Siemens SIMATIC S7-1200 Manual page 63

Each different memory location has a unique address. Your user program uses these
addresses to access the information in the memory location. The absolute address consists
of the following elements:
● Memory area (such as I, Q, or M)
● Size of the data to be accessed (such as "B" for Byte or "W" for Word)
● Address of the data (such as Byte 3 or Word 3)
When accessing a bit in the address for a Boolean value, you do not enter a mnemonic for
the size. You enter only the memory area, the byte location, and the bit location for the data
(such as I0.0, Q0.1, or M3.4).
In the example, the memory area and byte address (M = bit memory area, and 3 = Byte 3)
are followed by a period (".") to separate the bit address (bit 4).
Easy Book
Manual, 11/2011, A5E02486774-04
4.4 Memory areas, addressing and data types
Absolute address of a memory area:
A Memory area identifier
B Byte address: byte 3
C Separator ("byte.bit")
D Bit location of the byte (bit 4 of 8)
E Bytes of the memory area
F Bits of the selected byte
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