Siemens SIMATIC S7-200 - Direct Addressing of the CPU Memory Areas; Using the Memory Address to Access Data; Accessing a Bit of Data in the CPU Memory (Byte.bit Addressing)

CPU Memory: Data Types and Addressing Modes
5-2
S7-200 Programmable Controller System Manual
A5E00066097-02
5.1 Direct Addressing of the CPU Memory Areas
The S7-200 CPU 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.
Using the Memory Address to Access Data
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. Figure 5-1 shows an
example of accessing a bit (which is also called “byte.bit” addressing). In this
example, the memory area and byte address (I = input, and 3 = byte 3) are
followed by a period (“.”) to separate the bit address (bit 4).
I3 4
76543210
MSB
LSB
I 0
I 1
I 2
I 3
I 4
I 5
I 6
I 7
MSB = most significant bit
LSB = least significant bit
.
Area identifier (I = input)
Byte address: byte 3 (the fourth byte)
Period separates the byte address
from the bit number
Bit of byte, or bit number: bit 4 of 8 (0 to 7)
I 8
I 9
I 10
I 11
I 12
I 13
I 14
I 15
Figure 5-1 Accessing a Bit of Data in the CPU Memory (Byte.bit Addressing)