing functions. With the advent of open systems, there are communication functions in PLC at present, some products have communication with the lower computer, some products have communication with the same computer or the upper computer, and some products also have the function of data communication with the factory or enterprise network. When designing and selecting the model, it should be based on the requirements of practical application and reasonably select the required computing functions. Most applications only require logic operations and timing and counting functions, while others require data transfer and comparison, and when used for analog quantity detection and control, algebraic operations, numerical conversions, and PID operations are used. In other cases, operations such as decoding and encoding are required to display data.
(2) Control function
The control functions include PID control operation, feedforward compensation control operation, ratio control operation, etc., which should be determined according to the control requirements. PLC is mainly used for sequential logic control, therefore, most occasions often use a single loop or multi-loop controller to solve the control of analog quantities, and sometimes also use a special intelligent input and output WeChat public account PLC engineers can take the data to increase knowledge and improve the skill unit to complete the required control function, improve the processing speed of PLC and save memory capacity. For example, PID control unit, high-speed counter, analog unit with speed compensation, ASC code conversion unit, etc.
(3) Communication functions
Medium- and large-sized PLC systems should support a wide range of fieldbuses and standard communication protocols (e.g., TCP/IP) and should be able to connect to the plant management network (TCP/IP) if required. The communication protocol should comply with ISO/IEEE communication standards and should be an open communication network.
The communication interface of the PLC system should include serial and parallel communication interfaces (RS2232C/422A/423/485), RIO communication interfaces, industrial Ethernet, common dcs interfaces, etc.; Large and medium-sized PLC communication buses (including interface equipment and cables) should be configured with 1:1 redundancy, the communication bus should meet international standards, and the communication distance should meet the actual requirements of the device.
In the communication network of the PLC system, the communication rate of the superior network should be greater than 1Mbps, and the communication load should not be greater than 60%. The main forms of communication network of PLC system are as follows:
1) PC is the main station, and multiple PLCs of the same type are slaves to form a simple PLC network;
2) 1 PLC is the master station, and other PLCs of the same type are slave stations, forming a master-slave PLC network;
3) The PLC network is connected to the subnet of the DCS in the large DCS through a specific network interface;
4) Dedicated PLC network (dedicated PLC communication network for each manufacturer).
In order to alleviate the CPU communication task, PLC communication processors with different communication functions (such as point-to-point, fieldbus, and industrial Ethernet) should be selected according to the actual needs of network composition.
(4) Programming function
Offline programming mode: PLC and programmer share a CPU, when the programmer is in programming mode, the CPU only provides services for the programmer, and does not control the field equipment. After the programming is completed, the programmer switches to the running mode, and the CPU controls the field device, but cannot be programmed. Offline programming reduces system costs, but is inconvenient to use and debug.
Online programming mode: CPU and programmer have their own CPU, the host CPU is responsible for on-site control, and in a scanning cycle with the programmer for data exchange, the programmer will send the online program or data to the host, the next scanning cycle, the host will run according to the newly received program. This method is costly, but the system debugging and operation are convenient, and it is often used in large and medium-sized PLCs.
Five standardized programming languages: Sequential Function Diagram (SFC), Ladder Diagram (LD), Functional Module Diagram (FBD) and Statement Table (IL) and Structural Text (ST) two text languages. The selected programming language should comply with its standards (IEC6113123), and should also support multiple language programming forms, such as C, Basic, Pascal, etc., to meet the control requirements of special control occasions.
(6) Processing speed
The PLC works by scanning mode. From the point of view of real-time requirements, the processing speed should be as fast as possible, if the signal duration is less than the scanning time, the PLC will not be able to scan the signal, resulting in the loss of signal data.
The processing speed is related to the length of the user's program, the CPU processing speed, the quality of the software, etc. At present, the response of PLC contacts is fast and high, and the execution time of each binary instruction is about 0.2-0.4μs, so it can adapt to the needs of applications with high control requirements and fast corresponding requirements. The scanning cycle (processor scanning cycle) should meet: the scanning time of the small PLC is not more than 0.5ms/K; The scanning time of large and medium-sized PLCs is not more than 0.2ms/K.
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