Siemens SIMATIC ET 200AL, System Manual

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High-speed analog modules
8.1 Basics
Analog value processing
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Function Manual, 06/2014, A5E03461439-AC
Send clocks
When you configure isochronous mode, the input and output data is synchronized within a
send clock (cycle). The data of the input modules is acquired in cycle n and simultaneously
transferred to the IM. The data is available in cycle n+1 in the CPU and can be processed.
The CPU copies the data to the IM in the next cycle n+2. The output modules then output
the data simultaneously in the same send clock. The acquisition – processing – output
therefore takes place in three cycles. You can find more information on the configuration of
isochronous mode in the STEP 7 online help.
①
Measured values of the input modules are acquired and copied to the IM
②
Measured values are processed and output values determined
③
Output values are copied to the IM and output by output modules
Figure 8-1 3-cycle model
Oversampling
The use of the oversampling function in analog input or analog output modules requires an
isochronous configuration.
With analog input modules, the set send clock is divided into time-equidistant sub-cycles.
The send clocks can be subdivided into 2 to 16 sub-cycles. Each sub-cycle reads in a
measured value. The read-in measured values of a data cycle are copied to the IM in the
next send clock and are then available to the processing CPU one send clock later.
With analog output modules, the set send clock is also divided into time-equidistant sub-
cycles. The send clocks can be subdivided into 2 to 16 sub-cycles. Each sub-cycle returns
an output value. The output values are copied to the IM by the CPU within the same send
clock and are written to the process one send clock later.
The read-in and output values are transmitted in the user data of the analog module. This
way, the address space of the module is extended from 2 bytes of user data per channel to
16 x 2 bytes of user data per channel. If you subdivide the send clock into less than 16 sub-
cycles, the unused addresses are assigned the error value 0x7FFF during input. For output,
the values of the unused addresses are ignored.