The role of Siemens PLC variables - Database & Sql Blog Articles

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Variables can be used in the plc program, which is one of the features of the S7 series PLC. The following functions can be implemented by using variables in S7.
(1) Increasing the versatility of the program In the PLC programming stage, in order to improve the versatility of the program, the same logic block can be used for different control situations by variables.
For example, for the logic block shown in Figure 11-2.2, assume that its function is to implement the logic operation C=BA; D-D+l, and use variables to program.

When the logic block is called, if the variable A is assigned the value IO.1, the variable B is assigned the value I0.2, the variable D is assigned the value MW10, and the variable C is assigned the value QO.1, the execution result of the program is:
QO.1=10.2.IO.1;
MW10 - MW10+l.
When the logic block is called, if the variable A is assigned to Il.l, the variable B is assigned to I1.2, the variable D is assigned to MW20, and the variable C is assigned to Ql.l, the execution result of the program is:
Ql.l=11.2. Il. l;
MW20 = MW20+l.
(2) It is convenient for program check. After the variable is used, when the logic block is called, the corresponding input and output signals required by the logic block can be displayed in the call instruction (see Figure 11-2.3) to facilitate program design and inspection.

Since S7 has a specified format requirement for variables and symbol addresses, the logical block variables A, B, C, and D corresponding to Figure 11-2.2 have been used with symbol addresses IN A, IN-, respectively, in Figure 11-2.3. B, OUT_C, INOUT D are programmed.