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Entry type: FAQ Entry ID: 109752514, Entry date: 01/05/2018
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What should you watch out for when using the fail-safe communication blocks of S7 F Systems Lib V1.3 SP2 and higher?

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The "S7 F Systems Lib" provides blocks for the fail-safe sending and receiving of data via S7 connections. This enables fail-safe transmission of data of the F data types F_REAL and F_BOOL between different S7 F-CPUs.

With version V1.3 SP2 of the "S7 F Systems Lib"  the blocks F_RCVBO, F_RCVR and F_RDS_R have received a new input parameter - "COMMVER_USED" - with which the compatibility mode can be set. This input parameter is set by default to "1".

Behavior with "COMMVER_USED" = 1

  • This mode does not support communication with blocks from older versions of the S7 F Systems Lib (< V1.3 SP2).
  • The communication is not affected by a brief connection interruption as long as the duration of the interruption is less than the set F monitoring time (value at the "TIMEOUT" input parameter.
  • In the case of a communication overload, with this setting there might be sequence number errors that lead to an interruption of the communication and switching to the configured substitute values via the
    F communication blocks.

Note

You can avoid a communication overload be limiting the number of communication calls per second. Therefore, if possible, you should limit the number of communication block instances or move them to an OB with a slower cycle (longer cycle time).
Note that the permitted maximum number of communication calls per second is limited by the CPU used.
More information on the topic is available in the entry entitled How can you determine the quantity framework of the communication instances (PBC) for PCS 7 automation systems?.

Behavior with "COMMVER_USED" = 0

  • This mode supports communication with blocks from older versions of the S7 F Systems Lib (< V1.3 SP2).
  • The communication can be aborted by a brief connection interruption even if the duration of the interruption is less than the set F monitoring time. If there is abortion of the F communication, there is a switchover to the configured substitute values.
  • In this mode, a communication overload does not lead to an abortion of the F communication.

Note
More information about F communication blocks and the "COMMVER_USED" input parameter is available in the manual entitled SIMATIC Industrial software S7 F/FH Systems - Configuring and Programming in the section "F-Blocks for F-Communication between F-CPUs".

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