Linear Addressing - Siemens SIMATIC S5 Manual
Distributed i/o system
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9.1.1
Linear addressing
Definition
Linear addressing is possible in the P and Q areas of the CPU. Each input or
output of a DP slave has one and only one address in the P or Q area,
respectively (i.e. linear addressing).
Advantages
Linear addressing affords rapid access to the individual bytes of a DP slave.
You also have bit-serial access to input and output bytes 0 to 127 (63 with
941 CPU) via the process image (e.g. U E 1.0).
Restrictions
The following restrictions apply to linear addressing:
ET 200 Distributed I/O System
EWA 4NEB 780 6000-02a
IM 308-C – addressing, diagnostics and use of the FB IM308C
If you address an output using linear addressing, you cannot also write to
the output via the FB IM308C. The IM 308-C does not recognize the at-
tempt to address this output via the FB IM308C.
CPUs 941 to 944: The 941 to 944 CPUs build the process image word by
word. This can mean that under certain circumstances, two DP slaves are
addressed within a word. If you selected QVZ as the error reporting mode
in COM ET 200 Windows and you address via the process image, the fol-
lowing case may arise:
There are two DP slaves in a word and one DP slave fails: the CPU gener-
ates QVZ for the second DP slave as well, despite the fact that this DP
slave can still be addressed via the bus.
The following rules therefore apply to the CPUs 941 to 944:
– Align the start of address of a DP slave with an even address (e.g. 2, 4,
6, ...) and leave the odd address free. You can then use access via the
process image as well.
and
– Use load and transfer commands. The commands run byte-by-byte
checks and can tell whether or not a byte exists.
CPUs 941 to 944: If you select linear addressing for the Q area with
CPUs 941 to 944, you cannot use L QB /T QB. Instead, you access the
addresses through the standard function block FB 196.
If you address the inputs/outputs of a DP slave in the Q area you must
make sure that there is no input/output module at the same address in the
central programmable controller.
If you use the entire Q area, do not insert an input or output module in the
central programmable controller.
You can avoid the danger of inadvertent double assignments by reserving
input and output areas when you enter the master parameters with
COM ET 200 Windows (see Section 8.2.3).
9
9-5
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- IM 308-C Master Interface
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