1. Importance of Data TypesIn PLC programming, data types are like the foundation of a building, determining how data is stored, its range of values, and the rules for operations. Only by accurately grasping data types can we ensure precise data storage, efficient transmission, and correct calculations, thus ensuring the stable operation of the entire automation control system. For example, in a temperature control system, the temperature data collected by sensors must be stored using the correct data type to allow for precise comparison and calculation in the PLC, enabling accurate control of heating or cooling devices.2. Basic Data TypesBitDefinition and Characteristics: A bit is the smallest data unit in PLC, with only two possible values: 0 and 1, corresponding to the “off” and “on” states in electrical circuits. It is commonly used to represent discrete signals, such as the pressing (1) and releasing (0) of a button, or the opening (1) and closing (0) of a valve.Application Scenario: In industrial automation production lines, it is used to detect the operational status of devices, such as the start (1) and stop (0) signal feedback of a motor. In ladder diagram programming, bit data types often appear in the logical expressions of contacts and coils.ByteDefinition and Characteristics: 1 byte consists of 8 bits, with a value range of 0 – 255 (unsigned), representing binary numbers from 00000000 to 11111111. Bytes can be used to store simple numerical values, such as device IDs and status codes.Application Scenario: In small control systems, it is used to store simple coded information from sensors, such as a specific type of sensor represented by values from 0 to 255 indicating different detection distance ranges, stored and processed using byte data types.WordDefinition and Characteristics: 1 word equals 2 bytes, or 16 bits, with an unsigned value range of 0 – 65535, and a signed value range of -32768 to 32767. Word types are suitable for storing larger integers, such as the speed values of motors or the cumulative running time of devices (in seconds, especially when the values are large).Application Scenario: In motor control, if we want to set a target speed for a motor, the speed value can be stored and transmitted using word data types, facilitating speed control calculations in the PLC.DWordDefinition and Characteristics: A double word consists of 4 bytes, or 32 bits, with an unsigned double word value range of 0 – 4294967295, and a signed double word having an even larger range. Double words are used to store very large integers or the integer part of floating-point numbers (in some special operation scenarios).Application Scenario: In large industrial automation projects, to calculate the total energy consumption of equipment, since the energy consumption values may be very high, using double word types for storage ensures that data accuracy is not lost due to overflow.3. Data Type ConversionAutomatic ConversionPrinciple: In certain specific operations, PLC automatically performs data type conversions. For instance, when a byte type data is added to a word type data, the PLC automatically extends the byte data to word type before performing the calculation to ensure the accuracy of the results.Notes: Although automatic conversion is convenient, programmers must be aware of the conversion rules to avoid data precision loss or results that do not match expectations. For example, the byte type data has a limited range, and when extended to word type, the high bits are filled with 0s. If subsequent processing involves the high bits, planning should be done in advance.Forced ConversionMethod: In programming, sometimes it is necessary to forcibly convert one data type to another. Different brands of PLC have different forced conversion instructions. For example, in Siemens S7-1200 series, the “CONVERT” instruction can be used to convert integers to floating-point numbers.Application Scenario: When unified format processing of different types of data is required, such as in data communication, integer data collected from sensors needs to be converted to floating-point format for interaction with the host computer.4. Data Types and Programming PracticeVariable DeclarationRules: At the beginning of PLC programming, the variables to be used must be declared, specifying their data types. For example, in structured text programming, a variable used to store temperature values can be declared as: “VAR TEMP: REAL; END_VAR”, where the variable “TEMP” is declared as a real (floating-point) type for storing temperature measurement values.Function: Correct variable declaration helps the compiler check for data type errors in the program, improving the readability and maintainability of the program.Data Type Matching in OperationsRequirements: When performing arithmetic or logical operations, the data types involved must be compatible. For instance, a bit data cannot be directly multiplied with a word data. If mixed operations are to be performed, data type conversion must be done in advance.Example: When calculating the running power of a motor, the power calculation formula is P = U * I (voltage multiplied by current). Voltage and current data may be stored in different data types, and before the calculation, it must be ensured that they are of the same type, such as converting both to floating-point numbers for precise calculation to obtain an accurate power value.
