Chapter 2 Devices, Symbols and Instructions used in the POU in which the local symbols are declared. Besides, the identifier of a local symbol in a POU can be the same as the identifier of a local symbol in another POU. However, if this local symbol identifier is the same as a global symbol identifier, the system will automatically regard it as a local symbol. *Note : For further explanations regarding a POU, refer for ISPSoft User Manual. 2.3.2 Classes Symbols can be categorized into four classes. The features of the four classes are described below. VAR － General symbol The symbols of this class are for general operations only.
The significance of a symbol of this class depends on the data type of the symbol. INPUT － Symbol used as an input pin of a function block 2_ A symbol of this class is used as an input pin of a function block. It can only be declared in the function block. If a function block is called, the symbols of this class can receive the input values sent by the caller. In a ladder diagram, INPUTs are put at the left sides of function blocks with pins receiving the input values sent by the caller. OUTPUT － Symbol used as an output pin of a function block A symbol of this class is used as an output pin of a function block. It can only be declared in a function block.
After the execution of a function block is complete, the operation results will be sent to the caller through the symbols of this class. In a ladder diagram, OUTPUTs are put at the right sides of function blocks with pins sending the operation results to the caller. INOUT － Symbol used as a feedback pin of a function block A symbol of this class is used as a feedback pin of a function block. It can only be declared in the function block. Please refer to the following example. When the function block is called, the caller sends the value in D1 to DT_IO, which is a symbol of the INOUT class. After the operation comes to an end, the final value of DT_IO is sent to D1.
In a ladder diagram, the symbols of this class are put at the left sides of function blocks. 2.3.3 Data Types The data type of a symbol determines the format of the value of the symbol. Suppose there are two symbols VAR_1 and VAR_2. The data type of VAR_1 is BOOL, and the data type of VAR_2 is WORD. If VAR_1 and VAR_2 are used in a program, VAR_1 will represent a contact, and VAR_1 will represent a 16-bit device which can be involved in arithmetic operation or transferring the data. The data types supported by ISPSoft are listed below. 2-11