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I (process image input): The CPU samples the peripheral (physical) input points just prior to
the cyclic OB execution of each scan cycle and writes these values to the input process
image. You can access the input process image as bits, bytes, words, or double words. Both
read and write access is permitted, but typically, process image inputs are only read.
Table 4- 19
Bit
Byte, Word, or Double Word
By appending a ":P" to the address, you can immediately read the digital and analog inputs
of the CPU, SB or SM. The difference between an access using I_:P instead of I is that the
data comes directly from the points being accessed rather than from the input process
image. This I_:P access is referred to as an "immediate read" access because the data is
retrieved immediately from the source instead of from a copy that was made the last time the
input process image was updated.
Because the physical input points receive their values directly from the field devices
connected to these points, writing to these points is prohibited. That is, I_:P accesses are
read-only, as opposed to I accesses which can be read or write.
I_:P accesses are also restricted to the size of inputs supported by a single CPU, SB, or SM,
rounded up to the nearest byte. For example, if the inputs of a 2 DI / 2 DQ SB are configured
to start at I4.0, then the input points can be accessed as I4.0:P and I4.1:P or as IB4:P.
Accesses to I4.2:P through I4.7:P are not rejected, but make no sense since these points are
not used. Accesses to IW4:P and ID4:P are prohibited since they exceed the byte offset
associated with the SB.
Accesses using I_:P do not affect the corresponding value stored in the input process image.
Table 4- 20
Bit
Byte, Word, or Double word
Q (process image output): The CPU copies the values stored in the output process image to
the physical output points. You can access the output process image in bits, bytes, words, or
double words. Both read and write access is permitted for process image outputs.
Table 4- 21
Bit
Byte, Word, or Double word
By appending a ":P" to the address, you can immediately write to the physical digital and
analog outputs of the CPU, SB or SM. The difference between an access using Q_:P instead
of Q is that the data goes directly to the points being accessed in addition to the output
process image (writes to both places). This Q_:P access is sometimes referred to as an
"immediate write" access because the data is sent immediately to the target point; the target
point does not have to wait for the next update from the output process image.
Because the physical output points directly control field devices that are connected to these
points, reading from these points is prohibited. That is, Q_:P accesses are write-only, as
opposed to Q accesses which can be read or write.
S7-1200 Programmable controller
System Manual, 03/2014, A5E02486680-AG
Absolute addressing for I memory
I[byte address].[bit address]
I[size][starting byte address]
Absolute addressing for I memory (immediate)
I[byte address].[bit address]:P
I[size][starting byte address]:P
Absolute addressing for Q memory
Q[byte address].[bit address]
Q[size][starting byte address]
4.2 Data storage, memory areas, I/O and addressing
PLC concepts
I0.1
IB4, IW5, or ID12
I0.1:P
IB4:P, IW5:P, or ID12:P
Q1.1
QB5, QW10, QD40
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