Siemens SIMATIC S7-200 System Manual page 42

S7-200 Programmable Controller System Manual
Data in other memory areas (such as T, C, HC, and the accumulators) are accessed by using an
address format that includes an area identifier and a device number.
V B 100
Byte address
Access to a byte size
Area identifier
MSB
7
VB100 VB100
Most significant byte
MSB
15
VW100 VB100
Most significant byte
MSB
31
VD100 VB100
Figure 4-5 Comparing Byte, Word, and Double-Word Access to the Same Address
Accessing Data in the Memory Areas
Process-Image Input Register: I
The S7-200 samples the physical input points at the beginning of each scan cycle and writes
these values to the process-image input register. You can access the process-image input register
in bits, bytes, words, or double words:
Bit:
Byte, Word, or Double Word:
Process-Image Output Register: Q
At the end of the scan cycle, the S7-200 copies the values stored in the process-image output
register to the physical output points. You can access the process-image output register in bits,
bytes, words, or double words:
Bit:
Byte, Word, or Double Word:
Variable Memory Area: V
You can use V memory to store intermediate results of operations being performed by the control
logic in your program. You can also use V memory to store other data pertaining to your process
or task. You can access the V memory area in bits, bytes, words, or double words:
Bit:
Byte, Word, or Double Word:
Bit Memory Area: M
You can use the bit memory area (M memory) as control relays to store the intermediate status of
an operation or other control information. You can access the bit memory area in bits, bytes,
words, or double words:
Bit:
Byte, Word, or Double Word:
28
V W 100
Byte address
Access to a word size
Area identifier
LSB
0
Least significant byte
LSB
8 7 0
VB101
24 23 16 15
VB101
I[byte address].[bit address]
I[size][starting byte address]
Q[byte address].[bit address]
Q[size][starting byte address]
V[byte address].[bit address]
V[size][starting byte address]
M[byte address].[bit address]
M[size][starting byte address]
V D 100
Byte address
Access to a double word size
Area identifier
MSB = most significant bit
LSB = least significant bit
Least significant byte
8 7
VB102 VB103
LSB
0
I0.1
IB4
Q1.1
QB5
V10.2
VW100
M26.7
MD20