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Entry type: FAQ Entry ID: 69063762, Entry date: 10/07/2019

How do S7-300/S7-400/S7-1500 modules behave with CPU RUN/STOP transitions when taken out of the process image?

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When the operating mode changes from STOP to RUN, with the S7-1500 the substitute values fail and the values of the output modules flicker.

All outputs are disabled in the STOP operating mode and react as they were parameterized. For the S7-1500 modules (DQ32x24VDC/0.5A ST, for example) you can parameterize substitute values for the CPU STOP or "Keep last value". Substitute values are values that the output modules output to the process when the CPU goes into STOP. The following figure shows in the case of CPU STOP how to set the behavior in the Properties of an output module.

Fig. 1

Substitute value behavior when changing operating mode

  1. When the CPU goes from RUN to STOP, the parameterized substitute values are output at the outputs.
  2. When the CPU goes from STOP to RUN, the substitute values are deleted on all channels.
    Note the following here:


  • When the CPU starts up the IOs are released at the beginning of the OB1 cycle.
  • The output remains set to 0 until the first OB1 cycle has ended. However, the release for the output is already present in this time as well as multiple transfer of the values via the bus. In this way the value 0 is transferred.
  • For a smooth change of the CPU operating mode from STOP to RUN you need a redundant configuration (S7-400H system).
    To achieve a smooth change of operating mode you store the values in the retentively configured DB areas and program output to the IOs in the OB100.

In the case of the S7-1500 the process image is deleted with the transition from STOP to RUN. If the substitute values are to be output also in RUN, then the values in the OB100 have to be written again. In the Properties of the output modules you can set for the complete module whether the input/output addresses are to be updated in the process image of the CPU or are to be addressed only via direct access.

  • If you have set "Automatic Update" for the process image, the current value is output from the process image.
  • If you have set "None" for the process image, no new values are output automatically.

Fig. 2

If you have set "None" for the process image and the channels of an output module are to receive defined values after the STOP-RUN transition, you write these values in the Startup OB via direct access to the channels.

The following figure shows a sample SCL in which the value 1 is written to all 32 outputs of a DQ module. With ":P" you directly access the module channels.

Fig. 3

Creation Environment
The screens in this entry were created with STEP 7 (TIA Portal) V15.


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