Using Pointers For Indirect Addressing - Siemens Simatic S7 Series System Manual

PLC concepts
4.2 Accessing data
The following table provides an example of the fixed mapping convention (established by
STEP 7 Micro/WIN SMART and downloaded as part of the I/O configuration, in the system
block).
Table 4- 18 CPU mapping convention
CPU
Starting I0.0
address
Q0.0
4.2.6

Using pointers for indirect addressing

Indirect addressing uses a pointer to access data in memory. Pointers are double word
memory locations that contain the address of another memory location. You can only use
V memory locations, L memory locations, or accumulator registers (AC1, AC2, AC3) as
pointers. To create a pointer, you must use the Move Double Word instruction to move the
address of the indirectly addressed memory location to the pointer location. Pointers can
also be passed to a subroutine as a parameter.
An S7-200 SMART CPU allows pointers to access the following memory areas: I, Q, V, M, S,
AI, AQ, SM, T (current value only), and C (current value only). You cannot use indirect
addressing to access an individual bit or to access HC, L or accumulator memory areas.
To indirectly access the data in a memory address, you create a pointer to that location by
entering an ampersand character (&) and the first byte of the memory location to be
addressed. The input operand of the instruction must be preceded with an ampersand (&) to
signify that the address of a memory location, instead of its contents, is to be moved into the
location identified in the output operand of the instruction (the pointer).
70
Signal Expansion
board module 0
I7.0 I8.0
Q7.0 Q8.0
AI12 AI16
AQ12 AQ16
Expansion Expansion
module 1 module 2
I12.0 I16.0
Q12.0 Q16.0
AI32 AI48
AQ32 AQ48
Expansion Expansion
module 3 module 4
I20.0 I24.0
Q20.0 Q24.0
AI64 AI80
AQ64 AQ80
System Manual, 09/2015, A5E03822230-AC
Expansion
module 5
I28.0
Q28.0
AI96
AQ96
S7-200 SMART