Entry type: FAQ, Entry ID: 46636225, Entry date: 11/26/2010

What should you watch out for when parameterizing the watchdog time when a ring redundancy is established with the Media Redundancy Protocol?

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The media redundancy function ensures network and plant availability. Redundant transmission paths (ring topology) ensure that when one transmission path fails an alternative path is made available.

The Media Redundancy Protocol (MRP) is a component of the PROFINET standardization in compliance with IEC 61158. An overview of the PROFINET IO controllers and IO devices that support the Media Redundancy Protocol and for which the Media Redundancy Protocol can be enabled is available in Entry ID: 44383954.

If one transmission path fails, reconfiguration of the network, i.e. switchover to the redundant transmission path, can take up to 200ms. 200ms is the maximum reconfiguration time for 50 devices in the ring.  
If the watchdog time of the PROFINET IO device is less than the switchover time, then it might happen that OB86 temporarily reports subrack failure/return.

Increase the watchdog time by

  • Increasing the update time or
  • Increasing the number of accepted update cycles with missing I/O data.

The watchdog time is the product of the parameters "Update time" and "Number of accepted update cycles with missing IO data". The watchdog time must be greater than 200ms to avoid module failure through redundancy switchover.

Follow the instructions below to increase the watchdog time.

  • Open the hardware configuration of the SIMATIC S7-300 station that is functioning as PROFINET IO controller.
  • Mark the PROFINET IO device in the hardware configuration. Double-click the PROFINET interface in the slot table of the PROFINET IO device to open the "Properties" dialog of the PROFINET interface.

Fig. 01

  • In the "Properties" dialog of the PROFINET interface, you switch to the "IO Cycle" tab.
  • Set the values for the "Update time" and "Number of accepted update cycles with missing IO data" parameters so that the watchdog time is greater than 200ms.

Fig. 02

A sample application of ring redundancy with the Media Redundancy Protocol (MRP) is available in Entry ID: 33696406.