Siemens NAN 67 Series Manual de Serviço descarregar pdf (Página 97)

7-5

Ladder Logic (LAD) for S7-300 and S7-400

C79000-G7076-C504-02

A pointer is a device that identifies the location of a variable. A pointer

contains an address instead of a value. When assigning an actual parameter

for the parameter type “pointer,” you provide the memory address. STEP 7

allows you to enter the pointer in either a pointer format or simply as an

address (such as M 50.0). The following is an example of the pointer format

for accessing data starting at M 50.0:

P#M50.0

If you are working with an instruction whose address identifier indicates a

memory area of your programmable logic controller and a data object that is

either a word or a double word in size, you need to be aware of the fact that

the memory location is always referenced as a byte location. This byte

location is the smallest byte number or the number of the high byte. For

example, the address in the statement shown in Figure 7-2 references four

successive bytes in memory area M, starting at byte 10 (MB10) and going

through byte 13 (MB13).

Statement: L MD10

Address identifier Byte location

Figure 7-2 Example of Memory Location Referenced as Byte Location

Figure 7-3 illustrates data objects of the following sizes:

S Double word: memory double word MD10

S Word: memory words MW10, MW11, and MW12

S Byte: memory bytes MB10, MB11, MB12, and MB13

When you use absolute addresses that are a word or a double word in width,

make sure that you do not create any byte assignments that overlap.

MB10 MB11 MB12 MB13

MW11

MD10

MW10 MW12

Figure 7-3 Referencing a Memory Location as a Byte Location

Pointers

Working with Word

or Double Word as

Data Object

Addressing

1 2 ... 92 93 94 95 96 97 98 99 100 101 102 ... 317 318

Sem comentários

Siemens NAN 67 Series Manual de Serviço Página 97