1. Direct addressing
The S7 series plc programming instructions support both direct addressing and indirect addressing.
Direct addressing is done by directly giving the signal address, which can be used for all programming elements (I, Q, M, SM, V, L, S, T, C, AC, HC, etc.).
[Example 8-5] The program for outputting the contents of the flag memory MB10 on QB2 at one time by direct addressing is as shown in Fig. 8-4.1.
The MOV B in the figure is a byte move instruction in S7-200PLC, EN is the start condition, the IN end is the data address to be moved, the OUT end is the moving target position address, and SMO.O is the constant "1" signal inside the PLC. . A more specific description of this directive can be found in the relevant section of the plc programming.
2. Indirect addressing
In the indirect addressing mode of the S7-200, the operand specifies the address where the operation object is stored, and does not specify the operation object. Therefore, the indirect addressing mode needs to be performed by the "address pointer".
The so-called "address pointer" is a register dedicated to storing addresses when indirectly addressing. In the S7-200 PLC, the following points should be noted when establishing the address pointer:
1 Only variable V, local variable L or accumulators AC1, AC2, AC3 can be used as the address pointer (ACO is not available).
2 to establish the address pointer, the memory address required for indirect addressing must be moved to the address pointer in double word form; the memory address is marked with the symbol "&", and the "&" indicates that the address is only the address, not the memory. details.
3 Indirect search for j, should be preceded by the address pointer with a "*" mark, indicating that the operand is an indirect addressed address pointer.
4 The memory that is allowed to be accessed by the address pointer is the current value of I, Q, V, M, S and timer T, counter C, but the analog input/output AI/AQ, high-speed counter HC, special flag SM with local variable L Cannot be accessed via the address pointer. The indirect addressing of the 5S7-200 cannot be used for binary bit signals, high-speed counters HC, local variables L, such as indirect addressing for I0.2, Q0.2, HC1, LW2, etc.
[Example 1] A program in which the 4-byte data (12345678) in the variable memories VB200 to VB203 is moved to the flag registers MBO to MB3 with AC1 (accumulator 1) as an address pointer is as shown in Fig. 8-4.2.
The program is divided into four segments, and the first segment moves the address VB200 into the address pointer AC1 using the double word move instruction MOVD (the symbol in the ladder diagram is MOV__DW).
In the second stage, the content (the value 1234 in VB200 and VB201) stored in the address VB200 in the address pointer ACI is moved to the MWO by the word move instruction MOVW.
In the third paragraph, the address in the address pointer is incremented by 2 (the address of AC1 is changed to VB202).
The fourth segment moves the content (value 5678 in VB202, VB203) stored in the address VB202 in the address pointer AC1 to MW2.
The specific description of the MOV instruction in the figure can be found in the relevant section of Section 10.4 of Chapter 10.
[Example 2] The program for moving the contents of the variable memory VW determined by the offset memory VD1004 to the VW1900 with LD10 (local variable 10) as a pointer is as shown in Fig. 8-4.3.
The program is divided into three segments, and the first segment uses the double word move instruction MOVD (the symbol in the ladder diagram is MOV DW) to move the address VBO into the address pointer LD10.
In the second paragraph, the address VBO in the pointer is added to the content of the offset memory VD1004 (the address of the LD 10 is changed to VBO + offset), and the moving target is determined.
The third segment moves the content stored in the address "VBO+offset" in the pointer LD10 to the VW1900.
In this example, if the content in the VD 1004 is 1000, the content in the variable memory VW1000 will be copied into the VW 1900; if the content in the VD 1004 is changed to 1100, the content in the variable storage VW 1100 will be copied into the VW 1900.
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