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Siemens S7-200 PLCs use the following memory access settings:

Area Description Accessed as Bit Accessed as Byte Accessed as Word Accessed as Dword Can be Retentive Can be Forced
I Discrete inputs and image register read/write read/write read/write read/write no yes
Q Discrete outputs and image register read/write read/write read/write read/write no yes
M Internal memory bits read/write read/write read/write read/write yes yes
SM Special memory bits (SM0 to SM29) are read-only read/write read/write read/write read/write no no
V Variable memory read/write read/write read/write read/write yes yes
T Timer currents and Timer bits T-bit read/write no T-current read/write no T-current-yes

T-bit-no

no
C Counter currents and Counter bits C-bit read/write no C-current read/write no C-current-yes

C-bit-no

no
HC High-speed counter currents no no no read-only no no
AI Analog inputs no no read only no no yes
AQ Analog outputs no no write only no no yes
AC Accumulator registers no read/write read/write read/write no no
L Local variable memory read/write read/write read/write read/write no no
S SCR read/write read/write read/write read/write no no

Direct Addressing

The S7-200 stores information in different memory locations that have unique addresses. You can explicitly identify the memory address that you want to access. This allows your program to have direct access to the information. Direct addressing specifies the memory area, size, and location. for example, VW790 refers to word location 790 in V memory area.

To access a bit in a memory area, you specify the address, which includes the memory area identifier, the byte address, and the bit number preceded by a period. The figure below shows an example of accessing a bit (which is also called a byte.bit address). In this example, the memory area and byte address (1=input, and 3=byte 3) are followed by a period (.) to separate the bit address (bit 4). This is a picture of the dialog. This is a picture of the dialog.

You can access data in most memory areas (V, I, Q, M, S, L, and SM) as bytes, words, or double words by using the byte-address format. To access a byte, word, or double word of data in the memory, you must specify the address in a similar way to specifying the address for a bit. This includes an area identifier, data size description, and the starting byte address of the byte, word, or double-word values, as shown in the figure below. Data in other memory areas (such as T, C, HC, and the accumulators) are accessed using an address format that includes an area identifier and a device number. This is a picture of the dialog.

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