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Siemens AG
Entry type: FAQ, Entry ID: 49749632, Entry date: 04/27/2011

How do you activate the "Prioritized OCM communication" function to improve the OP communication performance?

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With OP communication you exchange data between operator stations, for example Operator Panel (OP), Touch Panel (TP) or WinCC, and SIMATIC modules with communications capability. The service is possible by way of MPI, PROFIBUS and Industrial Ethernet subnetworks.

With OP communication we provide you with functions that you need for operator control and monitoring. These functions are integrated in the operating system of the SIMATIC S7 modules. A CPU maintain multiple connections to one or various different operator stations.

When you activate the "Prioritized OCM communication" function you can significantly accelerate the OP communication.

The CPUs below support the "Prioritized OCM communication" function.

CPU Order number Firmware
CPU315-2 PN/DP 6ES7315-2EH14-0AB0 from V3.2
CPU315F-2 PN/DP 6ES7315-2FJ14-0AB0 from V3.2
CPU317-2 PN/DP 6ES7317-2EK14-0AB0 from V3.2
CPU317F-2 PN/DP 6ES7317-2FK14-0AB0 from V3.2
CPU319-3 PN/DP 6ES7318-3EL01-0AB0 from V3.2
CPU319F-3 PN/DP 6ES7318-3FL01-0AB0 from V3.2

You activate the "Prioritized OCM communication" function for the CPUs above in the Hardware Configuration.
In the Hardware Configuration you double-click the configured CPU to open the Properties dialog. Go to the "Cycle/Clock Memory" tab and activate the "Prioritized OCM communication" function.

Fig. 01


  • The consistency of the data to the user program is no longer guaranteed. The consistency must be provided by the user program.
  • The cycle time increases.

Data consistency with the PUT/GET functions

  • When you activate the "Prioritized OCM communication" function, the specified data consistency is no longer guaranteed, because the communication tags in blocks of up to 240 bytes are not copied consistently to and from the user memory in the cycle control point of the operating system. The communication tags are copied to and from the user memory during the runtime of the user program. The consistency must be provided by the user program.
    The following continue to be consistent:
    • Byte / Word / Double-word access
    • SFC14 "DPRD_DAT"
    • SFC15 "DPWR_DAT"
    • SFC81 "UBLKMOV"
  • If a defined data consistency is required, then the communication tags in the user program of the CPUs must not be greater than 240 bytes.

Further Information
The manual "SIMATIC S7-300 CPU 31xC and CPU 31x: Technical Data", sections 3.2.3 and 3.2.11 provide information about the "Prioritized OCM communication" function. The manual is available for downloading in Entry ID: 12996906.