How do PROFINET IO controllers behave in the case of error when accessing the data of a shared device?
In large or widely dispersed plants, numerous IO controllers are often implemented. The "Shared Device" function permits you to share the modules and submodules of an IO device between various IO controllers. This capacity to divide make flexible automation concepts possible. For example, you can group together close-lying IO modules in one IO device.
This entry describes how PROFINET IO controllers behave in the case of error when accessing the data of a shared device.
If a PROFINET IO controller fails, the working IO controller continues to access the shared device, but only the submodules assigned to it. The submodules of the failed IO controller are no longer read or written to.
Using the "Module-internal Shared Input/Shared Output (MSI/MSO)" function you can access up to 4 IO controllers on one submodule (input module or output module) of a shared device.
If an IO controller fails with MSI, the other assigned IO controllers can continue to have read access to the input data of an input module.
If the IO controller which has write access to the outputs of the submodule fails with MSO, then the outputs are no longer written. If one of the IO controllers which has read access to the outputs of the submodule fails with MSO, then the outputs continue to be written and read by the functioning IO controllers.
If the shared device fails, then each sharing IO controller gets a station failure reported. Other diagnostics depend on the assignment of the submodules. Only this IO controller receives the diagnostics information of the interface module.
The interface module of the shared device is assigned to one of the accessing IO controllers. The IO controller to which the interface module is assigned and which has established an application relation (AR). Only this IO controller receives the PROFINET diagnostics.
Special case of ET 200M as shared device
- If one of the assigned IO controllers fails, then the complete distributed IO device ET 200M outputs substitute values as shared device. This means that not only the IO output modules of the failed IO controller output substitute values, but also the IO output modules of the functioning IO controllers.
- Since the outputs of an IO module cannot output replacement values separately, in the shared device environment it holds that the IO only outputs process values if all the assigned IO controllers are in the "RUN" operating mode. Therefore, as soon as one IO controller goes into the "STOP" mode, the outputs of the IO module also output substitute values which are subscribed to by the other IO controllers. This behavior can be observed, for example, with the S7 function "Enable PA". "Enable PA" causes the CPU in the "STOP" operating mode to report as if it were in the "RUN" operating mode. In this way, the modules assigned to the other IO controllers still in the "RUN" operating mode are enabled again.
Special case of ET 200MP as shared device
In shared device mode, if one of the sharing IO controllers goes into the "STOP" operating mode or fails because of a disconnected connection, for example, then all the submodules of the output module behave as parameterized for substitute value (switch-off, for example). This means that even if one IO controller fails, the other IO controllers sharing the shared device can no longer control the assigned submodule of the output module.
- The maximum number of IO controllers depends on the interface module. Refer to the device manual of the interface module concerned to find out how many IO controllers can be supported.
- Entry ID 102325771 provides information about which IO devices support the Shared Device function and MSI/MSO.